Home Diseases Optometry - Rosenblum Yu.Z. What does a cylinder mean in a glasses prescription? Conversion of astigmatic lenses

Optometry - Rosenblum Yu.Z. What does a cylinder mean in a glasses prescription? Conversion of astigmatic lenses

A spherical lens cannot improve vision with astigmatism, because by correcting one meridian, it at the same time worsens the other. Spherical lenses enhance or weaken the refraction of the eye, but they cannot eliminate the difference in the refraction of the main sections. To correct astigmatism, cylindrical lenses are used, which are like a cast from a cylinder. They can be of two types - scattering and collecting light.

The higher the strength of the cylinder and the older the person who first wore cylindrical glasses, the worse they are tolerated. When first prescribing glasses, it is not recommended to prescribe cylinders with a force of more than 4.0 D.

As already mentioned, correction of an astigmatic eye can be achieved using two combinations of spherical and cylindrical lenses. The transition from one combination of a sphere and a cylinder to another combination is carried out by the transposition method.

CYLINDER TRANSPOSITION
1. Under the sphere of the new copybook, the algebraic sum of the spherical and cylindrical components is written.
2. 3The sign of the cylindrical component is reversed.
3. The direction of the cylinder axis changes by 90 degrees.

Examples:
Original copy: +1.0; +2.5 axis 100 degrees.
Transposition: +3.5;-2.5 axis 100 degrees.
Original copy: -1.75; -2.0 axis 120 degrees.
Transposition: -3.75;+2.0 axis 30(210) degrees.
Original writing: -1.25; +4.0 axis 90 degrees.
Transposition: +2.75; -4.0 axis 0 deg.

In case of intolerance to cylindrical lenses, a spherical equivalent can be prescribed.

When reading a prescription for astigmatic glasses, which is made in a spherocylindrical prescription, one must keep in mind that under the sign sph the refraction of one of the main sections of the astigmatic lens is written, under the sign cyl is the astigmatic difference, akh indicates the direction of that main section, the refraction of which is written under the sign of the sphere .

Determination of astigmatism using CROSS CYLINDERS

In cases where the patient is not resistant to axis displacement, the correct position of the cylinder axis is important in correction. You can clarify the position of the axis and the optical power of the cylinder using CROSS CYLINDERS (Jackson bi-cylinders or crossed cylinders). They use the dot group or "Grit" test found in most sign projectors, or the round sign on the visual acuity chart, the size of which should correspond to the visual acuity obtained. Control is carried out after vision correction; the frame must contain selected lenses. The sets include cross cylinders plus - minus 0.25 D and plus - minus 0.5 D. You can use any of them, but some believe that the 0.5 D cylinder should be used when determining the direction of the cylinder axis, as it is more sensitive, and 0.25 D - when determining cylinder forces.

Clarification of the cylinder axis - AXIAL TEST

Each eye is examined separately. The cross-cylinder, depending on its design, is located in the frame or attached to it so that its handle coincides with the axis of the correcting cylinder (the handle is on the axis!). In this case, at 45 degrees from the handle, the axes of the cross-cylinders will be located, which are indicated by a plus or minus sign, one on the right, the other on the left, i.e. artificial astigmatism is created and visual acuity is reduced. Next, the cylinder is rotated around its axis by the other side so that the plus and minus switch places. The image quality varies. The patient should be asked in which position the image is clearer or which image is more blurry (the real position of the axis has not been found) - the first or second. You need to remember at what position of the negative axis the image is better (when it is on the right or when it is on the left) and turn the handle of the correction cylinder approximately 5 degrees towards the negative axis. This manipulation must be repeated quickly (do not hold the CC for more than 2 seconds) several times, each time moving the cylinder handle by about 5 degrees until the patient says that he does not feel a difference in image quality when moving the cylinder, he sees the same in any position. This means that the image has entered the macular area, the axis is selected correctly and the study must be stopped.

Clarification of cylinder force - FORCE TEST

The study (Fig. 9) is carried out with the position of the axis of the cross-cylinder on the axis of the selected cylinder (axis to axis!). This means that we added 0.25D or 0.5D to the existing cylinder if they have the same signs as glass or reduced the refraction if the signs are opposite. We place either a positive or a negative cylinder on the glass axis. If the patient notices improvement in vision with increasing cylinder strength, then it should be increased. For example, if there was a cylinder + 0.75 D, and with a cross cylinder + 0.25 D vision improved, then in the recipe we change the cylinder to 1.0 D. In this case, we immediately need to change the spherical component, taking into account the changed force of the cylinder - by half its value (reduce , if the cylinder was increased or increased if the cylinder was decreased)

If there are doubts about the choice of cylinder size, then a smaller cylinder size is selected.

Astigmatism does not affect vision in all cases and does not always require correction, so decompensated astigmatism is corrected first.

correction of astigmatism in children

Even incomplete correction, compensating for astigmatism by more than half, significantly improves visual acuity.

