What is an optical center? Contact lens parameters
Despite the fact that doctors like to hide their secrets behind illegible handwriting and Latin, it is not at all difficult to help you understand the recipe.
The form of prescriptions can vary greatly, and information about the parameters of your eyes by different ophthalmologists can also be written down differently, but there are general rules.
Your task is to find the necessary designations in the prescription and, in case of ordering lenses on the website or in consultation with a specialist by phone, correctly interpret the corresponding values (the values for the right (OD) and left eyes (OS) can sometimes differ, and if they coincide, they can be abbreviated OU). It is very important not to confuse the signs of the numbers indicated in the recipe.
Your prescription for glasses
It is advisable that your prescription was issued no more than a year ago. This is especially important in young (under 18 years old) and mature age(after 40 years). If more than a year has passed since you visited an ophthalmologist, then we will either recommend that you do it again, or we can make glasses according to the old prescription if the parameters of the previous glasses suit you.
If you visit your ophthalmologist again, it would be a good idea to show him your previous prescription. This can help him 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 change processes.
We would like to draw your attention to one more circumstance. Prescriptions 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, a prescription for contact lenses contains mandatory additional parameters that are not present in prescriptions for glasses - the base curvature and diameter of the lenses. Secondly, contact lens placed directly on the cornea of the eye, and spectacle lens separates the eye from the eye by an air space called the vertex distance (10 to 16 mm). On the one hand, any lens, be it a spectacle lens or a contact lens, 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 necessary to correct your vision. Expressed in diopters and usually has values from -20.0 to 0 for nearsightedness (myopia) and from 0 to +20.0 for farsightedness (hyperopia).
CYL (cylinder)
In addition to the sphere, the recipe may contain an additional parameter - a cylinder. If it is present and not equal to zero (or DS), this means that you have a vision defect such as astigmatism (usually the eye has a shape close to a sphere, but sometimes it turns out to be elongated in one of the directions and has the shape of an ellipsoid, which makes it look like a rugby ball), and to correct it requires a lens that has different optical powers in different directions.
A cylinder is also expressed in diopters and means an increase (or decrease) optical power from the main value to the maximum (or minimum) in the perpendicular direction.
It so happened historically that some doctors writing a prescription take the maximum sphere as the main value and designate the cylinder with a “-” sign, while others denote the minimum sphere and cylinder, respectively, with a “+” 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 your recipe specifies a cylinder, 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 additive), which means how much the optical power of the lens should change for use at close distances (for example, for reading).
If it is present, then it’s time for you to think about glasses with multifocal lenses. The fact is that as you age, your eyes no longer cope with the discrimination small items close up, 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).
Addidacia is also expressed in diopters and ranges from +0.5 to +3.5. Often indicated for only one of the eyes, but it is implied to apply to both eyes.
In some recipes, instead of addition, several sphere values are used - for distance, for medium distances and for near.
Assignment of points
In addition, prescriptions may indicate the purpose of glasses:
- for distance (Dist)
- for medium distances (Inter)
- for near (or for reading) (Near)
- For constant wearing.
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 the lenses in the 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 frames.
It happens that the recipe specifies two distances. These are the distances from the center of the bridge of the nose to each eye separately. This designation option 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.
Examples of prescriptions for glasses
Example #1:
OD: sph-2.5 cyl +0.75 ax 45
OS: sph -2.0 cyl +0.50 ax 120
purpose of glasses: for distance, for work, for constant wearing
r.ts. – 68 mm
means that the right eye needs correction 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 the lens, but is important when making glasses), and for the left eyes need a -2.0 diopter lens and a +0.50 diopter cylinder installed in the frame at an angle of 120 degrees. The center-to-center distance of the pupils is 68 mm and the glasses are designed for constant wear.
Example #2:
OD: sph-3.5 - 1.0 x 90
OS: sph -3.5 - 0.5 ax 120
means that the right eye needs correction with a lens having a sphere value of -3.5 diopters and a cylinder of -1.0 diopters, 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 and office lenses) having a sphere of +2.25 diopters and an addition for near distances of 1.5 diopters.
