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Front-End Filter Improves Corner Smearing
  
 
HaruhikoT
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p.2 #1 · p.2 #1 · Front-End Filter Improves Corner Smearing


Nanh wrote:
HaruhikoT, How much difference does it make when you use the f = 1.5m filter and the f = 2m filter? e.g. if you used 1.5m on the G28 and the 2m on G21? How about in theory(for both cover glass thickness)? The filter is still quite expensive and I'd rather not buy more than a few.

Looking at the examples on flickr there is a quite big difference in sharpness in the center. It's as if the filter acts as a strong anti-aliasing filter. But nothing a little of unsharp mask can't heal, so I am still very enthusiastic about
...Show more

1.5m vs 2m, I've not tested yet that combination. Simulation shows visible astigmatism difference between 1.5m and 2m for both 1.8mm and 0.8mm coverglass. Now I'm going to post the Simulation How-To, so if you want to know details of theoretical (simulation) results, you can run it by yourself with OSLO free edition.

Just watched my samples again and noticed center sharpness difference you mentioned.
There seems to be some field curvature and also I've slightly missed focus. But I'm not sure the exact reason.



Oct 05, 2016 at 11:48 AM
HaruhikoT
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p.2 #2 · p.2 #2 · Front-End Filter Improves Corner Smearing


Thanks to many informative comments from you, I've just made some changes on my simulation settings to improve its accuracy.
Results (e.g. astigmatism) are slightly changed from my initial post.

In addition, new settings can handle not only Symmetrical but also Retrofocus design!

Here are my current simulation settings.
I also wrote Quick Guide to use OSLO free edition, so now you can simulate by yourself.
I hope this would help you to know optimal PC filter for your own various lenses.

Please note I'm just a hobbyist not a pro of optics, so if you find any mistakes or better settings, please feedback.

----------- save txt below -----------
// OSLO 6.6 0 0 0
LEN NEW "PC1.5m+G21+1.85mm" 21.5 8
EBR 3.839
ANG 45.0
DES "OSLO"
UNI 1.0
// SRF 0
AIR
TH 1.0e+20
AP 1.0e+20
NXT // SRF 1
GLA BK7
RD 778.5
TH 3.0
AP 25.0
NXT // SRF 2
AIR
TH 16.4
AP 25.0
NXT // SRF 3
AIR
TH 1.3
AST
NXT // SRF 4
AIR
LMO EGR
DRW ON
NXT // SRF 5
AIR
LME
TH 18.5
PFL 21.5
NOT "PERFECT"
NXT // SRF 6
GLA BK7
TH 1.85
AP 22.0
NXT // SRF 7
AIR
PY 0.0
AP 22.0
NXT // SRF 8
GLA BK7
AP 22.0
END 8
DLNR 0 5
DLNR 1 5
DLMN 1 -1.0
DLMX 1 1.0
DLNR 2 5
----------- save txt above -----------

Quick Guides:

1. Save txt above as *.len.
2. Install OSLO EDU and start GUI.
3. Select "open an existing.." and choose the *.len file
4. Choose GW (Graphics Window) and click the 4th-9th icons from the left.
You can see various analysis including astigmatism.
To adjust scale, right-click in the window and choose Re-calculate.

5. You can change P-C filter's parameter by modifying *.len file. See below:

NXT // SRF 1
GLA BK7 <- Filter's Material
RD 778.5 <- Filter's Curvature radius
TH 3.0 <- Filter's Edge thickness
AP 25.0 <- 1/2 of Filter's Diameter

6. You can also change Cover glass thickness. For Leica/Kolari, use 0.8.

NXT // SRF 6
GLA BK7
TH 1.85 <- Cover glass thickness

7. You can also change Primary lens parameter. See below:

From Zeiss G21 technical sheet I got:
Angular field= diagonal 2w 90
Focal length = 21.5mm
Exit pupil Position = 10.7 mm in front of the last lens vertex
Back focal distance = 12.1 mm
Entrance pupil Position = 16.4 mm behind the first lens vertex

I set ANG as 45.0 (1/2 of angular field)
I set 21.5 to "PFL" in SRF 5.
I set 16.4 to "TH" in SRF 2.