8-18 years - hypermetropic astigmatism is subject to full correction. For initial and progressive myopia, the principle of adding cylinders comes into force only in cases where they increase maximum visual acuity (astigmatism more than 1.0 D). Watch in dynamics. When the level decreases to physiological, the cylinders should be discontinued.

Mixed astigmatism requires complete or almost complete correction and constant wearing of glasses. When selecting glasses, we focus on maximum visual acuity. At the same time, one should not be afraid of strengthening the myopic sphere, given the tendency to hyperaccommodation in these individuals.

Correction of astigmatism in adults

18-45 years - the appearance of hidden hypermetropia or progression of myopia may require the introduction of cylinders. An adult who has not previously worn top hats accepts them with great difficulty and, rather than
The older a person is, the more difficult adaptation is. If a large cylinder is required, it must be introduced in stages - first the minimal one, then add 0.75 D in subsequent glasses. Warn the patient that these will be trial glasses; they can be made with inexpensive frames and lenses, and after getting used to them, replace them in the final version with better quality.

60 years or more - there is a transformation of astigmatism from direct to reverse. Cylinders are prescribed only in cases where they significantly improve visual acuity and comfort; the completeness of astigmatic correction depends on the tolerability of the cylinders.

If astigmatism is more than 4.0 D or first detected at the age of 12 years or older, the first glasses are prescribed with a cylinder smaller than that detected.

In adults, the direction of the cylinder axis plays an important role during adaptation. For direct type astigmatism, correction often does not cause difficulties. With reverse astigmatism, adding cylinders affects vision more than with direct astigmatism, but adaptation is usually easy. Because humans live in a vertically oriented world, even small degrees of reverse astigmatism can significantly reduce vision. Astigmatism with oblique axes greatly affects vision; the primary purpose of the cylinders is tolerated with great difficulty, and in some cases, due to gross distortion of space, adaptation does not occur at all. In such cases, they resort to either step-by-step adaptation to the cylinders, or the issue is resolved in favor of contact correction. With astigmatism with oblique axes, uneven accommodation occurs in different meridians, constant fluctuations in the optical alignment of the eye - either the anterior or posterior focal surface is aligned with the retina. The stronger the cylinder, the more the axes are deviated from the horizontal or vertical, the stronger the image distortion caused by meridional aniseikonia - the difference in the size of the images on the retina of one eye. With an oblique position of the axis, the correction cylinder causes more problems with binocular vision. The maximum inclination of the vertical lines occurs when the axis of the correction cylinder is oriented at 45 and 135 degrees. In this case, 1.0 D of astigmatism causes an image tilt of 0.4 degrees. Under conditions of binocular vision, image deformation causes unpleasant sensations in the patient. There are certain mechanisms for compensating for distortions in the shape of objects and their position in space: perspective assessment; solid knowledge of the shape and size of visible objects; “linking” the outlines of objects to a familiar environment; limitation of the depth of visual space Small cylinders (degree of astigmatism 0.5 or less) are corrected in the presence of complaints: head
pain, especially with prolonged exercise at a distance (driving), visual fatigue near, slight decrease in vision. If there are no hidden violations of convergence and accommodation, small cylinders are prescribed.

25-05-2014, 17:44

Description

FINAL INVESTIGATION IN NATURAL CONDITIONS

This stage is the main one and cannot be skipped: it is here that the decision on assigning points is made.

Repeated examination in natural conditions includes monocular and then binocular selection of lenses for distance and selection of additional lenses for near.

Monocular selection for distance. The exact lenses that were determined as a result of all clarifying tests for cycloplegia are placed in the sockets of the trial frame. The visual acuity of each eye is examined in turn. If visual acuity is not lower than that determined with a wide pupil, proceed to binocular selection. More often, however, due to the usual tone of accommodation, it is reduced.

In this case, you should first of all try to change the spherical component of the correction: weaken the positive or strengthen the negative lens. The strength of the additional negative lens restoring maximum visual acuity will indicate the magnitude of the usual accommodation tone.

In case of hyperopia, it plays a corrective role, and in case of myopia, on the contrary, it enhances ametropia. Therefore, the attitude towards it is different: with hypermetropia it should be taken into account when prescribing a correction, with myopia - strive for its complete relaxation. However, even with hypermetropia, a high habitual accommodation tone (above 2,5 diopter) is undesirable, since it is unstable and does not provide high vision in unfavorable conditions.

With myopia, the accommodation tone is up to 0,75 DPT is conventionally considered normal. To remove it, it is enough to perform “fogging” according to Sheard; sometimes it disappears immediately with both eyes open. Only in rare cases does the usual accommodation tone exceed critical values. In this case, one has to resort to long-term atropinization and wear glasses at the height of cycloplegia.

Usually, after several days of wearing glasses, the excess accommodation tone disappears and visual acuity reaches its maximum. It is very rare that long-term treatment of pseudomyopia with medications and exercises is required.

So, as a result of studying the accommodation tone, the same spherical correction is left or a slightly weaker one (in the case of hypermetropia).

