Optics question for Darryl, Harry, or anyone else that can answer

[Wes]

Ok. My question concerns high cylinders in best form lenses. Tschernings ellipse covers sphere correction, and Vogels formula gives an approximate result to the ellipse. We use these to make the best lenses we can. USing spherical equivalent we can usually keep the ocular surface between 4 and 7 diopters. What say you regarding high cylinders? DO you use spherical equivalent? ex: pl - 12 do you call that sph equivalent of -6 and put it on a 3 base? or +10 - 10 does that go on an 11 base? are there better or more practical solutions than this? I haven't come acoss many references regarding high cyl. Atorics would be best im sure but we don't have that capability yet. If you know of references I can purchase please suggest them. Wes

[Darryl Meister]

The spherical equivalent, while a convenient choice for base curve selection, will not necessarily provide the best overall optical performance. The optical performance of the meridian of maximum (strongest) power is usually more sensitive to base curve selection, particularly in plus powers. The meridian of minimum power is less sensitive.

For instance, for a +3.00 DS -2.50 DC lens, the +3.00 D power meridian will be more sensitive to base curve selection than the +3.00 + (-2.50) = +0.50 D power meridian. Just about any reasonable base curve will provide satisfactory results for the +0.50 D power meridian.

Consequently, the ideal base curve associated with the power meridian of the spherical equivalent does not necessarily represent the base curve that will provide the best overall compromise in optical performance, and the ideal base curve associated with a power somewhere between the spherical equivalent and the power of the maximum meridian may provide a slightly better optical compromise.

Of course, if you're putting this much thought into this, you can also consider the shape of the frame and the orientation of the cylinder axis. Consider a +3.00 DS -2.00 DC x 180 lens in an oval frame; the ideal base curve associated with the vertical meridian will contribute very little to optical performance.

[YrahG]

I like Darryl's answer, the highest power meridian would definately benefit greatest from the best possible curvature of course the few random powers with extreme cyls would be the exception to the rule. I agree that the shape would be a good factor in determination of curvature. Like mentioned if the lens only allows 15 degrees of ocular rotation along the vertical meridian then why use a base on that meridian that can give better performance out to 30 of ocular rotation. In a case such as that the lens could be optimized more for the other meridian with the vertical being optimized to 15 as a comprimise. I believe that FF technology along with talent in the IT field will eventually lead to designs on the fly based on every possible variables that could possibly be presented.

Another quick point, manufacturer suggested BC's may be the optimal curves for distance or infinity but for lenses used at closer distances the base curves are going to need tweaking. Another scenario where the base curves could need tweaking is in scenarios where the vertex is modified from the average measure, in this case the BC needs modification as well.

Sorry another quick note, optimal BC's are even more important on hi index/poly lenses with low abbes, the chromatic aberration in these lenses combined with any monochromatic aberrations could mean more sensitivity to chromatic aberrations resulting in loss of visual acuity and ultimately an unsatisfied client.

[Darryl Meister]

manufacturer suggested BC's may be the optimal curves for distance or infinity but for lenses used at closer distances the base curves are going to need tweaking... where the vertex is modified from the average measure, in this case the BC needs modification as well.

Very true. For those interested in this topic, I would encourage you to download the following program, if you haven't already, and experiment with Tscherning's ellipse and optical ray tracing: Spectacle Optics program.

[Wes]

You do indeed live up to your surname, Sir Meister. I downloaded your spectacle optics program the day it came out. Your contributions to the industry are phenomenal.
Wes

[Darryl Meister]

As always, I appreciate the positive feedback. :cheers:

[YrahG]

As a long time lurked I must say Darryl your contributions are phenomenal.

[Darryl Meister]

Stop, you guys are making me blush. :o

[Wes]

I was a lurker for a long time before I made a profile as well. Started posting, got my feathers singed a couple times by stodgy old codgers who are usually right, but jerks about it. Don't let em beat you down. I can tell already that you're a good poster.
Wes