Help me calculate/understand the physics of aperture, DOF and BG blur -

Started 7 months ago | Discussions thread
Darren James New Member • Posts: 12
Re: Help me calculate/understand the physics of aperture, DOF and BG blur -


Depth of Field

  • n.In photography, the distance in front of and behind the subject that appears to be in focus.

I’m not sure what definition you use, but unless it’s that one or one like it then I’m not sure you can claim it’s “the usual definition”.

Tom Axford wrote:

Darren James wrote:

If I understand correctly:

You’re saying if I take my D850 and any given lens, and stick it on a tripod and take a photo, then stick my D500 on the same lens changing nothing else, absolutely every factor the same, that the Depth Of Field changes?

I hope that’s not what you’re saying because that is completely incorrect. Wildly incorrect. A crop sensor is a sensor that is smaller, that’s it. No action is taken on the focus of the light by the detector. The only change resulting from replacing a large detector with a smaller one in the same plane is that you capture a crop of the image. Like, if you were to crop in post. Go, try it for yourself. I did.

You have overlooked the fact that the circle of confusion (CoC) changes for a different sensor size. Conventionally, the CoC is taken as the image diagonal divided by 1500. This gives 0.03mm for FF and 0.015mm for MFT. The DoF is always worked out in relation to the whole image. Cropping the image reduces the DoF if you use the usual definition of DoF.

 Darren James's gear list:Darren James's gear list
Nikon Coolpix S33 Canon EOS 5D Mark III Canon EOS 6D Nikon 1 V2 Canon EOS 70D +12 more
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