What does the axis mean in lenses. Recalculation of cylinders in astigmatism: principles of spectacle correction. Treatment and prognosis

Lens A transparent body bounded by two spherical surfaces is called. If the thickness of the lens itself is small compared to the radii of curvature of spherical surfaces, then the lens is called thin .

Lenses are part of almost all optical devices. Lenses are gathering and scattering . The converging lens in the middle is thicker than at the edges, the diverging lens, on the contrary, is thinner in the middle part (Fig. 3.3.1).

Straight line passing through the centers of curvature O 1 and O 2 spherical surfaces, called main optical axis lenses. In the case of thin lenses, we can approximately assume that the main optical axis intersects with the lens at one point, which is commonly called optical center lenses O. A beam of light passes through the optical center of the lens without deviating from its original direction. All lines passing through the optical center are called side optical axes .

If a beam of rays parallel to the main optical axis is directed to the lens, then after passing through the lens the rays (or their continuation) will gather at one point F, which is called main focus lenses. At thin lens there are two main foci located symmetrically on the main optical axis with respect to the lens. Converging lenses have real foci, diverging lenses have imaginary foci. Beams of rays parallel to one of the secondary optical axes, after passing through the lens, are also focused to a point F", which is located at the intersection of the side axis with focal plane F, that is, a plane perpendicular to the main optical axis and passing through the main focus (Fig. 3.3.2). Distance between the optical center of the lens O and main focus F called the focal length. It is denoted by the same F.

The main property of lenses is the ability to give images of objects . Images are direct and upside down , valid and imaginary , at magnified and reduced .

The position of the image and its nature can be determined using geometric constructions. To do this, use the properties of some standard rays, the course of which is known. These are rays passing through the optical center or one of the foci of the lens, as well as rays parallel to the main or one of the secondary optical axes. Examples of such constructions are shown in Figs. 3.3.3 and 3.3.4.

Note that some of the standard beams used in Fig. 3.3.3 and 3.3.4 for imaging do not pass through the lens. These rays do not really participate in the formation of the image, but they can be used for constructions.

The position of the image and its nature (real or imaginary) can also be calculated using thin lens formulas . If the distance from the object to the lens is denoted by d, and the distance from the lens to the image through f, then the thin lens formula can be written as:

the value D reciprocal of the focal length. called optical power lenses. The unit of optical power is diopter (dptr). Diopter - optical power of a lens with a focal length of 1 m:

The formula for a thin lens is similar to that for a spherical mirror. It can be obtained for paraxial rays from the similarity of triangles in Fig. 3.3.3 or 3.3.4.

It is customary to attribute the focal lengths of lenses certain signs: for converging lens F> 0, for scattering F < 0.

Quantities d and f also subject to certain rule signs:

d> 0 and f> 0 - for real objects (that is, real light sources, and not continuations of rays converging behind the lens) and images;

d < 0 и f < 0 - для мнимых источников и изображений.

For the case shown in Fig. 3.3.3, we have: F> 0 (converging lens), d = 3F> 0 (real item).

According to the thin lens formula, we get: so the image is real.

In the case shown in Fig. 3.3.4, F < 0 (линза рассеивающая), d = 2|F| > 0 (real item), , that is, the image is imaginary.

Depending on the position of the object in relation to the lens, the linear dimensions of the image change. Linear zoom lens Γ is the ratio of the linear dimensions of the image h" and subject h. size h", as in the case of a spherical mirror, it is convenient to assign plus or minus signs depending on whether the image is upright or inverted. Value h always considered positive. Therefore, for direct images Γ > 0, for inverted images Γ< 0. Из подобия треугольников на рис. 3.3.3 и 3.3.4 легко получить формулу для линейного увеличения тонкой линзы:

In the considered example with a converging lens (Fig. 3.3.3): d = 3F > 0, , Consequently, - the image is inverted and reduced by 2 times.

In the diverging lens example (Figure 3.3.4): d = 2|F| > 0, ; therefore, the image is straight and reduced by 3 times.

optical power D lens depends on both the radii of curvature R 1 and R 2 of its spherical surfaces, and on the refractive index n the material from which the lens is made. In optics courses, the following formula is proved:

The radius of curvature of a convex surface is considered positive, and that of a concave surface is negative. This formula is used in the manufacture of lenses with a given optical power.

