magnification, It's definition, types, clinical uses, Uses in Optical instruments like microscopes, telescopes, Uses in Optical instruments like direct Ophthalmoscopes, indirect ophthalmoscopes and slit lamps, In low vision

1. Magnification and clinical
correlation in Ocular
examination
Raju Kaiti, M. Optom
Consultant Optometrist
Nepal Eye Hospital, NEH

2. Introduction
– An increase in apparent size, perceived size or the
actual size of an object or its image in relation to the
object
– It has no unit since it is a ratio
– Apparent increment in the size of the object when viewed
through the magnifying system
– Magnifying ability = size of retinal image formed with the
aid of the lens /size of retinal image of the object viewed
directly

3. Types
• Linear Magnification
• Angular Magnification
• Nominal and Maximum Angular Magnification
• Iso-accommodative Magnification

4. Linear magnification
• Applies to linear dimensions
• Generally indicated by multilication sign “X”
• LM= I/O= V/U=l’/l
• Object height and image height
– Is measured perpendicularly from the optical axis
– Can be measured from any pair of off-axis conjugate points
• In terms of reduced vergence,
Reduced vergence =1/distance
• M=object vergence/Image vergence

5. Sign convention
• Image inverted with respect to the object-
negative magnification
• Image upright with respect to the object-
positive magnification

6. Angular magnification
Angle subtended at eye by image produced by lens
Angle subtended at unaided eye by object at LDDV(least
distance of distinct vision)

7. M=Tan β/Tan α
Tan β=h’/d-l’
Tan α=h/-q
M=qL/1-dL’

8. Angular magnification
– A plus lens is used as a simple magnifier
– The magnifying ability of the loupe= size of the
retinal image formed with the aid of the lens / that of
the object viewed directly
– General expression for the angular magnification, M
= qL/1-dL’
where q = LDDV,
d = vertex distance
L/L’= object vergence/ image vergence

9. Nominal and maximum angular
magnification
• Two cases of extreme magnification.
1. Image formed by the lens is at infinity
2. Image is formed at the least distance of
distinct vision.

10. Case 1

11. • L`= 0, q= -0.25m, L= -F
• M nominal =
F
4
• Nominal magnification or conventional
magnification
• No accommodation is used to see the image by an
emmetropic eye.
• Angular magnification is one- quarter of the lens
power.

12. Case II
• Angular magnification will be maximum
when image is formed at LDDV.
• L’=-0.25m, d=0 then M=(-0.25)(-4-F)
• M max= 1+F/4
• Lower angular magnification is obtained
when the image is formed further than 0.25
m from the eye.

13. Iso-accommodative magnification
• Is the magnification determined considering
same accommodative effort is in act when
eyes are both aided and unaided.
• Misoacc=1-(q+d)F, when image is formed at
LDDV
• As per British standard (BS 5043), d=10cm
and q=-25cm, then
• Misoacc= 1+0.15F
• Represents the practical magnification that
one might attain with a loop.

14. Magnification and the depth of
field
• Increase in magnification limits the depth of
field, so higher the power more critical is its
positioning in relation to the object.
• Flexible for persons with good amplitude of
accommodation.

15. Summary
• Angular magnification= qL/1-dL’
• Nominal magnification= F/4
• Maximum magnification= 1+F/4
• Iso-accommodative magnification= 1+0.15F

16. ?????
• A +8.00D loupe is held 5cm in front of the
eye, and is used to view an object placed 8 cm
in front of the lens. Calculate
1. The linear magnification
2. The angular magnification
3. What nominal magnification would be ascribed
to this lens?

17. ?????
• An emmetrope exerting 4D of accommodation
views the image of an object clearly when the
object is 6 cm in front of a +10.00 DS lens.
Calculate the distance between the lens and the eye
and determine the angular magnification.

18. ?????
• A 2.00D Myope uses a +8.00D lens as a loupe. In
order to view the image of an object held in front of
the lens, he accommodates 6.00D. If he holds the
lens 5cm in front of the eye, what angular
magnification does he obtain and what is the
distance between the lens and object?

