Depth of Field Variations Based on Sensor Size

[jeffkohn]

Originally Posted by Thomas Campbell

If I put my 150mm 2.8 macro on a tripod, put the 1Ds2 on it, adjust and write down the settings, take a picture, then put my 50D on that lens, without adjusting a thing, keep the same settings, the pic from the 50D will have the same DOF. The 1Ds2 will just have a 30% more FIELD OF VIEW. It is the same pic, only the 50D is cropped in more.

As I said earlier it gets somewhat more complicated when you compare cameras with different pixel densities (which would be the case with a 1Ds2 and 50D). But assuming the pixel densities were about the same, what you said would only be true if you printed the APS/DX picture so that it was the same size as the same portion of the full-frame picture. If you printed the APS/DX picture to the same print as as the entire full-frame image, the DOF would not be the same because of the greater enlargement.

It's not so hard to understand if you think about the relationship between sharpness and enlargement. We all know that if you print a 4x6 and a 20x30 of the same image, things that look sharp in the 4x6 might not look as sharp in the 20x30 due to the much greater enlargement. It's the same principal with DOF - things that look sharp-enough to be in-focus for a smaller print will start to look slightly out-of-focus as you enlarge the print to a greater degree.

jeffkohn added 2 Minutes and 43 Seconds later...Double Post Merged Below

Originally Posted by thejakestir

All I know is if I put a 85mm lens on my 5d and compose the shot how I like, lets say the persons eyes are 1/3 from the top of the frame.
And then take the same shot with a 85mm lens on the 20d and compose the shot the same way, eyes 1/3 from the top of the frame. Then the 5d will have shallower DOF.
Of course I had to move away from my subject with the 20d, but the FoV is what determines how you compose your photos. And the sensor size has a large part in the FoV.
So far all intents and purpose , larger sensors affect your DOF.

That's exactly right. You either back up to shoot with 20D, or you put a shorter focal length lens on it. Either way you get more DOF. This is why point-n-shoots have extremely large DOF, they use very short focal lengths due to the tiny sensor.

[texfoto]

Originally Posted by thejakestir

All I know is if I put a 85mm lens on my 5d and compose the shot how I like, lets say the persons eyes are 1/3 from the top of the frame.
And then take the same shot with a 85mm lens on the 20d and compose the shot the same way, eyes 1/3 from the top of the frame. Then the 5d will have shallower DOF.
Of course I had to move away from my subject with the 20d, but the FoV is what determines how you compose your photos. And the sensor size has a large part in the FoV.
So far all intents and purpose , larger sensors affect your DOF.

Bingo! This makes sense now.