If none of the spherical lenses restore the initially achieved visual acuity, then the cylinder must be checked. In this case, an improvement in visual acuity can be achieved by weakening or increasing its strength. In the first case, the strength of the Cylinder in the frame is reduced to the optimal value (giving the highest visual acuity), in the second case it is left the same, since the uneven accommodation tone, which increases astigmatism, is undesirable and must be eliminated.

Finally, it may turn out that in order to restore the previous visual acuity it is necessary to rotate the axis of the correcting cylinder: an axial test is carried out with a crossed cylinder and the new optimal position of the axis is determined. A significant difference from its original position is most often due to prolonged wearing of glasses with an erroneously installed cylinder, i.e., the axis of which does not correspond to the main section of the astigmatic eye.

In these cases, it is advisable to prescribe correction according to the new axis position established for cycloplegia. If the patient has not previously worn astigmatic glasses, then the discrepancy may be explained by uneven accommodation. In these cases, it is recommended to choose for the cylinder axis the average direction between its positions with a narrow and wide pupil.

Thus, examination under conditions of cycloplegia is necessary mainly for accurate diagnosis of the main sections of astigmatism and for determining the boundary values of the sphere and cylinder. Under natural conditions, it is determined how much these values can be reduced, while it is undesirable to correct the axis of a cylindrical lens.

Bipocular selection for distance. In order to make sure that the patient will tolerate glasses well, the participation of both eyes in the act of vision should be checked.

Such balance must mean that three conditions are met:

equal tension of accommodation of both eyes and focusing them on the object in question (accommodative-refractive equilibrium);
the direction of the visual lines of both eyes towards the object in question (muscular balance, or orthophoria);
the same size of images of this object on the retinas of both eyes (equilibrium in image size, or iseikonia).

To evaluate them, there are special devices in which, using Polaroid glasses, rasters or other devices, images are shown separately for the right and left eyes, and the subject himself does not know which eye and what he sees. The device was produced in the former USSR RRD-1(raster separator for distance).

On its screen they show one of two figures - a cross consisting of four identical rectangular beams, or “brackets” - two equal rectangular letters WITH, directed with ends towards each other (Fig. 96). The same figures are available in foreign projectors or special devices with Polaroid field separation (for example, Haase's Polateste).

Examined from a distance 5 m sees the vertical beams of the cross with one eye and the horizontal beams of the cross with the other; when showing “brackets”, he sees the right part of the figure with one eye and the left part with the other.

Accommodative-refractive balance is determined by the equal clarity of vision of all beams of the cross, muscular balance - by the symmetry of its image, and iseikonia - by the equality of the sizes of the “brackets”.

The first type of equilibrium is practically of greatest importance. Equal clarity of the beams of the cross is achieved by substituting spherical lenses ±0.25 diopter in front of the worse seeing eye. Muscle balance should be examined if the patient is dissatisfied with the selected glasses (i.e. 4 -th stage of the examination). The need to study iseikonia arises extremely rarely - only when correcting high-grade anisometropia.

In the absence of a device RRD-1 A sequential binocular test is recommended: the patient with selected lenses observes the smallest optotypes distinguishable by him; The person conducting the examination alternately covers the patient’s right and then the left eye with a shield. If the patient notices a difference in the clarity of the signs, the examiner adds +0,25 diopter or - 0,25 diopter in front of the worse seeing eye. This procedure is continued until the clarity of vision in both eyes is equal. The test is successful if the visual acuity of both eyes is equal (no amblyopia) and if the difference in their refraction is no more than 2,0 diopter

Before prescription of glasses, the patient is recommended to wear the selected lenses in a trial frame for 10-15 min. If the patient does not tolerate the correction well, then it is necessary first of all to reduce the difference in the power of the spheres of the two eyes by reducing the lens power in the worse eye. If this does not help, then reduce the strength of the cylinders in both eyes, and if this is not enough, then reduce the strength of the spheres. At the same time, a feeling of comfort is achieved due to a slight decrease in visual acuity.

Selection of glasses or additional lenses for near vision. The selection of separate lenses for near may be necessary for presbyopia, as well as for insufficiency of accommodation at a young age. The latter especially often accompanies initial myopia in children and adolescents.

The selection is carried out with two eyes open. Lenses selected for distance are inserted into the sockets of the trial frame, and positive spherical lenses identical for both eyes are added. The patient reads text No. 4 of the table to determine near visual acuity, located at 33 see before his eyes. It is best to use a device for this POZB-1 or other near vision device.

Choose lenses with which reading text is most convenient. There is no such strictly substantiated methodology as for selecting lenses for distance, but for selecting glasses for near. There are several ways to select the optimal correction.

Duochrome test - select lenses that give equal clarity of characters on a red and green background (at a distance 33 cm).

Distance reserve test - determine those lenses with which reading can be done without difficulty from a distance 33 ± 7 see, and then 60 years - from a distance 33 ± 5 cm, the glasses set the eyes to the middle of the “comfort zone”.

