calculation of Equivalent Viewing Distance and Effective Viewing Power

1. Calculation ofCalculation of
EVD & EVPEVD & EVP
Rabindra Adhikary
ravinems@iom.edu.np

2. Equivalent Viewing Distance (EVD)Equivalent Viewing Distance (EVD)
Defn
:
◦ It is the distance at which the object itself would
subtend an angle that is equal to the angle that
is being subtended by the image.
◦ Actual working distance from the spectacle plane

3. Calculation of EVD for the patientCalculation of EVD for the patient
EVD = Eye to Image distance Enlargement ratio
◦ Eye to image distance = Viewing distance
◦ Enlargement ratio (ER) = Best near Acuity (BNA)
Target near Acuity (TNA)
Equivalent viewing power = 1/ EVD (m)

4. Method of calculation of EVDMethod of calculation of EVD
Specify resolution at near by M notation
◦ Record print size & viewing distance
MeterMeter
EquivalentEquivalent
SnellenSnellen
EquivalentEquivalent
Size of TextSize of Text
0.4 M0.4 M 20/2020/20
0.5 M0.5 M 20/2520/25 Footnotes, Phone books,Footnotes, Phone books,
0.8 M (N0.8 M (N66)) 20/4020/40 Paperback PrintPaperback Print
1.0 M (N1.0 M (N88)) 20/5020/50 Newspaper PrintNewspaper Print
1.2 M (N1.2 M (N1010)) 20/6020/60 Magazine PrintMagazine Print
1.6 M1.6 M 20/8020/80 Children’s BookChildren’s Book
2.0 M2.0 M 20/10020/100 Large Print MaterialLarge Print Material
4.0 M4.0 M 20/20020/200 Newspaper subtitleNewspaper subtitle
5.0 M5.0 M 20/25020/250 Newspaper headlineNewspaper headline

5. Method of calculation of EVDMethod of calculation of EVD
Step one
◦ Take note of the smallest print read with
acceptable accuracy & Viewing distance
◦ Take note of accommodation or reading add
◦ Predict the target acuity (Resolution goal) :
Calculate ER ?
Step two
◦ Predict the distance required to meet the
resolution goal
◦ Calculation = Viewing distance for BNA/ER = EVD

6. Method of calculation of EVDMethod of calculation of EVD
Step three
◦ Verify the predicted EVD that allows resolution
goal
◦ Predict EVD (app. Add) at spectacle plane that
gives clear focus
Step four
◦ Consider the optical system that provides same
EVD (EVP)
 Spectacle magnifier, Hand Held magnifier, Stand
magnifier, Telemicroscope, Electronic Devices

7. Method of calculation of EVDMethod of calculation of EVD
 Example: Target acuity 0.8 M (N6)
Patient 1Patient 1 (Hari)(Hari) Patient 2Patient 2 (Ram Bdr.)(Ram Bdr.)
AgeAge 20 yrs20 yrs 70 yrs70 yrs
Smallest printSmallest print
readread
2.0 M (N2.0 M (N1616)) 4.0 M (N4.0 M (N3232))
Viewing distanceViewing distance 12 cm12 cm 32 cm32 cm
AddAdd NoneNone 2.50 Ds2.50 Ds
Acc. DemandAcc. Demand 8.0 Ds8.0 Ds 0.50 Ds0.50 Ds
Enlargement ratioEnlargement ratio 2.0/0.8 = 2.5 x2.0/0.8 = 2.5 x 4.0/0.8 = 5x4.0/0.8 = 5x
EVD (required)EVD (required) 12/2.5 = 5 cm12/2.5 = 5 cm 32/5 = 6.3 cm32/5 = 6.3 cm
EVP = 1/EVDEVP = 1/EVD 1/5 cm = +20 Ds1/5 cm = +20 Ds 1/6.3 cm = +16 Ds1/6.3 cm = +16 Ds

8. Possible optionsPossible options

9. Find the Option with equal
EVD(IEVD(Istst
))
Spectacle magnifier
◦ EVD actual working distance from the spectacle
plane
◦ Presbyopes
 EVD = Equivalent Focal length of add
◦ Non presbyopes (Accommodation should be
estimated)
 EVD = reciprocal of add power + the accommodation
◦ Patient 1 (Hari)
 +20Ds (EVP) needed to read 0.8 M ( Acco. +8.0 Ds)
 Required add = +20.00 – (+8.0) = +12.00 Ds will allow
him to work at 5 cm (EVD)
◦ Patient 2 (Ram Bdr.)
 +16 Ds (EVP) needed to read 0.8 m (Acco. +0.50 Ds)
 Required Add = +16 – (+0.50 ) = +15.50 Ds , EVD = 6.3
cm

10. Find the Option equal EVD (IIFind the Option equal EVD (IIndnd
))
Hand Held Magnifier
◦ As a Collimator (Option I)
 Reading material is in focal length I.e image at
infinity
 Retinal image size is independent of separation
between eye to magnifier
 Field of View (FoV)= A (EVD/Z)
◦ Where, A = Lens diameter , Z = the eye to magnifier
distance
 EVD = The focal length of the magnifier
◦ Presbyopes need to wear distance correction
◦ young patient need to relax accommodation
 As a collimator
◦ A +4.00 Ds magnifier has an EVD of 25 cm
◦ A +10.0 Ds magnifier has an EVD of 10 cm
◦ A +20 Ds magnifier has an EVD of 5 cm

11. Find the OptionFind the Option
(II(IIndnd
))
Hand Held magnifier
◦ Non collimated system
◦ Combined with an add or Accommodation.
◦ EVD = Eye to image distance/ ER
= Focal length of add/ ER (Variable)
◦ Eg: Patient is wearing add + 4.00 Ds, with +20.00
Ds HHM
◦ Patient need to adjust the magnifier and object
such that image is at 25 cm (Add =4.00 Ds)

