#1  
Old 25-11-2017, 08:54 PM
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skysurfer
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Canon EF 1.8 focus not evenly distributed ?

I made a shot of the Cygnus nebula region with the Canon EOS 6 with EF 85mm f/1.8 (set to f/2.5 to get rid of the worst vignetting) and an Astronomik UHC-S clipin filter, the latter due to moderate light pollution.
I focused using Live view via DSLR controller on my Android cellphone screen (much larger than the rear camera screen) and took a few test shots which I downloaded to my cellphone and zoomed in strongly and everything in sharpness (near the center) seemed OK, so I continued with the two minute frames.

But the left (south) is out of focus. And this is not a stacking error as the original frames (30 x 2minutes) have the same issue.

It is not the first time I got this error.

Here the images: the stacked (with Photoshop CC 2018) and postprocessed image and one of the original frames, downscaled to 1920x1280 in jpg.

It is not the sensor as most photos are just sharp to the edges, particularly taken with the 70-300L at full aperture f/5.6.
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  #2  
Old 26-11-2017, 09:16 AM
JA
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Hello SS,

I'm not suggesting that your problem is due to coma or astigmatism, but I would suggest you download this Lens abberation test page (scroll down). It's like a whole lot of evenly spaced stars and then try to replicate your situation to understand the issue more....

https://www.lonelyspeck.com/a-practi...erration-test/

All I can think of is that somehow the lens optical axis and a line perpendicular to the sensor surface are not parallel and hence one side of the image plane is closer to focus than the other. If so it should be more obvious at larger apertures, so comapre f1.8 with f16. Is there any sign of play or excess wear in the lens mount that might be causing this? With the lens mounted on the camera grab the lens barrel and try to push it from side to side. Any history of any loose lens elements or lens dropped at any time or repairs?

Best
JA
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  #3  
Old 26-11-2017, 10:29 AM
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Shiraz (Ray)
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could it be field rotation maybe? - how close was your polar alignment?
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Old 26-11-2017, 07:35 PM
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skysurfer
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Quote:
Originally Posted by Shiraz View Post
could it be field rotation maybe? - how close was your polar alignment?
With 5 minutes and 300mm (which has a much higher requirement on polar alignment) I don't have these problems, but, admittedly, as the exposures were only two minutes, I was a bit more relaxed with alignment, but I am sure it was witin 20', so that should not be the problem.
For me, aligning the north pole is more effort than the south pole as for the latter I just center BQ Oct in the FOV of the polar scope and I have never tracking errors, even with more minutes per frame with my ED110 (f=600mm).
There are no such 'bright' (+6.9) stars that close (10') to the north pole.

Quote:
Originally Posted by JA View Post
Hello SS,

-snip-
Is there any sign of play or excess wear in the lens mount that might be causing this? With the lens mounted on the camera grab the lens barrel and try to push it from side to side. Any history of any loose lens elements or lens dropped at any time or repairs?

Best
JA
No, I tried it again, no play or wear found. And the lens or camera body is never repaired, the lens I bought in 2009.

I have made some test photos with the ISO 12233 chart.

I used the same ISO (1600) and resolution (full 5472x3648) as the astrophotos. To enhance contrast, I overexposed the pictures with +1 stop. I mounted the camera on a tripod to eliminate motion blur and used a laser pointer which flat rear I put on the center of the chart and moved the camera and/or subject in such a way that the laser dot was in the center of the lens, to be sure that it is really perpendicular. Not an easy job.

Here the images (about 6MB each, hence I did not upload it to IIS, but my own website).

85 f/2.5 (which I use for astrophotos)
85 f/4
85 f/9

And some with the 70-300L @ 300mm

300 f/5.6 (full aperture)
300 f/11

It appears that the 85/2.5 is somewhat soft on the left side and on the right side but at a lesser extent, compared to the 85 f/9. But the 300 f/5.6 (full aperture) is also very sharp, better than 85 f/2.5. The left side of the 85mm at f/2.5 is the worst.
So I have three choices for small telephoto images of the sky:

* Squeeze at least to f/5.6 which prolong exposure time 2-3x
* Spend 5 grand (Australian) for a Zeiss Otus 55 f/1.4
* Accept these focus flaws
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