Relative accommodation reserve test - determine the weakest positive lens with which it is possible to read the text, then add it to it + 1,5 diopters, and in older people 55 years - +1,0 diopter

In case of presbyopia, the age of the subject also serves as a “compass”. Approximately the power of additional lenses is equal to the patient’s age minus 30 , divided by 10 :

Where D- power of additional lenses, diopters;

A- patient’s age, years (if older 60 years A is taken for 60 .

When accommodation is weakened due to age, the strength of additional lenses is determined by the degree of this weakening, i.e., the level of relative accommodation reserve. Usually it ranges from 1,5 to 3,0 diopter

It should be borne in mind that if absolute visual acuity is not reduced and reading is possible at a normal distance ( 33 cm), then the power of additional lenses should not exceed 3,0 diopter

After determining the strength of the lenses for distance, and if necessary, also for near, the interpupillary distance is measured and glasses are prescribed.

We do not describe the fitting technology for multifocal glasses (i.e., progressive lenses) here. It requires special sets of these lenses and a device to measure convergence when changing the direction of gaze.

Rules for prescribing corrective glasses. The effectiveness of optical correction of visual impairment due to ametropia largely depends not only on the correct selection of glasses, but also on the accuracy of their manufacture.

To avoid possible errors when writing prescriptions and making glasses, doctors and opticians must adhere to a unified designation system for optical correction devices.

Here are the basic rules for drawing up prescriptions for different types of corrective glasses.

General rules. Your glasses prescription contains the following required information:

date of discharge;
patient's last name (if he is younger than 14 years, then age is indicated);
doctor's name;
parameters of spectacle lenses, first for the right, then for the left eye;
the distance between the optical centers of spectacle lenses;
purpose of glasses (for distance, for constant wearing, for working at close range).

In addition, a prescription for glasses may contain the following information: basic parameters of the patient’s face necessary for choosing frames; the need for decentration of spectacle lenses; other features related to the lenses or frame.

There are special instruments to measure the distance between the centers of the pupils. In Fig. 97 shows a distance meter between the centers of the pupils ( IRG-65). The procedure for working with it is as follows. The device is positioned so that the index movement handles face down.

Place the device with the nose pad on the bridge of the patient’s nose and ask him to look at the fixation mark. In this case, the examiner sees both eyes of the patient and the scales and indices in front of him. Using handles located on the underside of the body, the examiner alternately sets the indices so that they divide the pupils of the eyes in half, and determines the interpupillary distance using a scale.

In the absence of a device IRG-65 The distance between the centers of the pupils can be determined using a measuring ruler for selecting eyeglass frames or a regular ruler. The doctor is at a distance 30-35 cm from the face of a patient who is looking at some distant object over the doctor's head. Then the doctor places a ruler on the bridge of the patient’s nose and sights the position of the center of the pupil of his right eye with his left eye, and of the left eye with his right; using the ruler divisions, determines the interpupillary distance in distance glasses.

The interpupillary distance is measured in a similar way using glasses for near, but at the same time the patient looks at the bridge of the nose of the doctor, who sights both pupils with one eye.

Calculations show that with interpupillary distance 60 mm and distance from eyes to object 33 cm the distance between the centers of the lenses in glasses for near vision should be 5 mm less than with distance glasses. It is this difference that is taken into account in sintered bifocal lens blanks: the center of the lower segment of the lens for each eye is shifted by 2,5 mm towards the nose in relation to the center of the main lens.

Rules for prescribing spectacle lenses. Spectacle lenses can be single- or multifocal.

Each of these types of lenses may include the following optical elements: spherical, astigmatic, prismatic, eikonic. In addition, spectacle lenses can be light-protective with different transmittance coefficients.

The following are the rules for prescribing single vision lenses. Spherical (stigmatic) lenses are prescribed as follows: after the designation sph(or in Russian - “sphere”) indicate the “+” sign for converging lenses and the “-” sign for diverging lenses and then the lens power in diopters ( D). The power of the lens is indicated as a decimal fraction, with a whole number of diopters after the decimal point 0 . For example:

When prescribing astigmatic lenses, after the number indicating the power of the spherical element, put a comma, then the symbol cyl(or in Russian - “cylinder”) and indicate the sign and power of the cylindrical element in diopters, as well as the position of its axis (non-active section) on the international scale TABO. For example:

As already mentioned, the eikonic element is practically not used for the correction of aniseikonia; The eikonic effect is used only for the correction of low vision.

Instead of a comma, sometimes a combination sign (=) is used, which resembles an equals sign, but with convex stripes. For example:

Lately the symbol D often omitted. For example:

Abroad, the designation of spherocylindrical combinations is usually simplified: first they put the sign and power of the spherical lens with two digits after the decimal point, then the sign and power of the cylindrical lens, instead of the word axis ( Oh) - multiplication sign - X.

The above script looks like this:

In the absence of a cylindrical lens, only the first number is given; in the absence of a spherical lens, instead of the first number, put 0,00 .

In our recipes, if there is no spherical element, its designation can be omitted. For example, instead of sph0.0 su1 +1.0Dah 10° allowed to write sy1 +1.0Dah 10°.

