01-19-2012, 08:18 PM
Hi popo,
[quote name='popo' timestamp='1326998992' post='15034']
I wonder if some of the more theoretically knowledgeable members can give me a hand here. I'm not quite up to speed on working out depth of focus (NOT depth of field) and possibly using the amount of de-focus to estimate the shift distance from ideal.
As background to this, I'm still dabbling with astrophotography. Using my modified 450D with the 135mm f/2 at f/2, I find that the focus is not consistent across the frame. It has a characteristic that a diagonal line is in focus, and deviating from that line the defocus grows. As such I suspect the lens axis is not perpendicular to the sensor, and this is not field curvature which would be a circular effect wouldn't it?
For example, using [url="http://www.tawbaware.com/maxlyons/calc.htm"]this[/url] calculator and a CoC value of 10.3 microns (double the pixel pitch of the 450D) I get a depth of focus of about 21 microns. The difference in size of a nominal point source (faint distant star) is about 4x greater in the worst defocus zone. Is the difference in distance between that and the edge of the depth of focus then (4-1)*0.5*depth_of_focus? I had to draw that out to visualise it! But that is in the ball park of 30 microns... I'm not sure I can hand adjust the sensor placement to that sort of accuracy!
If anyone hasn't fallen asleep or just gave up already, some sanity checking would be welcome.
[/quote]
cosine(5.52 degrees)*((4-1)*10.3 microns * 0.5) I would think, or 15.4 microns if I calculated this correctly. This based on a point source being 10.3 microns... <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />. This is one way of course. It would be similar but in opposite direction in the opposite corner. 5.52 degrees because that is half the AoV on APS-C of a 135 mm lens; I think you may have used the entire AoV, which is not correct, IMO.
In order to check whether it is the lens, or the camera mount or sensor, check another lens, I would say. Same result would indicate it is the camera, either sensor or mount (which can be adjusted by mere humans, apparently, but is a very time consuming process), even if it is the sensor in that case. Underneath the mount ring, there are shims of different thicknesses and inclinations. There are also shims behind the lens mount itself, to position it correctly. Of course only change those if it is the lens rather than the body.
Considering the minimal amount of tilt required, it is going to be tricky to do...
Kind regards, Wim
[quote name='popo' timestamp='1326998992' post='15034']
I wonder if some of the more theoretically knowledgeable members can give me a hand here. I'm not quite up to speed on working out depth of focus (NOT depth of field) and possibly using the amount of de-focus to estimate the shift distance from ideal.
As background to this, I'm still dabbling with astrophotography. Using my modified 450D with the 135mm f/2 at f/2, I find that the focus is not consistent across the frame. It has a characteristic that a diagonal line is in focus, and deviating from that line the defocus grows. As such I suspect the lens axis is not perpendicular to the sensor, and this is not field curvature which would be a circular effect wouldn't it?
Quote:Yes.The question then is how much deviation do I have?
However, this could be caused by a variety of issiues. It could be a decentered lens (slight tilt), be it very nicely centered, so not likely, or the lens mount, or the mount on the camera, or the sensor itself. All of the latter are possible.
For example, using [url="http://www.tawbaware.com/maxlyons/calc.htm"]this[/url] calculator and a CoC value of 10.3 microns (double the pixel pitch of the 450D) I get a depth of focus of about 21 microns. The difference in size of a nominal point source (faint distant star) is about 4x greater in the worst defocus zone. Is the difference in distance between that and the edge of the depth of focus then (4-1)*0.5*depth_of_focus? I had to draw that out to visualise it! But that is in the ball park of 30 microns... I'm not sure I can hand adjust the sensor placement to that sort of accuracy!
If anyone hasn't fallen asleep or just gave up already, some sanity checking would be welcome.
[/quote]
cosine(5.52 degrees)*((4-1)*10.3 microns * 0.5) I would think, or 15.4 microns if I calculated this correctly. This based on a point source being 10.3 microns... <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />. This is one way of course. It would be similar but in opposite direction in the opposite corner. 5.52 degrees because that is half the AoV on APS-C of a 135 mm lens; I think you may have used the entire AoV, which is not correct, IMO.In order to check whether it is the lens, or the camera mount or sensor, check another lens, I would say. Same result would indicate it is the camera, either sensor or mount (which can be adjusted by mere humans, apparently, but is a very time consuming process), even if it is the sensor in that case. Underneath the mount ring, there are shims of different thicknesses and inclinations. There are also shims behind the lens mount itself, to position it correctly. Of course only change those if it is the lens rather than the body.
Considering the minimal amount of tilt required, it is going to be tricky to do...
Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....