In many optical instruments light passes successively through two or more lenses. The image of the object given by the first lens serves as the object (real or imaginary) for the second lens, which builds the second image of the object. This second image can also be real or imaginary. The calculation of an optical system of two thin lenses is reduced to applying the lens formula twice, with the distance d 2 from the first image to the second lens should be set equal to the value l - f 1 , where l is the distance between the lenses. The value calculated from the lens formula f 2 determines the position of the second image and its character ( f 2 > 0 - real image, f 2 < 0 - мнимое). Общее линейное увеличение Γ системы из двух линз равно произведению линейных увеличений обеих линз: Γ = Γ 1 · Γ 2 . Если предмет или его изображение находятся в бесконечности, то линейное увеличение утрачивает смысл, изменяются только угловые расстояния.

A special case is the telescopic path of rays in a system of two lenses, when both the object and the second image are at infinity. long distances. The telescopic path of the rays is realized in spotting scopes - Kepler astronomical tube and Galileo's earth tube .

Thin lenses have a number of disadvantages that do not allow obtaining high-quality images. Distortions that occur during image formation are called aberrations . The main ones are spherical and chromatic aberrations. Spherical aberration manifests itself in the fact that in the case of wide light beams, rays far from the optical axis cross it out of focus. The thin lens formula is valid only for rays close to the optical axis. The image of a distant point source, created by a wide beam of rays refracted by a lens, is blurred.

Chromatic aberration occurs because the refractive index of the lens material depends on the wavelength of light λ. This property of transparent media is called dispersion. The focal length of the lens is different for light with different lengths waves, which leads to blurring of the image when using non-monochromatic light.

In modern optical devices, not thin lenses are used, but complex multi-lens systems in which various aberrations can be approximately eliminated.

The formation of a real image of an object by a converging lens is used in many optical devices, such as a camera, a projector, etc.

Camera is a closed light-tight chamber. The image of photographed objects is created on photographic film by a lens system called lens . A special shutter allows you to open the lens during exposure.

A feature of the operation of the camera is that on a flat photographic film, sufficiently sharp images of objects located at different distances should be obtained.

In the plane of the film, only images of objects that are at a certain distance are sharp. Focusing is achieved by moving the lens relative to the film. Images of points that do not lie in the sharp pointing plane are blurred in the form of circles of scattering. The size d these circles can be reduced by aperture of the lens, i.e. decrease relative borea / F(Fig. 3.3.5). This results in an increase in the depth of field.

projection apparatus designed for large scale imaging. Lens O projector focuses the image of a flat object (transparency D) on the remote screen E (Fig. 3.3.6). Lens system K called condenser , designed to concentrate the light source S on a diapositive. Screen E creates a truly enlarged inverted image. The magnification of the projection apparatus can be changed by zooming in or out of the screen E while changing the distance between the transparencies D and lens O.

The use of contact lenses is one of the most widely used vision correction methods used for nearsightedness, farsightedness and astigmatism. Through the use latest materials and technology, contact lenses are now a worthy alternative to glasses.

Contact lenses are characterized by the following main parameters:

Radius of curvature (BC, BCR)

This is a number with one decimal place. Refers to the radius of the curve inner surface contact lens. Usually, base curvature The lenses are the same for both eyes.

The curvature of the lenses, namely the front surface, is actually defined as the optical power of the lens (lens diopter). The optical power of a lens, measured in diopters, is expressed as negative or positive numerical values. The optical area of ​​a lens is central part contact lens with a given optical power. If these are well-chosen lenses for vision correction, then the curvature of the contact lenses should correspond as closely as possible to the curvature of the cornea of ​​the patient's eye. The basic curvature of contact lenses is the curvature of the back of the lens surface, its central part.

Most lenses have a spherical shape (central part), which is defined as the radius of curvature of contact lenses, measured in millimeters, a number with one decimal place. The curvature of the lenses, namely the front surface, is actually defined as the optical power of the lens (lens diopter). If these are well-chosen lenses for vision correction, then the curvature of the contact lenses should correspond as closely as possible to the curvature of the cornea of ​​the patient's eye. Generally, the base curvature of the lens is the same for both eyes. An ophthalmologist can determine which lenses of what radius of curvature you need by making the necessary measurements. For lenses with a non-spherical central back surface, the radius of curvature increases continuously from the center to the edge.

