i know that for asperics and high powered lenses you need to lower the OC by 1mm for every 2degrees of pantescopic angle. (correct me if i'm wrong)
What problems is the patient likely to experience if this is not done?
Many thanks
i know that for asperics and high powered lenses you need to lower the OC by 1mm for every 2degrees of pantescopic angle. (correct me if i'm wrong)
What problems is the patient likely to experience if this is not done?
Many thanks
Hi... yzf-r1
You are righ, Incorrect pantoscopic tilt can cause a cilinder, and then cause blur and so on.
The optical axis is suposed to pass through the eye's Center of Rotation.
I am sure Darryl are going to explain it better and give you all the math involved.
See you
Edgley
yzf-r1
If you want to really delve into this look up Martins formula for lens tilt. :) Changing the tilt of a lens induces oblique astigmatism (sometimes called marginal astigmatism depending who is doing the talking ;) ) Panto tilt changes the effective sphere power of the lens and induces cylinder power on the 180 merridian. For plus spheres it plus cyl and in a minus it's minus cyl.
Face form will also change the power of the lens but it induces cyl. in the 90 degree merridian...
the formula is S'={1+(sina)2/2n}
C'= S'(tana)2
Keys are S'= new spherical power
S= original sphere power
a= the degree of tilt
n= index of refrection of the lens material
C'= the induced cylinder on the axis of rotation
Hyperops are more of a problem than a myop when working in tilts and the effect on power, What it amounts to once you use this formula you can check to see the resultant power and how the amount of tilt in relation to the optical axes of rotation will neutrilize this unwanted power... (the one you mention oc 1 mm for every 2 degree of tilt
If you DO not lower and tilt than you induce power (get the answer with the formula on the amount) ....
Here is an example say we had a lens that is a +10, cr39 with 15 degree's of tilt .... the effective power if the oc was in front of the pupil and not lowered than the power at the oc would be...
S"= S{1+(sin a)2} = (+10.00)(1+{sin15}2/2(1.498})
=(10)(1+0.06699/2.996)=(10.22)(0.0718)=+10.22^
C'=S'(tan a)= (+10.22)(tan15)2= (+10.22)(0.0718)= +.73x180
The power if the oc was placed in front of the pupil and the frame did have 15 degree of panto tilt when they looked through the lens the effective power would be +10.22+.73x180 !!YIKES
That is why you lower the oc as you induce tilt ;)
Maybe not as smart as Darryl but us lab rats do know a thing or two if Darryl wants to check my math more "power" to him.. get it? ..ok it was a corny attempt at optical humor...
Jeff "grind if ya got em" Trail
You have done well, young Jedi. Soon, you too, will knows the ways of the Force... ;)Jeff Trail said: Maybe not as smart as Darryl but us lab rats do know a thing or two if Darryl wants to check my math more "power" to him.
...Though, while I understood exactly what you meant, use should probably use a little carrot ^ or something before your exponents in those formulas, since it just looks like you're mutliplying by 2 otherwise. You might also put parentheses around the 2n, since they are both part of the denominator, not just the 2. Something like,
S'={1+(sin a)^2/(2n)}
Best regards,
Darryl
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