that weird lens next to pinhole!!

[aneekatanveer]

hi guyz i was asked this question in my last assesment and i had no clue what the answer is iv looked on the net nd my supervisor couldnt remember either!!!i feel real du,b for asking this but there is a lens in the trail case set next to pinhole, its a long slit running through and no its not the maddox rod, its like a pinhole but long slit instead of a pin holre, i need to know its name and its use, iv searched the net and it said something about finding astigmatism, the article didnt make sense....any one got any ideas?thanks alot
neeki:idea:

[drk]

I'm sure Chip Anderson will know this...

It's called the "Stenopaic slit", and indeed, it has been used to refract for astigmatism.

Conceptualize it as an isolater of a principal meridian. Picture a theoretical case of simple myopic astigmatism:

+1.00
I
I_____ pl
I
I

If the patient were looking at a circular optotype ("O" on the eyechart), and the slit were mounted in a trial frame with a knob controlling axis, the patient could twist the slit until the vision was the clearest.* (Which direction would it be oriented in this case? Horizontally.) The refractionist then presents trial lenses and refracts this meridian (will be plano horizontally, here).

Then, the refractionist turns the slit 90 degrees away in regular astigmatism** and refracts that meridian (-1.00 in this case).

Final Rx here would be pl-1.00x180.

*Note that it does not have an appreciable pinhole effect: it's wider than that.

**In irregular astigmatism, we actually use this method of refraction, occasionally. We just refract the best two meridia (which are not 90 degrees apart), put the monstrosity in a lensometer and read the resultant!

[aneekatanveer]

thnx so much really appreciate it

[rbaker]

Used for resolving astigmatism during retinoscopy.

[HarryChiling]

We just refract the best two meridia (which are not 90 degrees apart), put the monstrosity in a lensometer and read the resultant!

Here you go this should help with that monstrosity.:D