Lens diameter (D, DIA, OAD)

The diameter of contact lenses is the distance between the edges of the lens measured through the center. The diameter of the lenses is one of the parameters that you need to know when choosing contact lenses for vision correction. Diameter is measured in millimeters and has numerical values from 13 to 15 mm. A number with one decimal place. Refers to the size of the contact lens and matches the parameters of your eye. As a rule, this parameter is the same for both eyes.

Optical power (OS) or otherwise Scope (SPH)

Lens refraction - Written as a number with a '+' or '-' sign, it refers to the refractive power of a contact lens. The optical power of the cylinder can be in the range from - 0.75mm. up to - 1.25mm. The tilt axis is measured in degrees (from 90º to 180º). The value for the right eye (OD) may differ from the value for the left eye (OS), both in magnitude and in sign. (For example [-1.75] or [+2.25])

Cylinder axis (AX), (for toric lenses)

This value is specified in degrees (°). Refers to the angle of your astigmatism. (For example 150°)

Cylinder power (CYL), (for toric lenses)

Written as a number with one or two digits after the decimal point, it refers to the power of your astigmatism. The cylinder measurement is given with a ‘-’ sign. (For example [-0.75] or [-1.50]).

Moisture content (H2O)

Moisture content, i.e. the water content of the contact lens material, is indicated in (%). The high water content of the lens material improves oxygen supply to the cornea and improves wearing comfort. The lacrimal pump is not efficient enough to supply the cornea with oxygen. The water in the lens material allows oxygen to move through the hydrogel material, allowing most of the oxygen needed by the cornea to flow through the lens. (For example, 38%)

Depending on the water content, the lenses are divided into:

- lenses with low content water (<50%)

Such lenses show excellent properties in vision correction in the range from -1 to -5 diopters. In addition, materials with a low water content are compatible with all types of contact lens care. They absorb little protein, which lengthens their lifespan. Lenses with low water content are more durable than lenses with high content water. Materials with low water content also have good stability and can be used for all three production technologies: turning, spin casting and mold casting.

- lenses with high water content (>50%)

These materials have a high oxygen permeability and are therefore excellently suited for the production of thicker and strong lenses for the correction of nearsightedness (myopia) and farsightedness (hypermetropia). However, lenses made from such materials have lower tensile strength compared to materials with a lower water content. These materials also have poor compatibility with disinfectants. Their use is contraindicated in patients using thermal disinfectants. Contact lens materials with a high water content tend to absorb protein, and since they are incompatible with enzymatic cleaners, this results in a shortened lens life. Contact lenses with a high water content are usually made by turning or casting.

- lenses with medium water content (about 50%)

Usually such lenses are made from ionic or non-ionic materials with a water content of 50 to 70%. This type of material combines the advantages of materials with low and high water content. Such materials have good physiological parameters and allow the production of thin comfortable lenses. The disadvantage of them is that they have increased protein absorption. In addition, they cannot be subjected to thermal disinfection.

Currently, hydrogel contact lenses are the most popular, however, silicone hydrogel lenses are also finding more and more positive responses among people resorting to contact correction vision.

Dk/t (Oxygen permeability)

An indicator of a contact lens that characterizes the access of oxygen to the cornea of ​​​​the eye. The oxygen permeability of a contact lens is characterized by the material, moisture content and thickness of the lens itself. The ratio of the volume of oxygen that has passed through a unit area of ​​a contact lens per unit time is denoted by Dk. The thickness of the lens in cm is denoted by t. (For example, Dk/t =138)

On average for hydrogel lenses Dk/t is usually in the range of 20-40 units. In principle, this is sufficient for daytime wear, although many studies indicate that the minimum Dk / t for maintaining eye health should be at least 80. In order for the lenses to be left on the eyes at night, even higher values ​​are required. The oxygen permeability of hydrogels is directly proportional to their water content: more content water, the more they pass oxygen to the cornea of ​​​​the eye, which has a positive effect on eye health. However, as the water content increases, hydrogel lenses become too soft, making them quite difficult to handle. That's why maximum content water in hydrogel lenses does not exceed 70%.

At silicone hydrogel lens transmission of oxygen is not related to the water content. As the name suggests, these lenses are made up of two materials: silicone and hydrogel. The transmission of oxygen through such lenses is determined not by the hydrogel, but by the silicone component, which works as an “oxygen pump”. Thus, the silicone part provides a very high oxygen transmission, and the hydrogel part provides a high wearing comfort of contact lenses. Silicone hydrogel contact lenses have a Dk/t of the order of 70-170 units, due to which some of them can be worn without removing for up to 30 days.