There are other options for writing prescriptions for glasses; they may also contain other additional designations. If you still have doubts about the correct understanding of the recipe, you can call or write to the site’s email and our specialists will try to help.
It is important that you are confident that your order is correct and that you will receive glasses that you are completely satisfied with.
1. Types of lenses. Main optical axis of the lens
A lens is a body transparent to light, bounded by two spherical surfaces (one of the surfaces may be flat). Lenses with a center that is thicker than
the edges are called convex, and those whose edges are thicker than the middle are called concave. A convex lens made of a substance with an optical density greater than that of the medium in which the lens
is located, is converging, and a concave lens under the same conditions is diverging. Various types lenses are shown in Fig. 1: 1 - biconvex, 2 - biconcave, 3 - plano-convex, 4 - plano-concave, 3.4 - convex-concave and concave-convex.
Rice. 1. Lenses
The straight line O 1 O 2 passing through the centers of the spherical surfaces delimiting the lens is called the main optical axis of the lens.
2. Thin lens, its optical center.
Secondary optical axes
A lens whose thickness l=|C 1 C 2 | (see Fig. 1) is negligibly small compared to the radii of curvature R 1 and R 2 of the lens surfaces and the distance d from the object to the lens, it is called thin. In a thin lens, points C 1 and C 2, which are the vertices of the spherical segments, are located so close to each other that they can be mistaken for one point. This point O, lying on the main optical axis, through which light rays pass without changing their direction, is called the optical center thin lens. Any straight line passing through the optical center of a lens is called its optical axis. All optical axes, except the main one, are called secondary optical axes.
Light rays coming near the main optical axis are called paraxial (priaxial).
3. Main tricks and focal points
lens distance
Point F on the main optical axis, at which the paraxial rays intersect after refraction, incident on the lens parallel to the main optical axis (or the continuation of these refracted rays), is called the main focus of the lens (Fig. 2 and 3). Any lens has two main focuses, which are located on either side of it symmetrically to its optical center.
Rice. 2 Fig. 3
The converging lens (Fig. 2) has real foci, while the diverging lens (Fig. 3) has imaginary foci. Distance |OR| = F from the optical center of the lens to its main focus is called focal. The focal length of a converging lens is considered positive, and that of a diverging lens is considered negative.
4. Lens focal planes, their properties
The plane passing through the main focus of a thin lens perpendicular to the main optical axis is called focal. Each lens has two focal planes (M 1 M 2 and M 3 M 4 in Fig. 2 and 3), which are located on either side of the lens.
Rays of light incident on a collecting lens parallel to any of its secondary optical axis, after refraction in the lens, converge at the point of intersection of this axis with the focal plane (at point F’ in Fig. 2). This point is called the side focus.
Lens formulas
5.Lens optical power
The reciprocal of D focal length lenses are called optical power lenses:
D =1/F (1)
For a converging lens F>0, therefore D>0, and for a diverging lens F<0, следовательно, D<0, т.е. оптическая сила собирающей линзы положительна, а рассеивающей - отрицательна.
The unit of optical power is taken to be the optical power of a lens whose focal length is 1 m; this unit is called diopter (dopter):
1 diopter = = 1 m -1
6. Derivation of the thin lens formula based on
geometric construction of the ray path
Let there be a luminous object AB in front of the collecting lens (Fig. 4). To construct an image of this object, it is necessary to construct images of its extreme points, and it is convenient to choose those rays whose construction will be the simplest. In general, there can be three such rays:
a) the AC ray, parallel to the main optical axis, after refraction, passes through the main focus of the lens, i.e. goes in a straight line CFA 1;
Rice. 4
b) the ray AO passing through the optical center of the lens is not refracted and also arrives at point A 1;
c) the ray AB passing through the front focus of the lens, after refraction, goes parallel to the main optical axis along the straight line DA 1.
All three indicated rays where a real image of point A is obtained. By lowering the perpendicular from point A 1 to the main optical axis, we find point B 1, which is the image of point B. To construct an image of a luminous point, it is enough to use two of the three listed rays.
Let us introduce the following notation |OB| = d – distance of the object from the lens, |OB 1 | = f – distance from the lens to the image of the object, |OF| = F – focal length of the lens.
