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Front-End Filter Improves Corner Smearing
  
 
aiken11
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p.93 #1 · p.93 #1 · Front-End Filter Improves Corner Smearing


BastianK wrote:
There you go: https://dl.dropboxusercontent.com/u/61479850/ILCE-7S-Zeiss%20ZM%2035mm%201.4%20%2B5m%20Opto.lcp
(Rightclick -> Save as)



Anyone has the updated link?
Thanks



Nov 12, 2017 at 07:51 PM
Fred Miranda
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p.93 #2 · p.93 #2 · Front-End Filter Improves Corner Smearing


hanay78 wrote:
Hello Samuli,

thanks for your answer!

I am not a pixel peeper. As I said I was deceived because the experiments i did with the filter were not successful. But probably this is due to the fact I was using the wrong lens.

Maybe the copy I had from ZM 25 was not the best. I have send it back, due to a non smooth focusing. So, I was not lucky at all.

That said, about the better issue:

I never have had the batis in my hand. I do not know how good it is, or how it renders. Both batis and zm
...Show more

Answers:

a) Just place the lens on a flat surface and you will easily see which side is the flat and convex.
b) You need to find a UV filter that lets a retention ring get screwed all the way down. I know the Tiffens are made this way.
c) Both Elpro 3 and 4 have the same retention ring (55mm). What changes between the 3 and 4 is the diopter strength.



Nov 14, 2017 at 05:46 PM
HaruhikoT
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p.93 #3 · p.93 #3 · Front-End Filter Improves Corner Smearing


Sorry for my late reply, I agree with you, at least in theory, an asphrical PCX having almost flat surface in the center and have some curvature towards edge may give better correction including midzone and also give less infinity problem.

I will try some simulation and post results when I have time.

In practical, the big problem is how can we get such specially designed lens at reasonable cost.

Thanks,
Haruhiko

thrice wrote:
This question is probably best directed to @HaruhikoT@
Would having an aspherical PCX mitigate the mid-field dip? Obviously we want the peak correction in the corners rather than the centre/mid. So a more flattened centre/mid would in theory be superior, if my pseudo-scientific reasoning is correct.

With the correct radii an aspherical PCX might not even affect the infinity focus of the lens, since we just want to pull the edges forward instead of the whole image.




Nov 17, 2017 at 10:51 AM
artur5
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p.93 #4 · p.93 #4 · Front-End Filter Improves Corner Smearing


Yes, that seems the big problem in this case : “reasonable” cost. Regular PCX aren’t that cheap already. Imagine a non spherical design custom made in small quantities. Many $$$ per unit, I guess. Arrived at this point, probably an A7x with ultrathin sensor mod (Kolari or others ) will be a cheaper solution for RF wides, at the expense of somehow degraded performance with native FE glass.
Splitting hairs, your simulations showed that a very weak PCX filter added to a modded A7x could be the ultimate solution for those stubborn Biogons that won’t sharpen in the extreme corners.



Nov 17, 2017 at 05:01 PM
thrice
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p.93 #5 · p.93 #5 · Front-End Filter Improves Corner Smearing


HaruhikoT wrote:
Sorry for my late reply, I agree with you, at least in theory, an asphrical PCX having almost flat surface in the center and have some curvature towards edge may give better correction including midzone and also give less infinity problem.

I will try some simulation and post results when I have time.

In practical, the big problem is how can we get such specially designed lens at reasonable cost.

Thanks,
Haruhiko



Thanks @HaruhikoT! I look forward to your testing results

I may be able to use my manufacturing contacts who manufactured my multicoated 1mm thick 5m PCX filters for aspherics as well. The price will be more but for the performance benefit perhaps it will be ok.



Nov 18, 2017 at 05:45 AM
ken.vs.ryu
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p.93 #6 · p.93 #6 · Front-End Filter Improves Corner Smearing


how is the sony 28 fe vs the contax g 28 with filter?


Nov 18, 2017 at 08:40 PM
ivancav
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p.93 #7 · p.93 #7 · Front-End Filter Improves Corner Smearing


Hi all,
I own a Sony A7II (without Kolari's modification), and I tried the front-filter solution to improve the images taken with a Zeiss Biogon ZM T* 21/2.8.

I just write to send some information about the combination.

I tried with the PCX 1.5m, thinking there will be not too much difference between the ZM Biogon 21/2.8 and the Contax Biogon 21/2.8 (both Zeiss lenses).
Here I post the results from some test-shots I took with the Zeiss ZM Biogon 21/2.8, with or without the PCX filter.

