Brandon Dube Offline Upload & Sell: Off
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HaruhikoT wrote:
Hi folks,
While Kolari mod is great solution for wide-angle rangefinder lens users,
personally I still hesitate to modify my A7 because such unofficial mod voids warranty.
Today I would like to introduce another solution to fix corner smearing issue.
I believe Astigmatism is the principal cause of corner smearing.
I tried some optical simulations and found that a plano-convex lens placed in front of the WA RF lens can control astigmatism.
I use OSLO free edition:
http://www.lambdares.com/oslo-university-program
For simplicity, simulation use perfect lens instead of complicated actual lens design.
I think symmetrical lens design such as Biogon matches this simplified simulation, but retrofocus design does not.
Here are the results.
Astigmatism at Extreme Corner:
21mm lens with 1.8mm thick cover glass (Original A7): 0.6mm.
21mm lens with 0.8mm thick cover glass (Kolari/Leica): 0.25mm.
1.5m plano-convex in front of 21mm with 1.8mm cover glass: 0.14mm.
28mm lens with 1.8mm thick cover glass (Original A7): 0.4mm.
28mm lens with 0.8mm thick cover glass (Kolari/Leica): 0.18mm.
2m plano-convex in front of 28mm with 1.8mm cover glass: 0.07mm.
So theoretically, plano-convex method outperforms Kolari/Leica in terms of astigmatism!
...Show more →
Hi,
Some may recognize my name from my work with Olaf Optical Testing / LensRentals. My primary engagement in optics is as an aberration theory researcher / student. My perspective may be of some interest here.
The coverglass generates aberrations because not all rays in a given cone, fan, bundle, or "pencil" of light that forms a point in the image hit it at the same angle if the beam is not from infinity. Snell's law says that a ray that is incident at an angle different to another ray is going to go somewhere different to the other ray. Thus, you get aberrations.
There are two types of aberrations some lens designers like to talk about. There are intrinsic aberrations, the ones that things like Seidel Sums and wavefront expansions calculate. There are also induced aberrations, or aberrations generated by aberration somewhere else in the system.
A perfect thin lens model captures the intrinsic aberrations of the coverglass but misses the induced aberrations due to the aberrations from the master or taking lens. For a well corrected master lens, you can assume those aberrations are pretty minimal.
Regarding symmetric vs retrofocus, there is nothing so different between the two as far as aberrations go. The big difference is that for a symmetric lens, the chief ray angle is similar to the object angle. The chief ray angle is what the exit pupil distance information on this subject you can find on the LensRentals blog, as done by Brian Caldwell, really is a function of. For a retrofocus lens, the chief ray angle is less than the object angle.
You can toggle your simulation between retrofocus and symmetric, with a perfect master lens, by separating the aperture stop from the thin lens and putting it a bit in front of it. If it is at the front focal point of the thin lens, the system is telecentric and you have the limiting case of extremity for a retrofocus lens, and something similar to the behavior of most fisheyes. In the stop at the lens case, you are close to the symmetric behavior though it is not exactly so.
The aberration I would worry about in these simulations is field curvature, or astigmatism. Unfortunately you can't generate pure astigmatism in the number of surfaces afforded to you by the free version of OSLO. Field curvature you can do by placing a meniscus lens with equal radii right on top of the cover glass. It will contribute nothing to the focal length and image location, but adjust the curvature of the field. There may be some small astigmatism generated as wel. This should give you 5 of the 6 surfaces (or maybe all 6 if the thin lens is 2 surfaces) you are allowed for free.
If you would like, I can perform some simulation in my license of Code V, though the work must remain very strictly noncommercial. I also have an archive of every design in Modern Lens Design, as I have served as a sub-editor of its 3rd edition. There are maybe 15 biogons in there and 15 retrofocus lenses.
Regardless, I am happy to provide guidance.
All the best,
Brandon
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