Subjective refraction...

[haya]

Hello all,
I have a question...

What is the effect on the refractive correction of inserting a cylinder of the correct power, but 10 degrees off axis in a trial frame during subjective refraction?

Is it possible for anyone to quote a numerical example?

:) haya...

[Darryl Meister]

Hi haya,

The exact effect will depend upon the cylinder power, since the change will become more pronounced with higher cyls. In general, changing the axis of a cylinder will produce three changes in the Rx:

1. A change in sphere power
2. A change in cylinder power
3. A change in cylinder axis

This occurs because changing the axis of the cylinder is like combining two obliquely crossed cylinders. This produces a new resultant spherocylindrical refractive error. This why you can do things like refine the prescribed cylinder axis with Jackson Cross Cyls, and why the ANSI Z80.1 Standard has the tolerances it does on cylinder axes.

Consider the specific case of changing the axis of a prescribed -1.00 DC x 180 cylinder by 10 deg. The resultant spherocylindrical error is created for the wearer is +0.17 DS -0.34 DC x 40. Changing the axis of a -2.00 DC x 180 by 10 deg creates a resultant spherocylindrical error of +0.35 DS -0.70 DC x 40. Note the induced sphere power in both cases.

You can play around with a few examples yourself if you download my OpticsLite spreadsheet from the OptiBoard Downloads section. In the Crossed Cylinders module, type in the spectacle correction as the first lens and the ocular error (same values but opposite signs as the spectacle correction) as the second lens. Then change the axis of the spectacle correction to see its effect.

Best regards,
Darryl

[Sara]

Hi haya,
Darryl has answered your question well.
There is a formula to find new sph-cyl from obliquely cross-cyls which you can apply here for your question.Unfortunately my keyboard does not have required keys to permit me to post that formula here.
Also you can get anwer from a graphical contruction known as "Stokes Construction".
Refer to any good optical textbook for formula and how to do it by using stokes construction.
Thanks,
Sara:)

BTW "haya" means "big yes"in my vernacular language:D

[haya]

I really appreciate your help, u guys!

I agree with you Sara, that Darryl Master (of Ophthalmic optics forum), sorry, Meister, has answered it well! :). He has as usual ANSWERED my question. Thanks! :bbg:

Sara, haya also means 'to live', in Arabic!!:D

Take care u guys,

:finger: i'll be back for sure.....

[haya]

Hope i haven't offended anyone, cos all of u guys are doing a great job (aka steve and shanbaum)

haya:hammer: head

[Sara]

hi Haya,
it's nice to know you know arabic,I'll come to you for lots of translations!!:D
Thanks,
Sara,the one who likes"haya";)

[haya]

To Sara,

Unfortunately I don't understand Arabic at all, but you can come to me for lots of Urdu!!:bbg:

Confused? I'm not an Arab, but i am muslim!!!

haya :hammer: head!!

[Sara]

Hi Haya,
The languages I can communicate are Gujrati,arabic,urdu,kiswahili and english.
The great linguist that I am !!:cool:
Thanks,
Sara

[haya]

Oh my!!

Sara, I am so envious!!

:drop:

I'll be back to pick YOUR brains too!!!

thanks mate :bbg:

haya