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Equivalent viewing power (EVP) and equivalent viewing distance (EVD) are measures used to characterize low vision aids that provide magnification. EVP is a measure of the optical power needed to view an object at a set distance, while EVD is the distance from the eye at which an object subtends the same visual angle as with optimal vision. There are multiple methods to calculate EVP and EVD, including using a patient's best corrected acuity and desired print size. EVP and EVD are more reliable specifications than magnification alone because magnification depends on the reference viewing distance used.
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Calculation of EVD and EVP
This document discusses the calculation of equivalent viewing distance (EVD) and equivalent viewing power (EVP) for low vision patients. It provides the following key points:
EVD is the distance at which an object would need to be viewed to achieve the same visual angle as the magnified image. It is calculated as the actual viewing distance divided by the enlargement ratio.
Various optical devices that can provide magnification are described, including spectacle magnifiers, handheld magnifiers, stand magnifiers, telescopes, and video magnifiers. The EVD for each device depends on factors like the magnifying power and the distance between lenses or from the lens to the image.
Examples are provided to demonstrate calculating the
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This document discusses low vision and provides definitions, classifications, common causes, and management strategies.
[1] Low vision is defined as visual impairment even after treatment that results in visual acuity worse than 6/18 but ability to use vision. It can be caused by conditions like macular degeneration, retinitis pigmentosa, cataract, and glaucoma.
[2] Low vision affects people's ability to perform visual tasks and can cause blurry or decreased vision, loss of peripheral vision, and light sensitivity. Evaluation involves assessing vision and goals, while management includes low vision devices and counseling.
[3] Common low vision devices include telescopes, magnifiers, and electronic
low vision aids
The document provides information on low vision, including definitions, causes, assessments, and aids. It defines low vision according to the WHO and Indian standards. Common causes that can benefit from low vision aids are discussed. Assessment of low vision patients involves testing visual acuity, visual fields, contrast sensitivity, and other factors. A variety of optical and non-optical low vision aids are described, including magnifiers, telescopes, illumination devices, software, and filters to reduce glare. The goals of low vision management and global prevalence of low vision are also summarized.
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- 1. 1
- 2. Equivalent Viewing Power (EVP) & Equivalent Viewing Distance (EVD). PRESENTOR: MUHAMMAD YASIN 2
- 3. Equivalent Viewing Power (EVP) Evp is a measure of the power of the lens or optical system . Evp is a term use to describe the magnifying effect of a lens which is equal to power resulting from the combination of a magnifier(assumed to be a thin lens)and a near addition(or the accomodation exerted) Evp is the reciprocal of equivalent viewing distance.
- 4. Methods to determine EVP. There are multiple ways to determine EVP:- A) RDM METHOD :- we use RDM to determine where the patient must hold the object. Because the object is at F, it will be one focal length from the eye, and the EVP is just the reciprocal of the focal length. We first calculate the relative distance using x = tested distance/M letters read Then get Evp using D=1/f
- 5. Methods to determine EVP. For example : A patient has a best corrected acuity of 3M measured at 30 cm, and the patient wants to read 1M print . calculate EVP? Sol: 30/3=x/1M x=10 cm. As D=1/f =100/10 =10 D. Any aid that has an EVP of +10.00 D can be used by this patient to read 1M print, regardless of the vertex distance.
- 6. Methods to determine EVP. B) Add power method : Calculations of EVP or EVD based upon the add power need to be based upon where the patient is actually holding the print. We can calculate by using following formula: F =ratio of current VA denominator/Desire VA denominator X 2.50
- 7. Methods to determine EVP. As an example, suppose that a patient has a best corrected visual acuity at 40 cm of 20/80, and the patient wants to read 20/20 print . Considering presbyopia . Calculate the power need? Sol: 80/20 X2.50 =4 x 2.50 =10.00D
- 8. Methods to determine EVP. If the patient was a presbyope, at 40 cm the patient would need a +2.50 D add to see the 20/80 print.To read 20/20 print, the patient would just need 4 times as much add or a +10.00 D lens. This is the same as moving the print from 40 cm to 10 cm, with a 10 cm distance corresponding to a +10.00 power add. Working with add powers is just the reciprocal of working with distances.
- 9. Methods to determine EVP C) Reference distance method : We can find out require power by using : Magnification =dF ‘ d’ is expressed in meters, and ‘F’ is in diopters. If the reference distance is 40 cm and the patient needs 4X magnification.Solve for the power? Sol: 4x= 4F F= +10 D
- 10. EQUIVALENT POWER:- The EVP of an Low vision aid is its equivalent power. It is given as :- Fe =F1+F2-(d)F1F2 F1= power of device F2= accomodation or addition use d= distance b/w F1 & F2
- 11. EQUIVALENT FORMULAS OF SOME DEVICES : A) SPECTACLES:- Fe=F1+F2 B) HANDHELD MAGNIFIERS:- Fe=F1+F2-(d)F1F2 C) STAND MAGNIFIERS:- Fe=(transverse magnification) (F2)
- 12. EQUIVALENT FORMULAS OF SOME DEVICES : D) TELEMICROSCOPES:- Fe=[F(reading cap)] x [MagTS] E)CCTV AND ELECTRONIC DEVICES :- Fe= (F2) x image size/object size
- 13. Equivalent Viewing Distance EVD is the distance from the eye that the patient has to hold the object so that it subtends the same angle as the smallest letter size that the patient can read at the reference distance. OR EVD is the distance from the eye at which the smallest letter size as viewed by the patient at the reference distance subtends the same angle as the image formed by a near aid.
- 14. Equivalent Viewing Distance A measure of the patient’s working distance that is related to visual acuity (equivalent viewing distance, which is the reciprocal of the equivalent viewing power) Example: A patient has a best visual acuity of 3M at 30 cm and wants to read 1M Print? calculate the value of EVD?
- 15. Equivalent Viewing Distance ANS: using formula Req. EVD= Required TPS(THRESHOLD print size)/current TPS x Current EVD. x=1o cm. We state that the EVD is 10 cm. If we position the 1M print 10 cm from the eye we will get 3X relative distance magnification (RDM) and the patient will just be able to read the print.
- 16. EVD & EVP IS RELIABLE THAN THE MAGNIFICATION The most obvious problem with using magnification to specify a near vision aid is that the magnification of a lens of given power is not a constant because of its dependence up on reference distance. For example, A lens of power +10.00 D, can have many different magnifications depending upon the reference distance used for measuring visual acuity.
- 17. EVD & EVP IS RELIABLE THAN THE MAGNIFICATION If the reference distance is 40 cm, then the magnification is 4X. If the reference distance is 25 cm, then the magnification is 2.5X. The solution, as previously mentioned, is to specify low vision near aids by either their power or by an equivalent viewing distance.
- 18. EVD & EVP IS RELIABLE THAN THE MAGNIFICATION However, most low vision aid suppliers still specify their visual aids by a magnification in terms of “X”, although the values specified may not be all that useful. For this reason we cannot avoid the use of magnification completely.
- 19. REFERENCES Bennett and Rabbetts' clinical visual optics intranet.opt.uh.edu A guide to low vision practitioners (American Academy) 19
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