Using fig. 4, we derive the formula for a thin lens. From the similarity of triangles AOB and A 1 OB 1 it follows that
(2)
From the similarity of triangles COF and A 1 FB 1 it follows that
and since |AB| = |CO|, then
(4)
From formulas (2) and (3) it follows that
(5)
Since |OB1|= f, |OB| = d, |FB1| = f – F and |OF| = F, formula (5) takes the form f/d = (f – F)/F, whence
FF = df – dF (6)
Dividing formula (6) term by term by the product dfF, we obtain
(7)
where
(8)
Taking into account (1) we obtain
(9)
Relations (8) and (9) are called the formula of a thin collecting lens.
At the diverging lens F<0, поэтому формула тонкой рассеивающей линзы имеет вид
(10)
7. Dependence of the optical power of a lens on the curvature of its surfaces
and refractive index
The focal length F and optical power D of a thin lens depend on the radii of curvature R 1 and R 2 of its surfaces and the relative refractive index n 12 of the lens substance relative to the environment. This dependence is expressed by the formula
(11)
(12)
If one of the surfaces of the lens is flat (for it R= ∞), then the corresponding term 1/R in formula (12) is equal to zero. If the surface is concave, then the corresponding term 1/R is included in this formula with a minus sign.
The sign of the right side of formula (12) determines the optical properties of the lens. If it is positive, then the lens is converging, and if it is negative, it is diverging. For example, for a biconvex glass lens in air, (n 12 - 1) > 0 and
those. the right side of formula (12) is positive. Therefore, such a lens in the air is converging. If the same lens is placed in a transparent medium with optical density
larger than glass (for example, carbon disulfide), then it will become scattering, since in this case it has (n 12 - 1)<0 и, хотя
, the sign on the right side of formula/(17.44) will become
negative.
8.Linear magnification of the lens
The size of the image created by the lens changes depending on the position of the object relative to the lens. The ratio of the image size to the size of the depicted object is called linear magnification and is designated G.
Let us denote by h the size of the object AB and H - the size of A 1 B 2 - its image. Then from formula (2) it follows that
(13)
10. Constructing images in a collecting lens
Depending on the distance d of the object from the lens, there can be six different cases of constructing an image of this object:
a) d =∞. In this case, light rays from an object fall onto the lens parallel to either the main or some secondary optical axis. Such a case is shown in Fig. 2, from which it is clear that if an object is infinitely distant from the lens, then the real image of the object, in the form of a point, is at the focus of the lens (main or secondary);
b) 2F< d <∞. Предмет находится на конечном расстоянии от линзы большем, чем ее удвоенное фокусное расстояние (см. рис. 3). Изображение предмета действительное, перевернутое, уменьшенное находится между фокусом и точкой, отстоящей от линзы на двойное фокусное расстояние. Проверить правильность построения данного изображения можно
by calculation. Let d= 3F, h = 2 cm. From formula (8) it follows that
(14)
Since f > 0, the image is real. It is located behind the lens at a distance OB1=1.5F. Every real image is inverted. From the formula
(13) it follows that
; H = 1 cm
i.e. the image is reduced. Similarly, using calculations based on formulas (8), (10) and (13), you can check the correctness of the construction of any image in the lens;
c) d=2F. The object is at double the focal length from the lens (Fig. 5). The image of the object is real, inverted, equal to the object, located behind the lens at
double focal length from it;
Rice. 5
d) F
Rice. 6
e) d= F. The object is at the focus of the lens (Fig. 7). In this case, the image of the object does not exist (it is at infinity), since the rays from each point of the object, after refraction in the lens, travel in a parallel beam;
Rice. 7
e) d
Rice. 8
11. Constructing images in a diverging lens
Let's construct an image of an object at two different distances from the lens (Fig. 9). It can be seen from the figure that no matter what distance the object is from the diverging lens, the image of the object is virtual, direct, reduced, located between the lens and its focus
from the side of the depicted object.
