Fan And Block - Week 1

Question 1

Does a point object have a point image?

Answer 1

No.

Instead, focal lines are formed along the two principle meridians. The two focal lines are separated by the ‘interval of sturm’

Question 2

What form do images take within the interval of sturm

Answer 2

Blue ellipses. (Except at the dioptric midpoint between the two focal lines)

The closest thing to a point image is the circle of lest confusion, which occurs at the dioptric midpoint

Question 3

In retinoscopy, how can you find a meridia?

Answer 3

The reflex of the pupil should be aligned with the light reflex

Question 4

In retinoscopy, which meridian should you neutralise first? Why?

Answer 4

The most positive meridian.

• this allows you to use negative cyl for the 2nd meridian

(Note: most positive in this case = the meridian where you add the most positive to neutralise.

Hence why if you have with/against, you neutralise with first)

Question 5

Explain where the COLC and focal lines fall for the following: 
A: Simple Myopic Astig 
B: Simple Hyperopic Astig
C: Mixed Astig
D: Compound myopic Astig
E: Compound Hyperopic Astig

Answer 5

Simple myop: COLC before retina, back focal line on retina
Simple hyper: COLC after ret, front focal line on retina
Mixed Astig: COLC on retina
Compound myo: COLC and both focal lines before retina
Compound hyp: COLC and both focal lines after retina

Question 6

How can a cyl lens correct astigmatism?

Answer 6

By reducing the interval of sturm to zero, such that a point focus can be obtained

Question 7

Do you usually determine BVS before or after ret?

Answer 7

After

Question 8

What is the goal of BVS?

Answer 8

To place the COLC on the retina, so anterior and posterior focal lines are equidistant from the retina

Question 9

What is the rough rule of thumb for estimating astigmatism with BVS?

Answer 9

Snellen denominator/9

Or

-0.50DC equivalent to 1 line

Question 10

What are the 2 main techniques used to determine astigmatism in subjective refraction?

Answer 10

1) The Fogging Method

2) The Jackson Cross Cylinder Method

Question 11

What charts/targets do we use for the fogging method?

Are there any other charts (out there) that can be used?

Answer 11

Fan chart - used to locate approximate axis
Locator (V) Chevron - used to refine axis
Corrector (T, L or block) chart - used to determine power of correcting cylinder

Note: This is also the order that these charts are used in.

Yes.

Question 12

Why do we begin the fogging method with BVS? What other similar technique can we begin with BVS?

Answer 12

Begin with BVS to place the COLC on the retina

Also can do this before X-cyl (for the same reason)

Question 13

What do the fan chart lines look like, in terms of clarity, when COLC is on the retina?

Answer 13

All lines on the fan chart should be equally blurred

Question 14

In which direction will the astigmatic foci move if you add:

1) +ve sphere
2) -ve sphere

What does this mean for the fan chart line clarity?

Answer 14

1) forward - therefore limbs of fan along the posterior focal line axis will appear clearer
2) backward - limbs of fan along the anterior focal line axis will appear clearer

Question 15

What are the rough steps to performing Fan and Block? (10) [note: this is from the prac sheet]

Answer 15

1. BVS - predict astig during this stage
2. Fogging: add +ve sph to fog based on estimated cyl
3. Ask px which lines of fan appear ‘clearest and blackest)
4. Rotate the V towards the ‘clearest’ lines
5. Ask which limb of the V is clearer
   - move V in direction of blurred limb (if both equal, check for reversal)
6. Rotate phoropter axis 90deg to the ‘clear’ block (blocks above V. i.e. if V at 45deg, and parallel block clearer, set phoropter axis to 135deg)
7. Increase -ve cyl power until the 2 blocks appear equal
8. Refine axis - add +0.50DS to confirm fog is maintained and minimised
   - (if lines marked with v are clearer, increase cyl, if lines - - along cyl axis become clearer, decrease cyl)
9. Letter chart, remove fog in appropriate steps until best corrected vision obtained
10. Endpoint checks

** way to remember
BFARARI RLE (sounds like some kind of Ferrari)

Question 16

How much +ve cyl do you add when fogging?

Answer 16

This is based on estimated cyl:

• if -1.00DC or less, add +0.50DS (fog by 2 lines)
• if more than -1.00DC, add +0.50DS + 1/2 px’s estimated degree of astig.

Question 17

List the instructions you should give during Fan and Block? (3) [kind of prac]

Answer 17

1. (during fogging): “Do any lines on the fan appear blacker and clearer?”
2. (referring to locator/V chart): “Now looking at the 2 limbs of the V, tell me which one is clearer: the one closer to the cross or the circle, or if they are both equal”
3. (after answer, if not equal): “Now i am going to rotate the V and as I rotate it, stop me as soon as you see both limbs of the V are equal”

Question 18

When looking at the 2 limbs of the V, what does it mean if:

1) Both are equal
2) One limb is clearer

Answer 18

2) check for reversal then move on

3) Rotate the V towards that direction

Question 19

True or False: You must always refine the axis again after power determination

Answer 19

True

Question 20

During axis determination, what does it mean when multiple lines appear clearer on the fan chart? What if all lines are equal? What if only a single line is clearer?

Answer 20

If single line clearer: indicates the meridian of the posterior focal line

If multiple lines clearer: the meridian of the posterior focal line lies in the middle (of these lines that appear clearer)

If all equal: suggests no cyl or over-fogged or under-fogged (meaning BVS may be inaccurate)

Question 21

for every -0.50DC added, what should you do?

Answer 21

Add +0.25DS to maintain the fog

Question 22

How many degrees apart are the limbs of the fan?

Answer 22

15 degrees each

Question 23

What does it mean to point the V

Answer 23

Point the V by having the corner of the V face the direction you want the v to point