The position of the axis of the corrective cylindrical lens should be indicated on the diagram TABO arrow.

For complex astigmatism, a sphere and a cylinder of the same sign should be drawn, and for mixed astigmatism, the opposite sign should be drawn. It is not allowed to write a combination of two cylindrical elements in one lens.

If the selection of glasses was carried out with a cylinder of one sign, and you need to write out a cylinder of another sign (for example, if for complex hypermetropic astigmatism, a trial selection is carried out with negative cylinders), then a transposition should be performed. In this case, a cylinder of one sign is replaced by a combination of a sphere of the same sign with a cylinder of the opposite sign with an axis located at an angle 90° relative to the original axis of the cylinder.

The rules of transposition are as follows: the sign of the cylinder is changed to the opposite, the direction of the axis is changed to perpendicular (i.e., it should be subtracted or added 90°), the sign of the sphere is reversed, and its strength is equal to the algebraic sum of the sphere and the cylinder in the original notation.

GOST 23265-78“Eyeglass lenses” for optical production and medical supply services provides a different system for designating the refraction of astigmatic lenses. It is not recommended for eyeglass prescriptions, but eye doctors and optometrists should know it to ensure that glasses are made correctly.

According to this system, to characterize an astigmatic lens, its three parameters are indicated in the following order:

the posterior vertex refraction is less than the refractive cross section (for positive lenses it is smaller in absolute value, for negative lenses it is correspondingly larger);
posterior apical refraction is greater than the refractive section;
direction of the main section with the lowest refraction on the scale TABO in degrees.

Examples of converting the sphere - cylinder system into the system GOST 23265-78 are given in table. 9.

In Russia, lenses with an astigmatic difference of up to 4,0 diopter and posterior apical refraction from- 30 to +20 diopter With an astigmatic difference up to 2,0 diopter intervals between the values of a cylindrical element are 0,25 diopter, over 2,0 diopter - 0,5 diopter

When prescribing glasses with prismatic action (after characterizing the spherical and cylindrical elements), the power of the prismatic element is indicated in prismatic diopters ( ? ) and the direction of the top-bottom line on the scale TABO. In this case the scale TABO continues until 360°.

Just like spherical and cylindrical lenses, prisms can be written in Latin and Russian transcription: prism - rr, base - bas. For example:

When the top-base line is in a horizontal position, it is allowed to indicate its direction with the words: “base to the nose” and “base to the temple” - “ bas nas " And " bas temp ».

In other positions of this line, its direction should be indicated on a circular scale TABO with obligatory designation by an arrow according to the diagram.

When prescribing glasses with a prismatic effect, the force of the corrective prism should be “distributed” approximately equally between the two eyes, that is, the prismatic element should be approximately the same in each eye, and the top-base line should have the opposite direction.

For example, if it is necessary to correct exophoria 6,0 Prisms should be prescribed:

When correcting combined heterophoria corrected with a prism 8,0 prdptr base 30° in front of the right eye, you should write:

According to existing standards, it is allowed to write prismatic elements with a force of 0,5 to 10 prdptr.

The following are rules for prescribing multifocal lenses. The most commonly used lenses are bifocal lenses. They are written out in the form of a fraction, the numerator of which indicates the characteristics of the elements of the upper part, and the denominator indicates the characteristics of the lower part of the lens according to the rules given above.

For example:

Since the segment of the lens used for working at near distances differs from the main part of the lens only by the addition of a spherical component (usually the same in both eyes), a second method of prescribing bifocals is often used: after indicating all the components of the lenses for distance, the refractive power of the additional spherical lens for distance is indicated. close.

For example:

Recently, most often they do not write “bottom” and “top”, but after the designation of lenses intended for distance vision for both eyes, they indicate: the addition for near is so many diopters. For example:

In English-speaking countries they write “n.a.” ( near addition).

Please note that only spherical bifocal lenses are commercially available ( GOST 23265-78).

Trifocal lenses are written in three rows separated by lines. For example:

Bifocal spheroprismatic glasses, prescribed to relieve accommodation and convergence, are prescribed as follows: after distance lenses (they must be non-astigmatic and the difference between the two lenses should not exceed 2,5 diopters) write " BSPO" For example:

After characterizing the refractive properties of spectacle lenses, their light transmission properties can be indicated. In this case, the light transmittance coefficient is indicated ( 25, 50 or 75% ). It is also possible to designate the color of the lens. For example:

Green filters 25%.

Smoke filters (neutral) 50%.

When prescribing glasses containing lenses of non-serial manufacture (including non-astigmatic and astigmatic elements exceeding the values provided by the standards; including prismatic elements; astigmatic lenses with light-protective properties; trifocal lenses), the patient should indicate the point where these glasses can be manufactured.

Rules for prescribing spectacle frames. In a typical prescription, one parameter related to the spectacle frame is indicated - the distance between the centers of the spectacle lenses, which corresponds to the distance between the centers of the patient's pupils. All other parameters of the frame are determined by trial fitting it to the patient’s face directly when placing an order for glasses in an optical store.