Rice. 9
Constructing images in lenses using secondary axes and the focal plane
(Constructing an image of a point lying on the main optical axis)
Rice. 10
Let the luminous point S be located on the main optical axis of the collecting lens (Fig. 10). To find where its image S’ is formed, let us draw two rays from point S: ray SO along the main optical axis (it passes through the optical center of the lens without refraction) and ray SB incident on the lens at an arbitrary point B.
Let's draw the focal plane MM 1 of the lens and draw the secondary axis ОF' parallel to the ray SB (shown by the dashed line). It will intersect the focal plane at point S'.
As noted in paragraph 4, a ray must pass through this point F after refraction at point B. This ray BF’S’ intersects with the ray SOS’ at point S’, which is the image of the luminous point S.
Constructing an image of an object larger than the lens
Let the object AB be located at a finite distance from the lens (Fig. 11). To find where the image of this object will be obtained, we draw two rays from point A: ray AOA 1 passing through the optical center of the lens without refraction, and ray AC incident on the lens at an arbitrary point C. Draw the focal plane MM 1 of the lens and draw the secondary axis ОF', parallel to the ray AC (shown by the dashed line). It will intersect the focal plane at point F'.
Rice. 11
A ray refracted at point C will pass through this point F'. This ray CF'A 1 intersects with the ray AOA 1 at point A 1, which is the image of the luminous point A. To obtain the entire image A 1 B 1 of the object AB, lower the perpendicular from point A 1 to the main optical axis.
Magnifier
It is known that in order to see small details on an object, they need to be viewed from a large visual angle, but the increase in this angle is limited by the limit of the accommodative capabilities of the eye. You can increase the angle of view (while maintaining the distance of best vision d o) using optical instruments (magnifying glasses, microscopes).
A magnifying glass is a short-focus biconvex lens or a system of lenses acting as one converging lens (usually the focal length of the magnifying glass does not exceed 10 cm).
Rice. 12
The path of rays in a magnifying glass is shown in Fig. 12. The magnifying glass is placed close to the eye,
and the object in question AB = A 1 B 1 is placed between the magnifying glass and its front focus, slightly closer to the latter. Select the position of the magnifying glass between the eye and the object so as to see a sharp image of the object. This image A 2 B 2 turns out to be virtual, direct, enlarged and located at the distance of best vision |OB|=d o from the eye.
As can be seen from Fig. 12, the use of a magnifying glass leads to an increase in the angle of view from which the eye views the object. Indeed, when the object was in position AB and viewed with the naked eye, the visual angle was φ 1. The object was placed between the focus and the optical center of the magnifying glass in position A 1 B 1, and the viewing angle became φ 2. Since φ 2 > φ 1, this is
This means that with the help of a magnifying glass you can see finer details on an object than with the naked eye.
From Fig. 12 it is also clear that the linear magnification of the magnifying glass
Since |OB 2 |=d o , and |OB|≈F (focal length of the magnifying glass), then
G=d o /F,
therefore, the magnification given by a magnifying glass is equal to the ratio of the distance of best vision to the focal length of the magnifying glass.
Microscope
A microscope is an optical device used for viewing very small objects (including those invisible to the naked eye) from a wide angle of view.
The microscope consists of two collecting lenses - a short-focus objective and a long-focus eyepiece, the distance between which can vary. Therefore F 1<
The path of rays in a microscope is shown in Fig. 13. The lens creates a real, inverted, enlarged intermediate image A 1 B 2 of the object AB.
Rice. 13
282.
Linear increase
Using micrometric
The eyepiece screw is placed
relative to the lens like this
in such a way that this is between
exact image of A\B\eye-
between the front focal point
som RF and optical center
Ptch eyepiece. Then the eyepiece
becomes a magnifying glass and creates imaginary
mine, direct (relatively pro-
interstitial) and increased
image of hhhv object av.
Its position can be found
using the properties of the focal
plane and secondary axes (axis
O^P' is carried out parallel to the
chu 1, and the OchR axis is parallel-
but ray 2). As can be seen from
rice. 282, use micro
osprey leads to significant
mu increase the angle of view,
under which the eye is viewed -
there is an object (fa ^> fO, which poses
wills to see details without seeing
invisible to the naked eye.
microscope
\AM 1L2Y2 I|y||
G=
\AB\ |L,5,| \AB\
Since \A^Vch\/\A\B\\== Hok-linear magnification of the eyepiece and
\A\B\\/\AB\== Gob is the linear magnification of the lens, then linear
microscope magnification
(17.62)
G== Gob Gok.
