varifocal blank size

Hi everyone.

Its been long since my last message.

I have had 2months of stress, work, study and exams. Finally its over . Well only for another 2 months that is then the process repeats itself.

Anyhow I have couple of questions: 1)how is the blank size aka minimum size uncut calculated for varifocals. Is it the same as single vision lenses/bifocals.

When examining patients for the eye test I always have difficulty measuring the back vertex distance. By definition its the ditance from corneal apex to the backmost trial lens in the trial frame but when it comes to measuring with accuracy I fail. Is it measured with both examiner eyes opened with examiner standing to the side of the patients face. How is it measured for trial frames.

What exactly is the purpose of prisms at the prism checking point in varifocals?

Thanks

hagi, I'd say that determining the minimum uncut blank size is the same as a single vision lens, i.e., subtract the pt's pd from the frame pd, then add twice the longest radius to the effective frame diamiter; that should be okay, except if the seg height is considerably above, or below the 180 mid line, then that has to be factored in; plus power varifocals, need to have a appropriate amount of prism thinning, since without prism thinning, the lenses would have an uncomfortable amount of base up prism...

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hagi said:
Anyhow I have couple of questions: 1)how is the blank size aka minimum size uncut calculated for varifocals. Is it the same as single vision lenses/bifocals.

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TR touched on it but it is not "exactly" like you would determine the effective cutout as you would a SV .. a fast easy way to do it in your head is do the usual way for a SV and than add the difference from the datum to the fitting hght. to the ED... i.e. your B is 40 and you have a 26 fit..I'll add that 6 (since above) to the formula..not perfect but it is a fast easy way to do it in your head..since most PAL's are stock 80' mm blanks (unless you are fitting short corridors) it really is not that much of an issue. :)

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When examining patients for the eye test I always have difficulty measuring the back vertex distance. By definition its the ditance from corneal apex to the backmost trial lens in the trial frame but when it comes to measuring with accuracy I fail. Is it measured with both examiner eyes opened with examiner standing to the side of the patients face. How is it measured for trial frames.

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most phoropters have this built in so I never really worried about it.. but as for a trial frame I always measured it from the side with a PD stick..

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What exactly is the purpose of prisms at the prism checking point in varifocals?

Thanks

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I don't know if you are asking about "prism thinning" as TR mentioned or if you are just wondering "why" we measure it at the 180 below the fitting cross? If so, the reason we have an induced amount of prism in the 90 degree meridian is due to surface design of the majority of PAL's (semi-aspherical)..at that point the lens goes from being aspherical to spherical (the 180) so in the process of surfacing you have a natural tilt to the blocked lens and end up with vertical prism, since it transfers OU usually it neutralizes it's self but you use prism thinning for two reasons, to reduce the amount of induced prism because of design as well as for cosmetic reasons..sort of a double bonus :)

Glad you get to "rest" your brain for a few months.. so how did the exams go?

Jeff "Santa left resin in my stocking instead of coal" Trail

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hagi said:

When examining patients for the eye test I always have difficulty measuring the back vertex distance. By definition its the ditance from corneal apex to the backmost trial lens in the trial frame but when it comes to measuring with accuracy I fail. Is it measured with both examiner eyes opened with examiner standing to the side of the patients face. How is it measured for trial frames.
Thanks

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Try a "distometer" http://west-op.com/distometer.html
For those interested in noastalgia mine says "Distometer by House of Vision, Chicago IL. Pat. Pending"

Some corneal reflection pupilometers also have a second vertical line in the viewfinders that can be used for measuring vertex distance. If yours has a second set of numbers along the slides for the p.d., its probably so equipped.

Many PALs are "decentered." If the blank size reads 77/82 or something like that, than the blank is decentered. Usually, the decentration is 2.5mm. As Jeff mentioned, cut-out usually isn't an issue with PALs unless you have a really large frame (and these days, 52mm eyesize is on the large size ;) ).

Thanks for replies and many thanks to JEFF for asking about examz.

How did they go? well good question jeff.6 exams in total, 1 I felt went brilliantly the others okish. The cruel thing is have to wait till july to find out the results. In the mean time have to prepare for 4 more practical exams for july. Big oucccchhhhh.

Wheres the fun in optics. Just tell me buddy cos I'm having a hard time searchingf for it. Maybe the fun starts after the exams. Prhaps, long hopes. Anyhow time for me to sleep.