If there is a discrepancy between the size of the face and the distance between the patient’s pupils, or if the patient wishes to choose a wide modern frame, the distance between the centers of the openings is much greater than its interpupillary distance, it becomes necessary to shift the centers of the lenses towards the nose.

The minimum diameter of an unfaceted lens required to obtain such decentration can be determined by the formula:

Where L- diameter of the unfaceted lens;

A- width of the spectacle frame opening;

b- width of the spectacle frame;

Dp- the distance between the centers of the lenses that a given patient should have wearing these glasses.

We remind you that the values " A" And " b» marked on the inside of one of the temples of the frame in the form of a fraction A/b. Decentration of lenses towards the temple is much less common. In this case, the minimum diameter of an unfaceted lens is determined by the formula:

If it is impossible to order in person, the glasses prescription should indicate the basic facial parameters necessary for selecting frames, namely:

A p And A l- distance from the center of the bridge of the nose to the centers of the pupils of the right and left eyes, mm;

B- nose bridge width, mm;

IN- distance between the bases of the ears, mm;

G- distance between temples, mm;

D- nose bridge height, mm;

E- temple length, mm.

These parameters can be determined using the spectacle frame fitting ruler included in the spectacle lens trial kits.

Distance A p And A l determined by grid scale 5 , while the zero point of the base recess 1 placed on the center of the bridge of the nose. Using the scale on this recess, the width of the nose bridge is determined. B. Distances IN And G measured using a scale 2 , while the doctor uses his left eye to visualize the position of the right auricle and right temple on the scale, and with his right eye - the position of the left auricle and left temple of the subject. Determining nose bridge height D and temple length E clear from the picture. However, it is still preferable to select frames in person.

Prescription form for glasses. To unify the prescription of glasses and more fully reflect all the characteristics of lenses and frames, two versions of the prescription form for glasses have been developed.

The first option is intended for prescribing the most popular single-vision spherical and astigmatic glasses (Fig. 98). The “Note” column may contain additional information regarding spectacle lenses and frames. This form is intended for in-person ordering, i.e. in this case, the patient must select the frame himself with the help of a qualified optician.

The second option - full (Fig. 99) - is intended for prescribing single-vision and bifocal glasses, containing, along with spherical and astigmatic, also prismatic elements. The front side is filled out in all cases: all the information about the spectacle lenses is entered into its cells. The position of the axes of the astigmatic element is indicated in the diagram by a line. The direction of the line top - base of the prismatic element is indicated in the same diagram by an arrow.

The reverse side of the form is filled out only when ordering glasses in absentia, when it is not possible to directly select the frames to suit the patient’s face. In this case, all the necessary dimensions of the face are indicated ( A p, A l, B, C, G, D, E), measured using a ruler or caliper, and the required distance from the back of the lens to the eye ( AND).

EXAMINATION OF A PATIENT WITH READY GLASSES

The examination is carried out no earlier than after 2 weeks after the patient started using glasses. This period is enough to get used to the glasses or identify their shortcomings.

If the patient is not satisfied with the glasses, then it is necessary to identify the reason for this. First, compliance with their recipe is established. The power of lenses is checked using a diopter meter or using the neutralization method. Then the correct position of the glasses on the face is determined. The correspondence of the centers of the lenses to the centers of the pupils is checked using a centriscope.

You can also mark the position of the lens centers on the dioptre meter.

The displacement of both lens centers by the same distance in the same direction (for example, up or down) is not a defect.

Shifting centers in opposite directions horizontally (i.e. increasing or decreasing the distance between the centers of lenses compared to the interpupillary distance) is permissible within the following limits: for lenses with a power of up to 0,5 diopter ±6 mm, for lenses 0,75...1,0 diopter ±4 mm, for lenses above 1,0 diopter ±2 mm.

With large deviations, a prismatic effect occurs and glasses-induced heterophoria occurs. And since exophoria causes less discomfort than esophoria, in glasses with negative lenses the patient can more easily tolerate a shift of centers outward, and with positive lenses, inward.

The patient tolerates the relative vertical displacement of the lens centers especially poorly. For lenses up to 2,0 diopter it should not exceed 2 mm, for lenses above 2,0 diopter - 1 mm.

For lenses of high refraction, for example when correcting aphakia, even these tolerances established by GOST may turn out to be too high. When patients complain of diplopia and it is eliminated with the help of prisms, the glasses have to be remade.

Next, check the distance of the lenses from the top of the cornea using a keratometer or a regular ruler. The accuracy of this measurement is not very high, since it is impossible to see the rear pole of the lens from the side. Measure the distance from the top of the cornea to the edge of the frame and add 1 mm. The normal distance of the lens from the eye is 12 mm. Its increase enhances the effect of positive and weakens the effect of negative lenses. The permissible deviations in the distance of lenses from the cornea are as follows: for lenses up to ±0.75 diopter ±10 mm, 1,0...3,75 diopter ±5 mm, 4...6,5 diopter ±3 mm, 7..8,5 diopter ±2 mm, 9...12,5 diopter ±1.5 mm, 13...20 diopter ±1 mm.