Lens center thickness

The thickness of the lens in the center is the actual thickness of the lens in its central part (in its geometric center). It affects, as mentioned above, the oxygen permeability

Coloration of the contact lens

Currently, soft contact lenses are produced both uncolored (transparent) and colored. Contact lenses can only be slightly tinted, which makes the handling procedure more convenient (they say: "tinted for ease of handling").

Color (for colored lenses)

The most widespread different colors contact lenses that can both radically change the color of the eyes, and slightly add saturation to the natural color. In addition to standard colors and shades, lenses with patterns are also used, which allows you to make your appearance extravagant.

The network of salons "Planet Optics" offers a wide selection of high-quality contact lenses from leading manufacturers, and experienced ophthalmologists will help you choose the right lenses and teach you how to wear them

The main step in choosing contact lenses is a visit to an ophthalmologist. The ophthalmologist chooses lenses based on a large number different parameters: the structure of the vascular bed, the incision and density of the eyelids, the amount and composition of the lacrimal fluid. In addition to these sizes, the contactologist learns about the presence eye diseases, allergies, irritations, dry eye syndrome. Only with the right examination, you can be sure that the lenses will be absolutely safe for you and will help correct your vision.

A prescription for contact lenses is different from a prescription for glasses. In addition to optical power (diopters, refractions), a contact lens prescription contains information relating the size of the lens to your eye . If you do not have a prescription, but you already wear contact lenses fitted by an ophthalmologist, a specialist in contact correction, You can find out the prescription parameters (optical power, radius of curvature, diameter) by reading them on the box in which the contact lenses were packed.

Optical power (sphere)
Refers to the strength of your contact lens, which written as a number with a "+" or "‒" sign and one or two digits after the decimal point (for example: + 2.5 or -4.25). The optical power of a contact lens is not the same parameter as for your glasses. The sign "+" and "‒" is a very important parameter. note that very often the optical power for your right eye (OD) may differ from the optical power for your left eye (OS), both in magnitude and in sign.

Radius of curvature (BC)
The base curvature radius is the radius of the curve on the inside surface of your contact lens. The radius of curvature is measured in millimeters and its value usually ranges from 7.8 to 9.5 mm. In most cases, this parameter is the same for both eyes. The base curvature is determined by special measurements using an autorefkeratometer or other ophthalmic equipment.
If you wear lenses with a radius of curvature smaller than the curvature of your eye, the lens will compress the cornea too much. causing her swelling. If the radius of curvature is greater - the lens will "float" over the eye larger than usual and may fall out.

Diameter (DIA)
Lens diameter is the distance from one edge of the lens to the opposite (through its center). Typically, soft contact lenses have a diameter of 13.0 to 15.0 mm. In most cases, this parameter is the same for both eyes. The diameter of the lenses is determined by the measurement of the cornea and is one of the main reference values ​​for fitting lenses.

If you have astigmatism, you may have been prescribed toric contact lenses by your doctor. In addition to the above parameters, a toric lens is characterized by two more quantities - a cylinder and an axis.

Cylinder (CYL)
The optical power of a cylinder is the difference between the values ​​of the optical power in the two main meridians (cylinder), and the axis of the cylinder determines its position. Cylindrical lenses help correct astigmatism, relieving headaches and eye pain. Typical range is -0.75 to -2.25. Please note that the measurement of the cylinder is given with the sign "‒" .

Axis
Refers to the angle of your astigmatism. This parameter is specified in degrees (°). Typical axis range: 90° to 180°. In accordance with the results obtained, astigmatism is divided into astigmatism with straight axes and with oblique axes.

Oxygen permeability of contact lenses (DK/t)
Oxygen is essential for the health of the cornea. She receives it from atmospheric air and lacrimal fluid, which is wetted during blinking. Contact lenses worn over the eye are a barrier that prevents the flow of oxygen from the atmosphere.
That's why, In order for your cornea to “breathe”, lenses must be made of a material with high oxygen permeability.
For daily wear lenses
DK/t must be at least 24 x 10-9
For extended wear lenses
DK/t must be at least 87 x 10-9
The higher the DK/t of the lenses, the better the oxygen supply to your eyes.