Aperture stop is placed 1.3mm (10.7+12.1-21.5) in front of perfect lens.
So I set 1.3 to "TH" in SRF 3.
This helps the case the focal length is not equal to exit pupil distance.

I set EBR(entrance beam radius) as 3.839 which comes from 21.5mm(fl) / 2.8(fno) /2.

Haruhiko



Oct 05, 2016 at 12:00 PM
hiepphotog
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p.2 #3 · p.2 #3 · Front-End Filter Improves Corner Smearing


HaruhikoT wrote:
Thanks to many informative comments from you, I've just made some changes on my simulation settings to improve its accuracy.
Results (e.g. astigmatism) are slightly changed from my initial post.

In addition, new settings can handle not only Symmetrical but also Retrofocus design!

Here are my current simulation settings.
I also wrote Quick Guide to use OSLO free edition, so now you can simulate by yourself.
I hope this would help you to know optimal PC filter for your own various lenses.

Please note I'm just a hobbyist not a pro of optics, so if you find any mistakes or better settings, please feedback.

----------- save txt
...Show more

Thank you Haruhiko. I got the the OSLO a couple of days ago but haven't figured out how to use it yet. This text base config really helps.



Oct 05, 2016 at 04:55 PM
nehemiahphoto
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p.2 #4 · p.2 #4 · Front-End Filter Improves Corner Smearing


hiepphotog wrote:
I think the G 21 would rival even that one and it is fine on the A7RII and the A7s. In this regard, Sony cam is better than Leica (something that Leica shooters can't stand....)

---------------------------------------------

The OP said his filter is AR coated. Putting another element in front of a lens would certainly reduce contrast. Without a direct comparison, I don't think most would recognize the mild loss in contrast.


Does the filter have to be AR coated to work for us, or just is it AR coated?



Oct 05, 2016 at 05:27 PM
hiepphotog
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p.2 #5 · p.2 #5 · Front-End Filter Improves Corner Smearing


nehemiahphoto wrote:
Does the filter have to be AR coated to work for us, or just is it AR coated?


Without AR coating, you might run into ghosting like the A7.



Oct 05, 2016 at 05:31 PM
nehemiahphoto
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p.2 #6 · p.2 #6 · Front-End Filter Improves Corner Smearing


hiepphotog wrote:
Without AR coating, you might run into ghosting like the A7.


I thought the sensor took care of that. The a7rii is much better than the a7 was, so I wonder if one got an AR-less filter how we would do on an a7rii versus a7




Oct 05, 2016 at 06:11 PM
hiepphotog
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p.2 #7 · p.2 #7 · Front-End Filter Improves Corner Smearing


nehemiahphoto wrote:
I thought the sensor took care of that. The a7rii is much better than the a7 was, so I wonder if one got an AR-less filter how we would do on an a7rii versus a7



I believe each glass element has to have at least AR to prevent any internal reflection. As for the A7RII, it's pretty much the top Sony body right now to use with these RF lenses: better vignetting and color cast handling (almost non-existent) than any other A7 body.



Oct 05, 2016 at 06:31 PM
nehemiahphoto
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p.2 #8 · p.2 #8 · Front-End Filter Improves Corner Smearing


hiepphotog wrote:
I believe each glass element has to have at least AR to prevent any internal reflection. As for the A7RII, it's pretty much the top Sony body right now to use with these RF lenses: better vignetting and color cast handling (almost non-existent) than any other A7 body.


Yes, the lenses bare that out--I purchased an A7r on release (or shortly after) and some older alt glass--it was bad, then I went to a7 and that was gentler (but still pretty bad). I read the a7rII has less deep photosite wells because of the new 42MP BSI sensor puts the wells closer to the surface, obscuring the rays less resulting in better astigmatism/color cast in the corners...not sure how accurate that is.



