Subjective refraction

Question 1

Can be done for

Answer 1

Sphere
Astigmatism

Question 2

Method of subjective refraction for spherical refraction

Answer 2

Ask patient to read letters and if they cant read add +1 and see if its better with or without
- If better without then move to minus and depending on how large the letters are use a large or small minus lens
- Then continue until there is no difference
- The using duochrome ask if red or greem is clear
If red clear overplus add – lens
- If green overminus add + lens
- If they say no difference add 1+ and as soon as you remove which is clear
- It is going red  clear rather than green  clear

Question 3

Astigmatism is estimated from BVS line

Answer 3

6/9 = 0.50DS or 1.00DC
6/12 = 0.75DS or 1.50DC
6/18 = 1.00DS or 2.00DC
6/24 = 1.50DS or 3.00DC
6/36 = 2.00DS or 4.00DC

Question 4

Use of subjective refraction

Answer 4

• Goal is to develop and refine a prescription through which the patient can see comfortably.
• Patient may prefer a refractive correction which differs from an objective procedure
  *It is a multi-step procedure

Subjective refraction:
* Start subjective refraction after retinoscopy
* Start subjective without retinoscopy

Question 5

What is the best vision sphere (BVS)?

Answer 5

-Best vision sphere (BVS) can be defined as the most plus or least minus spherical lens that provides best visual acuity.
-With BVS you move CLC onto the retina.

Question 6

Methods to estimate astigmatic interval

Answer 6

JCC
Fan and block

Question 7

What is the fan and block method

Answer 7

Add half the estimated astigmatic interval as +ve sphere
Ask patient to look at the fan and ask which line/s are clear. This tells you the direction of the back focal line.
All we need to do is add –ve cylinder, the same power of our estimated astigmatic interval with the axis in the same direction as the front focal line.
To refine the power, ask patient to look at the blocks and adjust cyl power until blocks appear equal.
Refine axis so all lines of the fan are equal.
Refine sphere using duochrome/letter chart.

Question 8

What is the JCC

Answer 8

To refine cyl axis
Using +0.50 Jcc. (red one) you want them to be at the axis you have calc from ret
Align handle with axis of working cyl and twirl asking if letters look clearer with lens 1 or 2.
Rotate working cylinder axis toward negative marking of x-cyl in preferred position.
If you twist it 180 and they say that Is better then move the axis 20 closer to that. If they then say it is better with lens one move back to original axis but only 10 closer. Continue doing this with the  increments getting smaller until they say no difference
Then to refine power use +0.25
If preffered power is with – 0.25 cyl then add – lens. If with +0.25 axis then remove – lens
Once they say same then test duochrome