12. Find the Option (IIFind the Option (IIndnd
))
Image alwz at 25 cm, but the ER/EVD varies
according to separation of the two lenses
HHM toHHM to
eyeeye
HHM toHHM to
focalfocal
plane ofplane of
addadd
(Image)(Image)
ERER EVDEVD EVPEVP ObjectObject
to HHMto HHM
25 cm25 cm 0cm0cm 1x1x 25 cm25 cm 4.00 Ds4.00 Ds 0 cm0 cm
20cm20cm 5 cm5 cm 2x2x 12.5 cm12.5 cm 8.00 Ds8.00 Ds 2.50 cm2.50 cm
15cm15cm 10cm10cm 3x3x 8.33 cm8.33 cm 12.00 Ds12.00 Ds 3.33 cm3.33 cm
10cm10cm 15 cm15 cm 4x4x 6.25 cm6.25 cm 16.0 Ds16.0 Ds 3.75 cm3.75 cm
5cm5cm 20 cm20 cm 5x5x 5.00 cm5.00 cm 20.00 Ds20.00 Ds 4.00 cm4.00 cm
0cm0cm 25 cm25 cm 6x6x 4.17 cm4.17 cm 24.00 Ds24.00 Ds 4.17 cm4.17 cm

13. Find the OptionFind the Option
(II(IIndnd
))
Operational rules for Hand Held Magnifier
(HHM)
◦ If HHM is separated from spectacle
◦ By one magnifier focal length
 Better resolution will obtain either from distance/Add
(no matter)
◦ By more than one magnifier focal length
 Need to view thru distance portion than add
◦ By less than one magnifier focal length
 Need to view thru add (or use accommodation)
◦ Held against the spectacle
 No matter either near / Add
 But object must be held relatively close to face

14. Find the Option equalFind the Option equal
EVD(IIIEVD(IIIndnd
))
Stand magnifier
◦ Object is fixed / Image
distance is fixed
◦ Enlargement ratio (ER) is
also fixed
◦ EVD = Eye to image
distance./ ER
◦ Eye to image distance
 Depends upon where the
patient’s eye is located
 Eye to lens distance + lens to
image distance

15. Calculation of ER in StandCalculation of ER in Stand
MagnifierMagnifier
Measure
◦ Equivalent power of lens (Fe) & Image distance (l’)
◦ Image vergence (L’) = 1/l’
◦ ER = L’-Fe / L’ (L’ –ve signifies divergence)
◦ Eg: If, Fe = +20 Ds, l’ = 20cm , L’ = -5 Ds
ER = -5 - (+20) / -5 = -25/-5 = 5 x
◦ If, Fe = + 12.50 Ds, l’ = 40 cm , L’ = -2.50 Ds
ER = -2.5 – 12.50 /-2.5 = -15/-2.5 = 6 x

16. Calculation of EVDCalculation of EVD
 EVD = eye to image distance/ER
 Eg: Fe = +20 Ds, l’ = 20 cm below lens surface
L = - 5 Ds, EVP = 25/5 = 5x
1. If eye is 5cm from magnifier,
Viewing distance = 5 + 20 = 25 cm (eye to image distance)
EVD = 25 cm /5 = 5 cm
2. If eye to magnifier is 20 cm, Eye to image = 20 + 20 = 40 cm
EVD = 40 cm / 5
(for same stand magnifier EVD will differ from eye to
magnifier separation)

17. Find the Option equal EVD IVFind the Option equal EVD IVthth
Telemicroscope
◦ Distance telescope with a near vision cap
◦ Object is at focal plane of near cap
◦ Cap acts as collimator – image at infinity

18. EVD = focal length of cap/EVD = focal length of cap/
telescope magnification (ER)telescope magnification (ER)
◦ Eg: a 3x telescope with a +4.00 Ds cap
Cap give EVD of 25 cm (its focal length)
But for Telescope with 3x ER
EVD = 25 cm/ 3 = 8.33 cm

19. Find the option with equal EVD VFind the option with equal EVD Vthth
Video-Magnifiers
◦ ER is given
◦ Viewing distance = eye to screen distance =
amount of reading add (presbyopes)
EVD = fig
Actual viewing distance / ER
Eg: CCTV enlarges by 10 x ,
◦ ER = 10 x ,
Viewing distance = 50 cm
◦ EVD = 50 cm / 10 = 5 cm

20. ConclusionConclusion
Manufacturer give unreliable
information on their magnifiers : don’t
trust them
Inappropriate & misleading definition of
Magnification
◦ Calculations will be wrong , trial will be
tedious
WHY to stick to EVD
◦ Reliable
◦ Reproducible
 No use of misleading magnification

21. Requirements for EVD calculationRequirements for EVD calculation
Equivalent power
of magnifier
Checking the
image distance for
Stand magnifier
(Multac factor???)

22. ProblemsProblems
A patient of 20 yrs has BNA 4M at 20 cm (Acc:
8 Ds) and TNA of 0.5 M – Ajit
A patient is 60 yrs has BNA of 3 M at 25 cm and
his TNA is 0.8 M - Jevanand
A patient is 45 yrs has BNA of 6M at 40 cm (with
+2.50 Ds) and his TNA is 1 M – Gopal
A patient is 30 yrs has BNA 5M at 30 cm (Acc: 6
Ds) and TNA of 0.6 M – Dipendra
A patient of 10 yrs has BNA 8M at 50 cm (Acc:
10 Ds) and TNA of 2M- Sanjay
 A patient is 80 yrs has BNA of 4 M at 25 cm
and his TNA is 1 M - Nabin
•Find all the possible options of optical
device/ Electronic devices matching EVD?