Let before say that after the filter was installed, I needed to remove some shims from inside the lens, to adjust infinity focus. An easy operation that everyone can do in ten or twenty minutes, with a cheap spanner wrench.

The PCX lens comes directly from OptoSigma. I also bought some things that I needed to mount the lens: a 46 to 55 filter adapter, and a 55 mm UV filter (and a 55 mm cap) allow me to take the lens in the right place.
And now the results (*): I see a strong improvement in sharpness, almost all over the frame: without the filter only at f/16 you will have sharp image all over the frame, and usable images only from f/8-f/5.6. With the PCX filter you will have sharp corners at f/8, but even at f/2.8 the image is almost completely sharp, everywhere except only the corners.
On the other side, using the PCX filter will add a slightly visible pincushion distortion.

(*) I took all the shots focusing at the center. But you can also consider that focusing at the edge (when applicable) can help increasing sharpness all over the frame. You can also consider that applying a field correction with photoshop or some post production software can again help correcting the image.

Thanks to all for all the important informations shared here,
Ivan



Nov 18, 2017 at 11:01 PM
Samuli Vahonen
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p.93 #8 · p.93 #8 · Front-End Filter Improves Corner Smearing


Hi Ivan,

Thanks for reporting here about ZM2.8/21, I added this lens to wiki as well.

Few questions:
1. Are the corners focused further distance from camera or closer to camera compared to center of image?
2. Which direction curved surface of PCX 1500mm is point at? (towards camera or away from camera)

If your answer is "1. further distance from camera (outward field curvature)" then you need shorter focal lenght PCX lens.

If your answer is "1. closer to camera (inward field curvature)" then you need longer focal length PCX lens.

The 1500mm PCX acts like 1500mm for the purpose we are discussing in this thread if the curved side is away from camera. If the orientation is opposite; curved side towards camera it will act like ~1200mm PCX giving stronger correction to field curvature.


Samuli



Nov 19, 2017 at 07:47 AM
ivancav
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p.93 #9 · p.93 #9 · Front-End Filter Improves Corner Smearing


Hi Samuli,
thank you for the advices, I tested the lens again to be sure of what I'm writing.

About your questions:
1- In my opinion, the corners are focused a bit further from the camera (I must say "in my opinion", because the image at the extreme corners isn't sharp enough to be so sure...).
2- The convex surface of the lens (SLB-50-1500PM) is pointed towards the camera. But when I made some tests to check which was the better orientation, I didn't notice a big difference...

Thank also for the wiki page, is very useful!

Bye,
Ivan


Samuli Vahonen wrote:
Hi Ivan,

Thanks for reporting here about ZM2.8/21, I added this lens to wiki as well.

Few questions:
1. Are the corners focused further distance from camera or closer to camera compared to center of image?
2. Which direction curved surface of PCX 1500mm is point at? (towards camera or away from camera)

If your answer is "1. further distance from camera (outward field curvature)" then you need shorter focal lenght PCX lens.

If your answer is "1. closer to camera (inward field curvature)" then you need longer focal length PCX lens.

The 1500mm PCX acts like 1500mm for the purpose we are discussing in this
...Show more




Nov 20, 2017 at 03:07 PM
 

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hanay78
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p.93 #10 · p.93 #10 · Front-End Filter Improves Corner Smearing


Hello,

can you post a photo of the setup, or describe how did you mount the lens? Thank you very much in advance



Nov 21, 2017 at 12:14 AM
ivancav
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p.93 #11 · p.93 #11 · Front-End Filter Improves Corner Smearing


Hi hanay78,
I mounted the lens in this way:
- Lens (Zeiss Biogon T* ZM 21/2.8)
- filter adapter from 46mm thread to 55 mm
- Optosigma SLB-50-1500PM (just put it on the filter adapter)
- 55 mm UV filter, just to hold and protect the sigma lens
(and finally a new 55mm lens cap).
This way works better if you put the lens with the curved face towards the camera; it can works even with the curved face looking forward, but you have to be careful while you screw the 55 mm filter (not too tight).
In that way you don't need anything else to hold the PCX lens in the right position.

I tried another way too, using a filter adapter to hold the optosigma lens (55 to 52 mm) and only after that an uv filter (52 mm): it didn't work well because the corners were a bit dark... (it is a wide lens...)

So my advice is to use the setup above.