If the glasses fully comply with the prescription and their position on the face is also correct, then intolerance may be due to physiological reasons.

The most common of them is heterophoria. Heterophoria in glasses is studied with a point light source and a Maddox cylinder. Prisms that correct heterophoria are selected and glasses with prismatic elements are prescribed.

The main symptom of incorrect distance of glasses from the eye is a deterioration in visual acuity in ready-made glasses, compared to that achieved during a trial selection. In these cases, you should try attaching an additional lens from ±0.25 to ±1.0 diopter The lens with which the highest visual acuity is restored will indicate the magnitude of the error. In this case, you should weigh how best to eliminate it: choose a new frame or insert new lenses into the frame chosen by the patient.

In this regard, it should be noted that when prescribing glasses with strong lenses (with a spherical lens above ±8.0 diopters) it is recommended to examine the patient with the selected frame. The approximate tolerance of future glasses can be assessed by attaching selected lenses to it using the so-called Halberg clip.

Another reason for intolerance is the distortion of the perception of spatial relationships introduced by glasses. Positive lenses give an apparent magnification of objects, negative lenses make them appear smaller. Astigmatic lenses have the property of distortion, as a result of which the shape of objects changes - they stretch horizontally or vertically, some objects move away, others come closer.

This can, for example, make it difficult to walk up stairs: the steps seem too close or too far away. The difference in the power of the lenses of the two eyes can cause symptoms characteristic of aniseikonia: with a difference in the power of spherical lenses - a feeling of interference with vision, sometimes double vision of objects (general aniseikonia), with a difference in the power of cylindrical lenses, an increase and approach to the observer of one side of objects, the right or left (meridional aniseikonia).

In case of such complaints, the power of the corrective lenses should be slightly weakened: in case of isometropia - on both eyes, and in case of anisometropia - in the worse eye. At the same time, they make a compromise: at the expense of some deterioration in visual acuity, they achieve a feeling of comfort.

Vertex correction – vertex distance

Vertex correction, vertex distance – everyone who wears glasses has probably heard such expressions. What is vertex distance and vertex correction? Why do you need to know and take into account the vertex distance and how to correctly calculate the vertex correction? Is there a difference between the diopters of glasses and lenses, and what should be the difference in diopters between contact lenses and glasses? In this article we will try to answer these questions.

Vertex distance is the distance from the back of the spectacle lens to the top of the cornea. Normally, the vertex distance should be 12-15 mm. You can measure the vertex distance using a special or regular ruler. It is this vertex distance that guarantees that the image passing through the surface of the spectacle lens will fall on the retina and, accordingly, the person will see the objects in question well and clearly in the glasses.

What happens if the vertex distance shifts?

If the vertex distance does not correspond to the norm, the corrective power of the spectacle lenses changes.

Dispersive minus lenses

Spectacle lenses with minus diopters are divergent; they are designed to compensate for the strong refractive refraction in myopic people, in whom, without correction, the image is located in front of the retina. Properly selected minus lenses, taking into account the vertex distance of 12-15 mm, move the image onto the retina and ensure clarity of vision. Increasing the vertex distance with minus lenses (moving away from the eyes) leads to the fact that the image will again move and be in front of the retina, which means the clarity of vision will deteriorate.

If the vertex distance is reduced (closer to the eyes) with minus lenses, then you can get an excessively strong correction.

Converging plus lenses

Plus lenses are converging and are used to correct weak refraction in hyperopia and presbyopia.

With hypermetropia, the image falls behind the retina, and correction with converging plus lenses moves it onto the retina and makes the image clear.

When the vertex distance from the eyes shifts, that is, when it increases, the refractive power of the plus lens will increase, that is, excessive correction, since the image will move in front of the retina. When the plus lens approaches the eyes, the image will move behind the retina and visual acuity will decrease again.

Thus, changing the vertex distance when correcting with plus and minus lenses will lead to opposite results. Therefore, even if adequate spectacle correction is selected, but the vertex distance is not observed, visual acuity in glasses may differ from the selected correction.

This is why it is important to consider the vertex distance when selecting glasses and ensure that the frames fit correctly before the glasses are even made.

Vertex distance and contact lenses

Vertex distance is an indicator characteristic only for spectacle correction. When corrected with contact lenses, there is no vertex distance, since the contact lens is located directly on the surface of the cornea of the eye. However, the selection of contact lenses is carried out using a trial frame and spectacle lenses from a diagnostic kit, where, of course, there is a vertex distance.

Vertex correction

The difference between the diopters of glasses and contact lenses

Should this vertex distance be taken into account when selecting contact lenses? Is there a difference between the diopters of glasses and contact lenses? What should the diopter ratio be?

In order for the diopters of contact lenses to correspond to the diopters of spectacle lenses, the concept of vertex correction was introduced.

