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Science Forum Index » Medicine - Vision Forum » My novice simulation of lens design parameters..
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| eekthorp |
 Posted: Mon Jan 08, 2007 5:15 am |
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Guest
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Hi again guys,
My last group post a few days ago got the first paragraph cut off,
sorry about that. Since then I've read some more and played with
some interesting tools some of you have written, especially Darryl's
Optical Analysis tools (thanks Darryl and the rest of you for all your
interesting posts in the groups!). I'm curious what you guys think
about my (novice but hopefully correct) conclusions/generalizations,
and sorry for the long post, again..?
Assuming I'm driving the tools right, for my sph perscription at
30degrees off-axis most lens designs will have about 0.25D (RMS error)
of oblique-astig distortion, and this is just a features of having a
-8D prescription! So, it seems that the Oswalt curve gives best
off-axis performance, but this isn't necessarily error-free off-axis
performance. c'est la vie, I guess? I also tried the wollaston
best-form, but the 'optical analysis' tool gave me a worse rms
error, shouldn't wollaston give better off-axis performance?
Hopefully, I'm using the tool right (?!)
While playing with the tools, optical quality seems to be basically the
same between the basic design parameters (<0.1D variation over
materials at 30degrees), so based on this conclusion I might as well go
with thin, and stick with a good manufacturer..
I imagine Hoya has a top-notch process(?). So going with Hoya nulux
(AS) 1.7 w/ a 36 ABBE will get thinner and marginally better CAE than
poly. Sola Finelite 1.6 offers 42 ABBE, but I'm not sure this small
ABBE change would be perceptible.. ? So if I go with the 1.7 Hoya, I
guess the next question is if I should stick with the highly
recommended Cristal or go with the hoya super-hard AR coat which only
appears to be avail for a few lenses..
.........
And here's my attempt at answering my own questions I asked last
week..
* Which lens cut (spheric/aspheric/atoric), if any, improve
lens-oblique-astigmatism over others?
Spherics.. at least for people w/o astig
http://www.opticampus.com/cecourse.php?url=lens_design/
"[aspherics] Provide both the visual advantages of best form lenses
and the cosmetic advantages of flat lenses" and "Do not provide
better optical performance than best form lenses, but simply provide
comparable performance without the restrictions imposed by best form
base curve selection".
I don't see any advantage in a flatter front curve for high myopes,
so spheric should be 'better'. I'm still not sure why aspherics
are offered at high myope powers.. I don't see any benefit, unless
they are rolling off the powers at the edge to further improve
cosmetics at expense of optics past 30degrees, which might be a good
reason. (?)
I assume the local optical optitians, et al, were recommending
aspherical to improve my vision because they've heard "aspherics
improve vision over spherics" but not the other half which says
"when a flater (not-best-form) curve is used with a spherical
design."
I'll have to dig more to say anything about what happens when you have
an astig though. Is spheric-best form still king? I think one of
Darryl's articles stated that atorics are best, but is that comparing
atoric to spherical best-form-curves or spherical w/ flatter than best
form curves?
* Which types of lens material (high/low index, high/low ABBE) improve
lens-oblique-astigmatism?
According to Darryl's Optical analysis tool, index and abbe do not
impact OAE (again assuming I'm driving the tool right). It appears
that high-index degrades off-axis error but it shouldn't be
perceptible.
I wonder if high-index has a noticeable disadvantage, practically, due
to physical processing/surface defects having larger impact on lens
quality? On the other hand, I looked through a poly and hoya 1.67 blank
off-axis and they were very similar. There's a small chance the
hoya's marginally better. Not sure what to make of this. Maybe it
reinforces that error is constant, that manufacturing technique matters
more than the basic design parameters, or maybe it means looking
through 60mm blanks at lower -6D 'stock perscription' powers isn't a
very controlled experiment. :)
* In addition to ABBE, is high/low index or spheric/aspheric/atoric cut
better for chromatic aberration?
CAE is only dependent on ABBE and not cut or index, for a given
prescription.
According to Darryl's tools (assuming I'm driving it right, heh).
So, optimizing ABBE may make some sense, since it's the one quality
parameter you can have some control over for an ophthalmic lens.
* Since error should only be about 0.25D, I wonder if my current
off-axis distortion is due to a lens issue, or maybe the distortion is
exaggerated since the current pair of lenses are too weak, since it
sure seems like I'm getting more than 0.25D in my -8D eye..
Thanks,
eekthorp |
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