Naturally, after have mounted the Optosigma PCX lens, you probably have to remove some shims (I removed one thick and one thin shim) to adjust infinity focus. To do so you need a spanner wrench, you can find the instructions in this forum...

Bye,
Ivan



Nov 21, 2017 at 07:55 AM
Slalom
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p.93 #12 · p.93 #12 · Front-End Filter Improves Corner Smearing


hi Ivan,
The Planoconvex lenses have different effects in the two orientations, you probably need to find a way to hold it with the curved face inward and centred, Then you may be able to reverse it to see if it is better:

G Biogon 21mm + 55mm Blank Filter + SLB-50-1500PM(reverse orientation,rubber tape adhered on the edge for centering) + ELPRO4's Retention Ring.

that is from:
https://vahonen.info/secure/wiki/index.php/Sony_A7_-_Thick_filter_stack_issues#Mitigation_option_2:_Filters

good luck



Nov 21, 2017 at 09:00 AM
ivancav
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p.93 #13 · p.93 #13 · Front-End Filter Improves Corner Smearing


Hi Slalom,
thank you. I know about the different effects of the orientation of the lens, I did some tests and I found it's better with the convex face towards the camera.
About centering, I put a little ring of paper around the PCX lens: with my 55mm UV filter is enough to make the lens fit perfectly and stay centered. But I think it depends on the effective thickness of the ring of the filter...
The rubber tape is surely a smarter solution!

Ivan

Slalom wrote:
hi Ivan,
The Planoconvex lenses have different effects in the two orientations, you probably need to find a way to hold it with the curved face inward and centred, Then you may be able to reverse it to see if it is better:

G Biogon 21mm + 55mm Blank Filter + SLB-50-1500PM(reverse orientation,rubber tape adhered on the edge for centering) + ELPRO4's Retention Ring.






Nov 21, 2017 at 11:00 AM
JT AU
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p.93 #14 · p.93 #14 · Front-End Filter Improves Corner Smearing


Dear thread contributors,

I have also tested the SLB-50-1500PM reversed on the Contax G21 and Zeiss ZM 21mm F4.5 and the additional lens results in greatly improved sharpness at all apertures as noted by other contributors.

Can anyone point to a thread on how to correct the magenta vignetting in Photoshop? I know nicoimages posted some brief instructions using opaque perspex to generate a magenta correction lcalibration' image however I cant find the thread.

Any advice or link appreciated. Thanks from down under. J




Nov 26, 2017 at 01:01 PM
JT AU
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p.93 #15 · p.93 #15 · Front-End Filter Improves Corner Smearing


PS I am using an A7r kolari V1


Nov 26, 2017 at 01:02 PM
Mathieu18
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p.93 #16 · p.93 #16 · Front-End Filter Improves Corner Smearing


Look up CornerFix as a stand alone, or Adobe has a Flat Field Lightroom plug-in. I know you asked specifically about PS but those are the big two people use. Seems a PS action could be created as well maybe? With a de saturation layer? Never tried to create one with a mask though...

JT AU wrote:
Dear thread contributors,

I have also tested the SLB-50-1500PM reversed on the Contax G21 and Zeiss ZM 21mm F4.5 and the additional lens results in greatly improved sharpness at all apertures as noted by other contributors.

Can anyone point to a thread on how to correct the magenta vignetting in Photoshop? I know nicoimages posted some brief instructions using opaque perspex to generate a magenta correction lcalibration' image however I cant find the thread.

Any advice or link appreciated. Thanks from down under. J





Nov 26, 2017 at 03:40 PM
sebboh
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p.93 #17 · p.93 #17 · Front-End Filter Improves Corner Smearing


JT AU wrote:
Dear thread contributors,

I have also tested the SLB-50-1500PM reversed on the Contax G21 and Zeiss ZM 21mm F4.5 and the additional lens results in greatly improved sharpness at all apertures as noted by other contributors.

Can anyone point to a thread on how to correct the magenta vignetting in Photoshop? I know nicoimages posted some brief instructions using opaque perspex to generate a magenta correction lcalibration' image however I cant find the thread.

Any advice or link appreciated. Thanks from down under. J



i use the flat field plug in LR, not sure if it works for PS too. i imagine taking a shot through a kleenex and using it as a color subtraction layer would work though.

to make my correction files i just focus the lens to infinity, boost exposure to +3, cover the lens with a kleenex (white paper might do), point the camera at an evenly illuminated chunk of sky, and take a picture.




Nov 26, 2017 at 06:24 PM
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