19. Some other form of
magnifications

20. Spectacle and relative spectacle
magnification
• SM= change in retinal image size brought
by correcting lens.
• Ratio of retinal image size with the
correcting lens to that without the
correcting lens.
• SM= retinal image size in corrected eye/basic
height of retinal image in uncorrected eye
• %SM= (SM-1)*100

21. • Depends on 2 factors
• Shape factor and power factor
• SM=
• 1 1
• 1-(t/n)*F1 1-dF`V

22. • F1 = power of the front surface
• F`V= back vertex power
• t = thickness
• n= refractive index
• d = distance from the back vertex and the
entrance pupil of the eye

23. Shape factor
• 1
1-tF1/n
SM increases with increase in front surface
power and increases with increase in
thickness.
SM decreases with decrease in index of
refraction of the lens material.

24. Power Factor
• 1/1-dFv’
Increases with increase in back vertex power
for Plus lens.
Decreases with increase in back vertex
power for minus lens.
If + lens id moved closer to the eye, d
decreases and SM also decreases.

25. • If –lens is moved closer to the eye d
decreases and SM increases
• SM>1 for + lens
• SM<1 for -lens

26. SM and vertex distance
• Retinal image size α Sec. Focal length.
• 1/power of the correcting lens
• As the vertex distance of a +lens increases
,the SM increases and vice-versa.
• As the vertex distance of a – lens increases,
the SM decreases and vice-versa

27. Factors affecting Retinal Image
• Change in prescription
• Change in vertex distance
• Change from spectacle to contact lens
• Change in thickness
• Change in bend or form of lens

28. SM in Astigmatism
• Differs in two principal meridians.
• The retinal image is larger in direction of the axis of
the –cyl that corrects the astigmatism.
• Difference between the two meridians= 1.5%per
dioptre of astigmatism.(spectacle)
• Difference between the two meridians=0.3%per
dioptre of astigmatism(contact lens)

29. Relative Spectacle Magnification RSM
• Ratio of the retinal image size of the corrected
ammetropic eye to that of the schematic emmetropic
eye or standard emmetropic eye.
• Depends on whether the ametropia is axial or refractive
• The image size is essentially the same as that for the
emmetropic eye in axial ametropia, but is magnified in
hyperopia and minified in myopia compared with that
for the emmetropic eye in refractive ametropia

30. • RSM= fe` /f`ST= FST/Fe
• FST= Equivalent power of the standard
emmetropic eye
• Fe = Equivalent power of the system
• Fe= FSP+ FA
• Where FSP = power correcting of spectacle
lens
• FA= refracting power of the ametropic eye

31. RSM in axial/refractive ametropia
• FA= FST
• RSM= FST / FSP + FST- dFSP*FST
• If spectacles are in the anterior focal point of the
eye, then RSM=1
• FA not equal to FST
• Axial length of the ametropic eye is equal to the
standard emmetropic eye.
• RSM= 1 /1- dFSP
• Significance is contact lens in ametropia

32. RSM in astigmatism
• If corrected by spectacle lens the retinal
image size is greatest in the axis meridian of
the correcting minus cyl and least in the
power meridian.
• If corrected by contact lens????

33. Magnification in Low vision
• Relative Distance Magnification (RDM)
• Relative Size Magnification (RSM)
• Projection Magnification
• Angular Magnification

34. Relative Distance Magnification (RDM)
• RDM=initial object to present distance / the same object
to new distance
• If an object is moved from the reference distance of
40cm to 10cm RDM=40/10=4
• Bring the object closer => increases the angular
subtends of the object=> appears larger
• Trees near the road side appears larger than that of the
far-distance
• Use of plus lens for accommodation

35. Relative Size Magnification (RSM)
• Magnification obtained by increasing the size of
the object at its original position.
• E.g. large print books, magazines, large display
screen
• RSM= angular size of enlarged object /angular
size of initial object
E.g. if at 40cm an object is 0.5mm high but is increased
to 2.0 mm high
RSM= 2/0.5=4

36. Projection magnification
• The magnification produced from the
formation of an enlarged image on a screen ,
of an opaque or transparent object e.g.
overhead projector, CCTV

37. Use of magnification in some
optical systems

38. THE MICROSCOPE
1)SIMPLE MICROSCOPE and 2) COMPOUND MICROSCOPE
1)SIMPLE MICROSCOPE- A single convex lens of
short focal length can be used to see magnified image
of a small object and is called a magnifying glass or
simple microscope
• principle -when a small object is placed betn
optical
centre & focus of a convex lens, its virtual erect &
magnified image is formed on the same side of the lens
• The lens is so held that the image is formed
at the LDDV

39. • Magnifying power = angle subtended at eye by
image produced by lens/ angle subtended at unaided eye
by object at LDDV
• M= 1+D/fe,
D=LDDV, fe=eye piece lens
Uses-
• Jewelers & watch makers
• To see slides

40. • COMPOUND MICROSCOPE
– Objective piece - Short aperture and short focal
length
– Eye piece - short focal length and large aperture
– Principle -
• When a small object is placed just outside the
focus of the object lens its real , inverted and
magnified image is produced in the other side of
the lens between its f and 2f .