Oct 05, 2016 at 06:38 PM
hiepphotog
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p.2 #9 · p.2 #9 · Front-End Filter Improves Corner Smearing


nehemiahphoto wrote:
Yes, the lenses bare that out--I purchased an A7r on release (or shortly after) and some older alt glass--it was bad, then I went to a7 and that was gentler (but still pretty bad). I read the a7rII has less deep photosite wells because of the new 42MP BSI sensor puts the wells closer to the surface, obscuring the rays less resulting in better astigmatism/color cast in the corners...not sure how accurate that is.


Definitely not astigmatism, the A7RII is pretty much similar to my A7S, which is similar to an A7R I tested. From my extensive test, the A7RII is superior to all in term of color cast.



Oct 05, 2016 at 06:44 PM
arduluth
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p.2 #10 · p.2 #10 · Front-End Filter Improves Corner Smearing


An additional, uncoated element will be more susceptible to flare... but this isn't the fault of the sensor. Any issues with the sensor issues would be above and beyond the flare you'd be introducing to the lens with uncoated elements.

The lens that the OP is using has some kind of AR coating on it. That's the way it comes. Unless you're into more veiling flare and more ghosting, you want all lens surfaces to be coated, whether you're using digital or film.



Oct 05, 2016 at 07:06 PM
 

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rscheffler
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p.2 #11 · p.2 #11 · Front-End Filter Improves Corner Smearing


One problem with an uncoated 'add-on' lens is if one surface is flat, which is the case for these lenses used by the OP, it could potentially reflect light back in through the lens. For example, a bright light source in the image will reflect off the sensor's surface, even if the sensor stack is AR coated. This reflected light could then reflect off the uncoated add-on lens in a way that redirects back at the sensor, causing flare.

This was a problem with pre-digital Canon super-teles... they used a flat protective front element 'filter' and even though AR coated, resulted in degraded contrast images in some backlit situations. The first generation of Canon's image stabilized super-teles incorporated a meniscus shaped front protective element that instead reflected away light reflected off the sensor instead of back at it.



Oct 05, 2016 at 10:43 PM
realVivek
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p.2 #12 · p.2 #12 · Front-End Filter Improves Corner Smearing


Haruhiko, Have you tried reversing the plano-convex lens to check for any differences in performance?

TIA!



Oct 06, 2016 at 09:55 AM
HaruhikoT
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p.2 #13 · p.2 #13 · Front-End Filter Improves Corner Smearing


Hi realVivek,

Yes, reversing filter makes correction a bit stronger.
In general, the shorter focal length P-C lens has the stronger correction effect.
Simulation results show approximately, Reversed 2.0m lens has similar effect to the Forward (curved surface faces to subject) 1.8m lens.
Please note "stronger" does not mean better results. Over-correction also harms corner sharpness.



Oct 06, 2016 at 12:46 PM
HaruhikoT
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p.2 #14 · p.2 #14 · Front-End Filter Improves Corner Smearing


Update: I found 2.55mm should be fair value as A7's cover glass thickness in the simulation.

Lensrentals says a7R's Optical Thickness of the sensor stack is 2.55mm:
http://www.lensrentals.com/blog/2014/06/sensor-stack-thickness-when-does-it-matter/
>The sensor optical measurements are made by Brian Caldwell and measure the optical equivalent as if the sensor stack was made of glass with a 1.52 refractile index.

Previously I set the value 1.8mm or 1.85mm, however some simulation results assuming exit pupil distance didn't match my field-test experience. So I changed that value to 2.55mm, eventually simulation results (e.g. spot diagrams) look so close to test samples.
I believe 2.55mm suits for all A7 series otherwise genuine FE lenses optimized for the stack must not work.

And this is my latest simulation results:
(Edited Oct.8: Sorry, I deleted results in this post because the LUX35 is designed for digital, so it must take some filter stack thickness into account. My simulation here was not counted the condition so it was misleading)

Maybe ZM 1.4/35's optimal P-C is also 4m/8m, if its exit pupil distance is same as the LUX35.

Haruhiko


Edited on Oct 08, 2016 at 11:11 AM · View previous versions



Oct 07, 2016 at 04:24 PM
retrofocus
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p.2 #15 · p.2 #15 · Front-End Filter Improves Corner Smearing


One of the best approaches I have seen so far to resolve the issue with RF wide angle lenses on A7 camera based sensors! Seems as efficient (maybe even better?) than the Kolari modification changing the sensor glass cover itself. Thanks for sharing!