From Fig. 282 it is clear that
» |L1Y,1 |0,I||
\AB\ 150.1 ‘
where 10.5, | = |0/7, | +1/^21+1ad1.
Let us denote by 6 the distance between the back focus of the lens
and the front focus of the eyepiece, i.e. 6 = \Р\Р'г\. Since 6 ^> \OP\\
and 6 » \P2B\, then |0|5|1 ^ 6. Since |05|| ^ Rob, we get it
b
Rob
(17.63)
The linear magnification of the eyepiece is determined by the same formula
(17.61), as does magnification of a magnifying glass, i.e.
384
Gok=
A"
Gok
(17.64)
(17.65)
Substituting (17.63) and (17.64) into formula (17.62), we obtain
byo
G==
/^rev/m
Formula (17.65) determines the linear magnification of the microscope.
Having been manufacturers of contact lenses for more than 23 years, with our own retail network of Ophthalmology Centers, we understand how important an individual approach is in the selection of contact lenses and an objective assessment of eye health before starting to use contact lenses. Only competent professional selection of contact lenses will ensure comfortable use and eye health for many years.
Therefore, before ordering contact lenses from our online store, please read the following rules and recommendations:
If you have never used contact lenses before, you need to consult an ophthalmologist, a specialist in contact correction. This is very important if you want to maintain long-term eye health.
A list of specialized contact vision correction offices in your region, where you can undergo an examination and choose contact lenses CONCOR, located
An ophthalmologist will examine the health of your eyes, select the type of contact lenses that is right for you, conduct a study of the fit and tolerance of the lenses on your eyes, and also tell you about the rules for using contact lenses (how to put on and remove correctly, how to care for them) and conditions wear that is specific to you.
2.Before placing an order in our online store, make sure that:
- You are regularly examined by an ophthalmologist (at least 1-2 times a year);
- The contact lenses you want to purchase from us were selected for you by an ophthalmologist, a specialist in contact correction;
- You have been using these contact lenses for more than three months under the supervision of an ophthalmologist, a specialist in contact vision correction.
- You are sure that you need contact lenses with exactly the parameters that you are going to specify when placing your order.
3. To place an order through our online store, you must know the following parameters of your contact lenses:
Diopter or optical power of your lens (sphere, sph)
expressed in negative or positive values. It is written as a number with a "-" sign if it is a negative value, or with or without a "+" sign if it is a positive value. And with one or two digits after the decimal point (for example: 2.0 or -2.25).
The optical power of your lens determined by an ophthalmologist, applying lenses with different diopters to your eyes until your vision becomes clear. The optical power value for the right eye (OD) may differ from the value of the left eye (OS) both in magnitude and sign.
Please note that the optical power of a contact lens differs from the same parameter for glasses. These are different parameters, since the contact lens is worn directly on the cornea itself, and the glasses are at a certain distance from it.
Radius of curvature (BC; R)
The cornea of the eye is the convex transparent part of the eyeball, which has its own radius of curvature.
The radius of curvature of a contact lens is the curvature of the inner surface of the contact lens.
The contact lens is placed directly on the cornea and the radius of curvature of the contact lens affects how the lens “sits” on the eye. The lens should not be too mobile or, on the contrary, fit too tightly to the eye.
Poor fit of a contact lens due to a discrepancy between the radius of curvature of the lens and the shape of the cornea can cause discomfort when wearing lenses, disruption of tear metabolism and the cause of eye diseases.
The radius of curvature of the contact lens is determined by the ophthalmologist.
However, please note that even if you know the radius of curvature of your previous contact lenses, it is important to know that contact lenses from different manufacturers will have a different fit on your eyes.
Therefore, when purchasing a new brand of contact lenses, you must consult an ophthalmologist. The doctor will be able to choose the right fit. During fitting, the lens is placed on the eye and the doctor uses a slit lamp and special tests to evaluate its fit on the cornea.
