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Thank you for asking many questions in the previous thread.
Here are my thoughts.
What is the primary lens attribute that determines the optimal Plano-convex filter choice?
As hiepphotog wrote, I also think focal length and exit pupil distance of the primary lens should define optimal focal length of the P-C filter.
At perfect lens, focal length = exit pupil distance.
My simulation results will be accurate in this condition.
At actual lenses, focal length may not equal to exit pupil distance.
Currently I have no idea how to simulate this case.
So maybe quicker way is just purchasing various focal length filters and trying them.
How do you know which glass type to choose for different lenses and cover glass thickness? By trying?
Simply by changing radius of the filter in the simulator. I also tried concave and meniscus lens but plano-convex seems to be the best.
Is it possible to figure out what's the approximate best glass type to use for ZM 35mm 1.4 even though it's not a perfect symmetric design?
In case of 35mm perfect lens and 1.8mm thick cover glass, 3m filter will be good.
I don't know how long ZM35's exit pupil distance is, but if it is larger than 35mm, longer focal length (perhaps 4m or 5m) would be better.
Would the result be different if the cover glass is placed at a constant distance to the sensor,
and the point where light first touches the cover glass is shifted back by the reduced amount?
About the cover glass, only thickness seems to be important.
Simulation shows the distance between cover glass and the sensor doesn't affect any aberration.
Are there any other effects generated by the filter glass? Distortion? CA? Focal length changes?
Yes. Most visible effect is Distortion. 21mm simulation shows +2% Distortion at the extreme corner.
You may also notice distortion changes by comparing my flickr sample photos.
CA and Focal length change are also available but seems to be quite small.
Would the 50mm filter glass fit in arbitrary 52mm filter rings? Any special techniques to fix the glass?
Does the 55 to 52mm step down ring + 52mm filter setup induce any mechanical vignetting on the Contax G 21?
Yes there is small gap between 52mm thread and the filter.
I've just put the filter carefully to the center, but this may cause decentering.
I think I can wrap adhesive tape or something around the filter to fit 52mm thread, but I haven't tried yet.
I know the filter's diameter 50mm may be bit too small for the G 21.
Some light fall off may occur, but in my eyes, vignetting looks similar to the one without the filter.
1) If you're using the G21 with a techart adapter, will AF (the lens fluctuating in distance from the senor) hurt IQ?
2) I can't tell tell how good the IQ on the Biogon 21...with the P-C (plano convex) mod, is it excellent?
3) Can we expect better performance with the a7rII than the a7 or a7r?
My techart works just fine after modification. I don't think it hurts IQ.
Actually my test samples were shot by MF adapter. Techart is not suitable for serious focusing.
I posted that setup just because I like the styling. MF adapter looks tacky IMO.
Even with a optimal P-C lens, still I can't expect the Biogon 21's overall IQ outperform latest FE lenses.
There are heavy vignetting and color cast. RAW correction can fix them but also suffer dynamic range.
Also the P-C lens degrades contrast as hiepphotog pointed out.
I expect this P-C filter method can be applied to all a7 series and gives similar improvement of astigmatism.
I've heard that a7rII's BSI sensor improves the vignetting and color cast. So overall IQ will be better at a7rII.
In practice, how big of a difference between 0.11mm vs. 0.14mm? I don't imagine that much.
Here are spot diagrams. The left one has 0.14mm astigmatism, and the right has 0.11mm.
A7 has 24MP = 166 px/mm = 8 lines / 0.1mm resolving power.
Especially at 35deg, I expect Kolari + P-C 3m will have visible improvement over Original A7 + P-C 1.5m.
(Updated Oct.7: Screenshot is deleted because old simulation is not accurate. See my latest post.)
Edited on Oct 07, 2016 at 03:48 PM · View previous versions