Accurately comparing FF vs APS-C sensor performance? An open discussion.

Started Apr 8, 2013 | Discussions thread
Alan_S
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Re: You misunderstand
In reply to OldClicker, Apr 17, 2013

OldClicker wrote:

One more try.

VirtualMirage wrote:

What realistic assumptions are these calculators doing wrong?

Assumptions aren't wrong - all models make assumptions.  It is only the misuse of the model that is wrong.  I believe the DoF calculators assume something like 20/20 vision at 1 meter for an 8x10" print at 300 DPI.  If these are not true, then the model is being misused and may be wrong.

I hesitate to weigh in, but here goes. OldClicker's statement on use/misuse of a model seems to sum it up well; here is an example of two "uses" and different views of apparent DOF from the same image (I posted these images in a previous discussion on this topic, but the seem relevant here as well -- and please, no C&C, these are NOT "good" shots, just a quick easy example for this conversation )...

The online DOF calculator I found concludes that there is area of acceptable sharpness of 14.5 feet at 50 yards, f/5.6, FF a900. The DOF calculator seems accurate enough viewed uncropped (1st image), but looking at a 100% crop (2nd image) I find the "acceptable area of sharpness" reduced to no more than 2 feet. These were taken with an a99 at about 50 yards:

uncropped, f/5.6, 400mm @ 50 yds... Looks like DOF calculator is close, @ 14.5'

same exact shot cropped 100% -- now DOF "acceptable area of sharpness" looks like <2'

Bottom line, since I tend to do some cropping on wildlife shots (sometimes, a LOT of cropping), I would not rely on the DOF calculator for an accurate representation of the acceptable area of sharpness.

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- AlanS

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