Moisture content of the contact lens
The water content of a contact lens is an important parameter. In hydrogel lenses, water is a conductor of oxygen. But the higher the moisture content of the lens, the more the lens "dries" the eye, taking moisture from the membranes of the eye . Such a lens behaves like a sponge, absorbing water. Wearing lenses with a moisture content of 50% or more can lead to the development of dry eye syndrome. The lower the moisture content of the lens, the better, but in this case, you need to choose silicone hydrogel lenses with high oxygen permeability (where silicone is the oxygen conductor, not water).

Be healthy!

Despite the fact that doctors like to hide their secrets behind illegible handwriting and Latin, helping to understand the recipe is not at all difficult.

Prescriptions can vary greatly in form, and information about the parameters of your eyes can also be recorded differently by different ophthalmologists, but there are general rules.

Your task is to find the necessary designations in the prescription and, in the case of ordering lenses on the website or in consultation with a specialist over the phone, correctly interpret the corresponding values ​​(the values ​​​​for the right (OD) and left eye (OS) can sometimes differ, and if they match, they can be abbreviated as OU). It is very important not to confuse the signs of the numbers indicated in the recipe.

Your eyeglass prescription

It is desirable that your prescription was written no more than a year ago. This is especially important in young (under 18) and adulthood(after 40 years). If it has passed since your visit to the ophthalmologist more than a year, then we will either recommend that you do it again, or we can make glasses according to the old recipe, if the parameters of the old glasses suit you.

If you again turned to an ophthalmologist, it would be useful to show him the previous prescription. This can help him to form a correct idea of ​​the health and condition of your eyes, not only at the current moment, but also taking into account the dynamics of vision changes.

We draw your attention to one more circumstance. A prescription for glasses and contact lenses should not be confused with each other. The eyes are the same, but the principles of vision correction are different.

Firstly, in the prescription for contact lenses there are mandatory additional parameters that are absent in the prescriptions for glasses - the base curvature and the diameter of the lenses. Secondly, contact lens placed directly on the cornea of ​​the eye, and spectacle lens separates the air space from the eye, called the vertex distance (from 10 to 16 mm). On the one hand, any lens, whether spectacle or contact, works with the eye as a single optical system. On the other hand, it turns out that the parameters of these optical systems are different.

SPH (sphere)

The sphere is perhaps the main, and for many, the only optical parameter of the recipe. It characterizes the optical power of the lens needed to correct your vision. It is expressed in diopters and usually has values ​​from -20.0 to 0 for nearsightedness (myopia) and from 0 to +20.0 for farsightedness (hypermetropia).

CYL (cylinder)

In addition to the sphere, an additional parameter, a cylinder, can be specified in the recipe. If it is present and not zero(or DS), this means that you have a visual defect such as astigmatism (usually the eye is shaped close to a sphere, but sometimes it is elongated in one of the directions, and has the shape of an ellipsoid, which makes it look like a ball for rugby), and to correct it, a lens is needed that has different optical power in different directions.

The cylinder is also expressed in diopters and means an increase (or decrease) in optical power from the main value to the maximum (or minimum) in the perpendicular direction.

It so happened historically that some prescribing doctors take the maximum sphere and the cylinder as the main meaning with the “-” sign, while others - the minimum sphere and the cylinder, respectively, are designated with the “+” sign. These tricks should not confuse you. When filling out the form, it should be important for you to strictly repeat what the doctor wrote in the prescription.

AX (axis)

If a cylinder is specified in your recipe, then one more parameter must be present - the axis. It is measured in degrees from 0 to 180 and indicates the angle at which the lens should be installed in the frame.

ADD (addition)

Pay attention to the presence in your prescription of such a parameter as addition (or addition), which means how much the optical power of the lens must change for use at close distances (for example, for reading).

If it is present, then it is time for you to think about glasses with multifocal lenses. The fact is that with age, your eyes are no longer able to distinguish between small objects near, and you have a choice: either use several glasses (for distance, for near, for a computer), or use modern achievements optical industry, allowing you to see equally well at all distances (glasses with such lenses, as a rule, require adaptation).

Addidakia is also expressed in diopters and ranges from +0.5 to +3.5. Often labeled for only one of the eyes, but it is implied to refer to both eyes.

Some recipes use several orb values ​​instead of adding - for distance, for medium distances, and for near.

Assignment of points

In addition, prescriptions may indicate the purpose of points:
- for distance (Dist)
- for medium distances (Inter)
- for near (or for reading) (Near)
- for permanent wear.