41. • The image produced by objective piece acts as
object for eye piece .
• The position of the eye lens is so adjusted that
the final image is formed at LDDV
• Me = 1+D/fe & Mo = v/ u
• M = Mo x Me = v/u (1+D/ fe)
• M= fo/fe= -Doc/D obj
At LDDV-
• M=fo/fe(1+fe/D)

42. TELESCOPE:
1. ASTRONOMICAL TELESCOPE :
– It produces virtual and inverted image
– Used to see heavenly bodies
•Principle -
– The objective forms the real and inverted image of the
distant object at its focal plane
– The position of eye piece is adjusted till the final image is
formed at LDDV.
– Normal adjustment - final image is formed at infinity
• M=fo / fe
– When final image is formed at LDDV -
• M= fo/fe(1+fe/D)

43. 2. TERRESTRIAL TELESCOPE:
– Produces an erect image
– Erecting lens is placed in between objective and eye piece
– Normal adjustment -
• M=fo/fe
– At LDDV-
• M= fo/fe(1+fe/D)

44. 3. GALLILEO’S TELESCOPE :
– It provides an erect image of the distant object by use of two
lenses
– The objective piece ( convex lens )form the real and inverted
image of the distant object on the other side of lens at the focal
plane of objective.
– This image acts as a virtual object for the eye piece(concave
lens) . Final erect image formed at infinity
– The difference between two lens equals to fo-fe
• M= fo/fe

45. OCULAR SYSTEMS
&
CLINICAL
SIGNIFICANCE

46. • Introduction :
• The ratio of size of image to the size of object
• M = image size / object size
= object vergence / image vergence
= image distance / object distance
• Human eye as the optical system the size of the image on
the retina is being compared with the size of the object of
regard
• Retinal image magnification (RIM) =
• Magnified retinal image size / original retinal size

47. – RIM has three components :
• Relative size magnification (RSM)
• Relative distance magnification (RDM)
• Lens vertex magnification (LVM)
– RSM and RDM can be achieved without the use of lens
where as LVM depends on the kind of lens placed before
the eye and its location
– It allows the use of magnifiers in such a way that image on
the retina are usable and functional although not in perfect
focus

48. • Clinical Significance :
• Direct ophthalmoscopy-
• Image is erect, virtual and
(about 15 times ) magnified in emmetrope ( more in
myopes less in hypermetropes).
• Indirect ophthalmoscopy -
• Image is real ,inverted
and magnified , which depends upon the dioptric
power of the convex lens , position of the lens in
relation to eye ball and refractive status of eye ball.

49. • Slit lamp Bio-microscope - Image is erect ,virtual and
magnification can be adjusted according to need as 10 X , 16 X
and 20 X.
•Low magnification:
– 7X - 10X : General eye
– Lids.
– Bulbar conjunctiva/sclera.
– Cornea/limbus.
– Tears.
– Anterior chamber/iris/crystalline lens.
•Medium magnification:
– 20X - 25X : Structure of individual
layers
– Epithelium/epithelial breakdown.
– Stroma.
– Endothelium.
– Contact lens fit/lens condition.
• High magnification:
– 30X - 40X : Details
• Epithelium
– vacuoles
– microcysts
– dystrophies.
• Stroma
– striae
– folds.
• Endothelium
– Polymegathism
– guttata
– blebs
– cell density.

50. In low vision aids –
• Spectacle Magnifier: RDM
• Hand magnifier :
• Useful in short term visual task
• Magnification depends upon equivalent power and how the
magnifier is used (RDM and Angular Magnification)
• Stand magnifier :
• The fixed focused eye having a fixed distance from object of
regard
• Magnification depends on the power of the magnifier (RDM
and Angular Magnification)

51. •Paper wet magnifier :
• Reading aid in which thick plano- convex lens is held
in contact with the reading material.
• Magnification is relatively low
•In Aniseometropia -
• Contact lens produces low magnification than spectacle
thus removes the aniseokonia .
•In Closed circuit television (CCTV) -
• Projection and relative distance magnification are
used .
•In Telescope -
• Angular magnification is used.

52. THANK YOU