01-19-2012, 06:49 PM
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? The question then is how much deviation do I have?
For example, using this 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.
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? The question then is how much deviation do I have?
For example, using this 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.
<a class="bbc_url" href="http://snowporing.deviantart.com/">dA</a> Canon 7D2, 7D, 5D2, 600D, 450D, 300D IR modified, 1D, EF-S 10-18, 15-85, EF 35/2, 85/1.8, 135/2, 70-300L, 100-400L, MP-E65, Zeiss 2/50, Sigma 150 macro, 120-300/2.8, Samyang 8mm fisheye, Olympus E-P1, Panasonic 20/1.7, Sony HX9V, Fuji X100.