Interpupillary distance (PD or RC)

Interpupillary distance is the distance in millimeters between the centers of the pupils of your eyes. It is used to center lenses in frame openings so that the center of the pupil coincides with the optical center of the lens. Otherwise, you are guaranteed discomfort when wearing glasses. This is especially important when installing complex lenses (toric, multifocal, etc.) into the frame.

It happens that two distances are indicated in the recipe. These are the distances from the center of the bridge of the nose to each eye separately. This designation is called monocular. It often happens that these values ​​do not match.

One more feature should be mentioned. The interpupillary distance for distance, as a rule, exceeds the value of the same parameter for near by 2 mm. This is due to the fact that when focusing on objects located close to the eyes, their optical axes converge.

Sample eyeglass prescriptions

Example #1:

OD: sph-2.5 cyl +0.75ax 45
OS: sph -2.0 cyl +0.50 ax 120
purpose of glasses: for distance, for work, for permanent wear
r.c. – 68 mm
means that the right eye needs to be corrected with a lens having a sphere value of -2.5 diopters and a cylinder of +0.75 diopters installed in the frame at an angle of 45 degrees (the axis or angle does not matter when ordering a lens, but is important when making glasses), and for the left the eye needs a -2.0 diopter lens and a +0.50 diopter cylinder set into the frame at an angle of 120 degrees. center distance pupils - 68 mm and glasses are designed for permanent wear.

Example #2:

OD: sph-3.5 - 1.0 x 90
OS: sph -3.5 - 0.5ax 120
means that the right eye needs to be corrected with a lens that has a sphere value of -3.5 diopter and a cylinder of -1.0 diopter installed in the frame at an angle of 90 degrees (sometimes the names of the cylinder and axis are omitted, but are implied), and the left eye needs a lens with the same optical power value -3.5 diopters and a cylinder -0.50 diopters, installed in the frame at an angle of 120 degrees.

Example #3:

OU sph +2.25 +1.5 add
means that both eyes need the same multifocal lenses (such lenses include bifocal lenses, progressive lenses and office lenses) with a sphere of +2.25 diopters and a near add of 1.5 diopters.

There are other options for writing prescriptions for glasses, they may contain other additional designations. If there are any doubts about the correct understanding of the recipe, you can call or write to the site's mail and our specialists will try to help.

It is important that you are sure that the order is correct and that as a result you will receive glasses that will suit you completely.

Lens - it is a transparent body bounded on both sides by spherical surfaces. A lens is considered thin (thin lens) if its thickness is much less than the radii of curvature R 1 and R 2 of both surfaces.

AT lens types.

Collecting - Scattering

(lens thickness at the middle (lens thickness at the middle

More than at the edges). less than at the edges).

d M

WithN

The main optical axis of the lens - this is a straight line (av) drawn through the centers of spherical surfaces.

Optical center of the lens - this is the point O, lying on the optical axis, through which any beam passes without changing its direction.

Focal plane - called the M plane N, through the focus of the lens perpendicular to the main optical axis.

Side optical axis- this is any straight line (cd) passing through the optical center of the lens, but not coinciding with the main optical axis.

Passages through a converging lens are collected

In focus F. Distance from the optical center of the lens F

to its foci is called the focal length - F.

Every lens has two focal points on either side of it.

Rays parallel to the optical axis, after

passing through a scattering lens

vayutsya. If the rays coming out of the lens are

live in the direction opposite to their direction F

then the continuations of the rays will intersect at the focus - F,

located in front of the lens .. The distance from the opti -

The center of a lens to its foci is called the focal length. For concave lenses, the focal length is expressed negative number.

Thin lens formula:

F - - distance from the lens to the image.

F - - focal length of the lens, this is the distance from the optical center of the lens to its foci

.

Optical power of the lens - D-

The unit of optical power is - diopter (1 diopter).

1 diopter is the optical power of such a lens, the focal length of which is 1 meter.

For a converging lens D  0, for a diverging lens D   .

Linear magnification of a thin lens - Г-

This is the ratio of the linear size of the image to the linear size of the object.

H - linear dimensions of the image.

H - linear dimensions of the object.

G=
.

Building an image in a lens.

Beam 1 - parallel to the main optical axis; after refraction in the lens, it passes through the focus;

Beam 2 - passing through lens center; this beam does not change its direction after the lens.

Beam 3 - focal Ray; after refraction in the lens, it is parallel to the main optical axis.

Picture Feature:

1. 
   enlarged, reduced
2. 
   Straight, inverted.
3. 
   Real, imaginary.