Vertex correction is a mathematical value.

To determine the vertex correction for contact lenses, that is, how much the diopters of contact lenses should differ from the diopters in glasses, there is a special table for converting diopters corrected for vertex distance. Every contact correction room should have such a table. With its help, the optometrist determines how much the diopters of contact lenses need to be changed so that they correspond to the selected spectacle correction, taking into account the vertex distance.

Vertex correction is calculated differently for minus and plus lenses.

As we have already said, minus lenses will become stronger as the vertex distance decreases, that is, as they get closer to the eyes. Therefore, the diopters of minus contact lenses must be smaller to match the selected spectacle correction.

Conversely, lenses with positive values become stronger as the vertex distance decreases (closer to the eyes). Therefore, plus contact lenses must be larger than the corresponding spectacle correction.

Diopter difference value

Another important point that affects the calculation of the vertex correction is the diopter value. How much less or more diopters should be, and what is the permissible difference in diopters, depends on the size of the diopters.

The higher the diopters, the greater the value of the vertex correction, and the more the diopters of glasses and contact lenses do not match and differ. Up to diopter values -/+ 3.75D, the vertex correction value is not taken into account and the diopters in contact lenses correspond to the diopters in glasses.

Allowable diopter difference

Vertex correction is calculated for diopter values above -/+4.0D:

From -/+4.0D to -/+5.75D, the difference between the diopter values of contact and spectacle lenses is 0.25D.
From -/+6.0D to -/+7.5D, the vertex correction value is 0.5D.
From -/+8.0D to -/+10.0D, the difference between the diopters of contact and spectacle lenses is 1.0D.
From +/- 10.5D to +/-11.5D, the vertex correction is 1.5D.
From +/-12.0D the vertex correction will be equal to 2.0D.

Thus, negative contact lenses will be smaller than spectacle lenses by a vertex correction value above -4.0D, and up to -3.75D they will be the same as glasses. Plus contact lenses up to +3.75 will have the same diopters as the corresponding spectacle lenses, and above +4.0D they will be larger than the spectacle lens by the amount of vertex correction.

	Prescription for glasses or contact lenses			Transposed recipe
	Sph, D	Cyl, D	Ax, hail	Sph, D	Cyl, D	Ax, hail
OD - right eye	Select -20.00 -19.50 -19.00 -18.50 -18.00 -17.50 -17.00 -16.50 -16.00 -15.50 -15.00 -14.50 -14.00 -13.50 -13.00 -12.50 -12.00 -11.50 -11.00 -10.50 -10.00 -9.50 -9.00 -8.50 - 8.00 -7.50 -7.00 -6.50 -6.00 -5.75 -5.50 -5.25 -5.00 -4.75 -4.50 -4.25 -4.00 -3.75 -3.50 -3.25 -3.00 -2.75 -2.50 -2.25 -2.00 -1.75 -1.50 -1.25 -1. 00 - 0.75 -0.50 -0.25 0.00 +0.50 +1.00 +1.50 +2.00 +2.50 +3.00 +3.50 +4.00 +4.50 +5.00 +5.50 +6.00 +6.50 +7.00 +7.50 +8.00 +8.50 +9.00 +9.50 +10.00 +1 0.50 +11.00 +11.50 +12.00 +12.50 +13.00 +13.50 +14.00 +14.50 +15.00 +15.50 +16.00 +16.50 +17.00 +17.50 +18.00 +18.50 +19.00 +19.50 +20.00	Select -6.00 -5.75 -5.50 -5.25 -5.00 -4.75 -4.50 -4.25 -4.00 -3.75 -3.50 -3.25 -3.00 -2.75 -2.50 -2.25 -2.00 -1.75 -1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0.00 +0.25 +0.50 +0.75 +1.00 +1.25 +1.50 +1.75 +2.00 +2.25 +2.50 +2.75 +3.00 +3.25 +3.50 +3.75 +4.00 +4.25 +4.50 +4.75 +5.00 +5.25 +5.50 +5.75 +6.00	Select 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 13 0 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 18 0
OS - left eye	Select -20.00 -19.50 -19.00 -18.50 -18.00 -17.50 -17.00 -16.50 -16.00 -15.50 -15.00 -14.50 -14.00 -13.50 -13.00 -12.50 -12.00 -11.50 -11.00 -10.50 -10.00 -9.50 -9.00 -8.50 - 8.00 -7.50 -7.00 -6.50 -6.00 -5.75 -5.50 -5.25 -5.00 -4.75 -4.50 -4.25 -4.00 -3.75 -3.50 -3.25 -3.00 -2.75 -2.50 -2.25 -2.00 -1.75 -1.50 -1.25 -1. 00 - 0.75 -0.50 -0.25 0.00 +0.50 +1.00 +1.50 +2.00 +2.50 +3.00 +3.50 +4.00 +4.50 +5.00 +5.50 +6.00 +6.50 +7.00 +7.50 +8.00 +8.50 +9.00 +9.50 +10.00 +1 0.50 +11.00 +11.50 +12.