Oct 07, 2016 at 04:37 PM
hiepphotog
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p.2 #16 · p.2 #16 · Front-End Filter Improves Corner Smearing


HaruhikoT wrote:

Update: I found 2.55mm should be fair value as A7's cover glass thickness in the simulation.

Lensrentals says a7R's Optical Thickness of the sensor stack is 2.55mm:
http://www.lensrentals.com/blog/2014/06/sensor-stack-thickness-when-does-it-matter/
>The sensor optical measurements are made by Brian Caldwell and measure the optical equivalent as if the sensor stack was made of glass with a 1.52 refractile index.

Previously I set the value 1.8mm or 1.85mm, however some simulation results assuming exit pupil distance didn't match my field-test experience. So I changed that value to 2.55mm, eventually simulation results (e.g. spot diagrams) look so close to test samples.

And this is my latest simulation results:
When you use
...Show more

Another excellent update Haruhiko! If there is enough demand, a custom made batch of 8m lens is not out of the question.

For the filter case, that is a huge drop in the center performance with the PCX filter (plano-convex instead of plano-concave). How does the 8m filter work on the stock camera? I'm surprised to see that the Lux 35 seems to perform less well using the PCX than the Kolari mod. Your real-life G 21 test shows that the PCX mod works wonderfully with one of the toughest lenses out there and the result is even better than the Kolari.

As most people here are interested in teh ZM 35/1.4, can you rerun the simulation with the ZM 35/1.4 instead? I think we can round up enough people to try this on the field.



Oct 07, 2016 at 04:46 PM
hiepphotog
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p.2 #17 · p.2 #17 · Front-End Filter Improves Corner Smearing


retrofocus wrote:
One of the best approaches I have seen so far to resolve the issue with RF wide angle lenses on A7 camera based sensors! Seems as efficient (maybe even better?) than the Kolari modification changing the sensor glass cover itself. Thanks for sharing!


Definitely more practical due to cost and the ease of self-service. As a long time Kolari user, getting the right colors is still a pain for me even with X-rite and custom WB. So far this way would preserve the original IR response (or at least it seems so).



Oct 07, 2016 at 04:49 PM
naturephoto1
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p.2 #18 · p.2 #18 · Front-End Filter Improves Corner Smearing


hiepphotog wrote:
Another excellent update Haruhiko! If there is enough demand, a custom made batch of 8m lens is not out of the question.

For the filter case, that is a huge drop in the center performance with the PCX filter (plano-convex instead of plano-concave). How does the 8m filter work on the stock camera? I'm surprised to see that the Lux 35 seems to perform less well using the PCX than the Kolari mod. Your real-life G 21 test shows that the PCX mod works wonderfully with one of the toughest lenses out there and the result is even better than the
...Show more

Some of us would have the campability to compare the 35 1.4 Distagon ZM on a stock A7r and a Kolari modded A7r M. Mike Broomfield as an example has both a stock camera and an A7rM V2 and I have a stock camera and an A7rM V3.

Rich


Edited on Oct 07, 2016 at 05:10 PM · View previous versions



Oct 07, 2016 at 04:55 PM
GMPhotography
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p.2 #19 · p.2 #19 · Front-End Filter Improves Corner Smearing


I'm interested in a the ZM 35 1.4 filter for it. I have a stock A7r II. Just point me to the correct filter solution and I'll buy it.


Oct 07, 2016 at 05:06 PM
hiepphotog
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p.2 #20 · p.2 #20 · Front-End Filter Improves Corner Smearing


naturephoto1 wrote:
Some of us would have the campability to compare the 35 1.4 Distagon ZM on a stock A7r.and a Kolari modded A7r M. Mike Broomfield as an example has both a stock camera and an A7rM V2 and I have a stock cameras and an A7rM V3.

Rich


I have no doubt Rich . I'm hopeful with this mod.



Oct 07, 2016 at 05:08 PM
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