Flashcards in Fan And Block - Week 1 Deck (23):
Does a point object have a point image?
Instead, focal lines are formed along the two principle meridians. The two focal lines are separated by the 'interval of sturm'
What form do images take within the interval of sturm
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
In retinoscopy, how can you find a meridia?
The reflex of the pupil should be aligned with the light reflex
In retinoscopy, which meridian should you neutralise first? Why?
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)
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
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
How can a cyl lens correct astigmatism?
By reducing the interval of sturm to zero, such that a point focus can be obtained
Do you usually determine BVS before or after ret?
What is the goal of BVS?
To place the COLC on the retina, so anterior and posterior focal lines are equidistant from the retina
What is the rough rule of thumb for estimating astigmatism with BVS?
-0.50DC equivalent to 1 line
What are the 2 main techniques used to determine astigmatism in subjective refraction?
1) The Fogging Method
2) The Jackson Cross Cylinder Method
What charts/targets do we use for the fogging method?
Are there any other charts (out there) that can be used?
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.
Why do we begin the fogging method with BVS? What other similar technique can we begin with BVS?
Begin with BVS to place the COLC on the retina
Also can do this before X-cyl (for the same reason)
What do the fan chart lines look like, in terms of clarity, when COLC is on the retina?
All lines on the fan chart should be equally blurred
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?
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
What are the rough steps to performing Fan and Block? (10) [note: this is from the prac sheet]
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)
How much +ve cyl do you add when fogging?
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.
List the instructions you should give during Fan and Block? (3) [kind of prac]
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"
When looking at the 2 limbs of the V, what does it mean if:
1) Both are equal
2) One limb is clearer
2) check for reversal then move on
3) Rotate the V towards that direction
True or False: You must always refine the axis again after power determination
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?
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)
for every -0.50DC added, what should you do?
Add +0.25DS to maintain the fog
How many degrees apart are the limbs of the fan?
15 degrees each