Contact Lens Diameter (D)
This is the size of your contact lens - the distance between the edges of the lens, measured through the center.
The diameter of the lens is determined by an ophthalmologist by measuring the cornea. 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.
If you need to correct astigmatism with contact lenses, then you will need toric contact lenses.
Toric contact lenses, in addition to the above parameters, have two more values:
Cylinder (cyl)
The amount of your astigmatism. Determined by an ophthalmologist.
Tilt axis (Ax)
This value refers to the angle of your astigmatism. Determined by an ophthalmologist and specified in degrees (o). Typical axes range from 0o to 180o.
If you need correction of keratoconus, your doctor will prescribe keratoconus lenses.
In this case, you will need to know
Keratoconus Lens Type
K1, K2 or K3. The type of keratoconus lens is determined by an ophthalmologist.
Please note that orders for toric and keratoconus lenses cannot be placed in the online store; these orders are accepted only from doctors.
When ordering tinted contact lenses additional parameter is , hue, background, iris and saturation.
4. You should definitely consult an ophthalmologist before ordering contact lenses from an online store if:
- You have good vision and would like to simply change your eye color using colored or tinted contact lenses. You need not only to choose the color of contact lenses, measure the radius of curvature of the cornea, but also make sure that wearing contact lenses is not contraindicated for you.
- You want to try other contact lenses (even if they have exactly the same parameters as your previous lenses). Contact lenses are made from different materials and using different technologies. Their features are taken into account by the doctor when selecting contact lenses. Moreover, in order to avoid complications when wearing, the contact doctor must monitor the health of your eyes in the first month of wearing new contact lenses.
- You already use contact lenses and would like to purchase additional colored lenses. You need to choose the color of your contact lenses to achieve the desired result. The final result is greatly influenced by the initial eye color. In addition, these lenses often differ in the size of the shading of the pupillary zone, and if you select them yourself, a situation may arise when the world through the lens is visible in green (blue, etc.) color.
We are confident that if you follow the above recommendations, you will be satisfied with our products!
Please note that the buyer is responsible for the content and accuracy of all information specified when placing an order for contact lenses in the online store.
(concave or dissipative). The path of rays in these types of lenses is different, but the light is always refracted, however, in order to consider their structure and principle of operation, you need to familiarize yourself with the same concepts for both types.
If we draw the spherical surfaces of the two sides of the lens to complete spheres, then the straight line passing through the centers of these spheres will be the optical axis of the lens. In fact, the optical axis passes through the widest point of a convex lens and the narrowest point of a concave lens.
Optical axis, lens focus, focal length
On this axis there is a point where all the rays passing through the collecting lens are collected. In the case of a diverging lens, we can draw continuations of the diverging rays, and then we will get a point, also located on the optical axis, where all these continuations converge. This point is called the focus of the lens.
A converging lens has a real focus, and it is located on the opposite side of the incident rays; a diverging lens has an imaginary focus, and it is located on the same side from which the light falls on the lens.
The point on the optical axis exactly in the middle of the lens is called its optical center. And the distance from the optical center to the focal point of the lens is the focal length of the lens.
The focal length depends on the degree of curvature of the spherical surfaces of the lens. More convex surfaces will refract rays more strongly and, accordingly, reduce the focal length. If the focal length is shorter, then the lens will provide greater image magnification.
Lens optical power: formula, unit of measurement
To characterize the magnifying power of a lens, the concept of “optical power” was introduced. The optical power of a lens is the reciprocal of its focal length. The optical power of a lens is expressed by the formula:
where D is the optical power, F is the focal length of the lens.
The unit of measurement for the optical power of a lens is diopter (1 diopter). 1 diopter is the optical power of a lens whose focal length is 1 meter. The shorter the focal length, the greater the optical power, that is, the more the lens magnifies the image.
Since the focus of a diverging lens is imaginary, we agreed to consider its focal length to be a negative value. Accordingly, its optical power is also a negative value. As for the converging lens, its focus is real, therefore both the focal length and the optical power of the converging lens are positive quantities.
