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  #21  
Old 17-01-2019, 06:49 AM
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In live view mode the focus is evaluated via sensor (and firmware... probably by FFT), not by autofocus phase sub-system which is separate and works through small mirror (behind/below the main one) and sensors placed at the bottom of the mirror cage.
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Old 20-01-2019, 09:17 PM
Jasp05 (Aaron)
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I feel I may have jumped the gun a bit with my "achieving focus with my lenses through live view".

Although it works for daytime (not at infinity), I had a play tonight and it seems I don't have enough back focus to achieve a sharp focus at shorter focal lengths.

On my 18-55mm for example I can't get focus at 18mm, but I can at 55mm.

My 55-250mm I can't get focus at 55mm but can at 250mm.

So it seems the sensor spacing needs some adjusting to compensate.
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Old 30-01-2019, 08:17 PM
Jasp05 (Aaron)
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Seen as i have to adjust the sensor spacing after modification to enable focus with my lenses again. How do you go about testing while making adjustments?

Should only need the back cover off to make adjustments. So i guess i could take test exposures after making an adjustment?

And given there are 3 screws how important is having no tilt on the sensor?

How would i test for sensor tilt also?
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Old 30-01-2019, 08:26 PM
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Another approach to get infinity focus is to adjust lens mechanics.


Adjusting tilt is tedious job, I would start with micrometer like this one.
CCD inspector may help to evaluate and indicate where the problem is..
The final adjustment can be done by placing thin shims on the camera flange (cigarette paper, or aluminium foil) to compensate for the residual tilt
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Old 30-01-2019, 11:51 PM
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Would it be worth trying to make an artificial star for testing fwhm once adjustments have been made?


Or can i just rely on focus from examining the image after a test shot is done. At least the star test would give me a way to measure if focus is better?
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Old 31-01-2019, 07:22 AM
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Quote:
Originally Posted by Jasp05 View Post
Would it be worth trying to make an artificial star for testing fwhm once adjustments have been made?

Or can i just rely on focus from examining the image after a test shot is done. At least the star test would give me a way to measure if focus is better?

Probably worth it... this is (I believe) how manufacturer adjust those things (not artificial star, but definitely test pattern is used).


(For example, I have a problem with one of my lenses (Samyang 500mm F/6.3), it suffers from coma in frame corners, while centre is OK (and it is only my specimen that shows this problem, others are OK ).

I plan to use red laser module and another catadiopric lens (MTO 1100) as colimator, so my artificial star will be at optical infinity. Then I will try to adjust coma by moving the back lens group and checking the image quality using live view.)



Test shot is easiest way to determine if focus is OK or not.
Maybe you should use autofocus at distant object while it is still light, then switch to MF and wait for a dark to take star test?



At some point when modifying my 450D I was thinking about checking the distance (and tilt) of the sensor with some sort of microscope at high magnification, this arrangement should have a very narrow focus depth, but there was no need for that, tilt was acceptable, and I am using that camera exclusively for AP.

Another way to test tilt would be to try to use laser beam reflected from sensor glass cover (assuming it is parralel with sensor plane).

Something lik ethis one: http://electron6.phys.utk.edu/optics...ferometers.htm


I have laser distance module that uses self-interference with reflected beam, thus detecting the change in distance.. it is VERY sensitive, resolution is well below 1um).. To use it for that purpose, first I need to locate it (somewhere in my cubboards), then I need to make flange adapter.... this is a project for future.

Last edited by bojan; 31-01-2019 at 07:42 AM.
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