3. This lecture presents the optical theory and
practice associated with contact lens fitting and
usage.
Contact lenses and spectacles are also
compared as optical devices.
4.
5. BACK Vertex power is the distance of the
second principal focus from the back vertex of
the lens.
6.
7. Minus power – is less in contact lenses
compared to spectacles.
Plus power – is higher in CL compared to
spectacles.
8.
9. Image size in an optical system is inversely
proportional to power of the lens.
10. SM = Image size with Spectacle
Image size without Spectacle
11. CLM = Image size with Contact Lens
Image size with Spectacle
12. Image size in any optical system is directly
proportional to the focal length of the system.
In hyperopia, the contact lens focal length is
shorter than the equivalent spectacle focal
length (shorter by the vertex distance in fact).
13. Consequently, the image size is smaller when
contact lenses are worn.
The reverse is the case in myopia. The contact
lens focal length is longer than the equivalent
spectacle focal length, and therefore the
contact lens image size is larger.
14. CLM = 1-d FSp
Examples with d = 14 mm
+ 10.00 D, CLM =
- 10.00 D, CLM =
0.86
1.14
15. RSM = Image size with Spectacle
Image size in Emmetropia
16. SM is a real-world comparison of corrected (focused)
and uncorrected (blurred) retinal image sizes.
CLM is a more realistic comparison of contact lens
corrected versus spectacle lens corrected retinal
image sizes.
RSM is a hypothetical magnification comparing
image sizes in a corrected ametropic eye and a
theoretical emmetropic schematic eye.
17.
18.
19.
20.
21. Spectacle wearing myopes accommodate less
than spectacle wearing hyperopes (2.114 D
versus 2.786 D).
With contact lens wear, the accommodation
required in ametropia is approximately the
same as for an emmetrope (≈ 2.415 D).
22. The accommodative demand of a myope is
greater in contact lenses (2.415 D) than with
spectacles (2.114 D).
The accommodative demand of a hyperope is
greater with spectacles (2.786 D) than with
contact lenses (2.415 D).
23. If a myope is switched from spectacles to
contact lenses the change may precipitate the
need for a near correction.
If a hyperope is switched from spectacles to
contact lenses the need for a near correction
may be postponed
24.
25. A hyperope wearing contact lenses converges
less than when wearing spectacles.
This is because of the base-out prism effect
induced by spectacles acting as an exercising
26. A myope wearing contact lenses converges
more when wearing contact lenses than when
wearing spectacles.
When wearing spectacles, they behave as a
base-in relieving Prism
27. Δ = h in cm
q in m (q is the distance from C of R to Fixation Plane)
30. Tear lens under a flexible lens is very thin and
has no power
Tear lens under a rigid lens depends on
material rigidity and the fitting relationship
(apical touch) (Parallel) (apical Clearence)
- TL No TL + TL
31.
32. Cornea/tears interface is optically insignificant
bcz difference in refractive index of cornea
and Tear is insignificant
Tear lens is “sphericalized” by the back
surface of a spherical lens
This results in a major reduction of corneal
astigmatism with a spherical lens
Almost 90% Corneal astigmatism can correct
with Spherical Rigid Lens
34. 1. Failure to vertex correct spectacle Rx to derive ocular
Rx
2. Failure to vertex correct over-Rx (if>+4.00 D)
3. K readings only represent the central 3 mm
4. K readings incorrect
5. Tear lens power varies slightly with Ks
6. BVPTRIAL may be incorrect - verify it
35. 7. Subjective over-Rx is incorrect
8. BOZR of trial lens incorrect - verify it
9. Trial lens is decentred and/or tilted
10. Trial lens flexure in situ, rigid and soft
11. Corneal molding by the lens
36. 12. Corneal shape not spherical or sphero-cylindrical
13. Variable toric tear lens due to trial lens
movement/decentration/tilting/rotation
14. Tear lens under a thick, high power, low water soft
lens
15. Environment-induced changes in thick, soft trial
lens
16. One or more of the above in combination
