DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
moving_comfort Veteran Member • Posts: 8,211
The cropping issue, illustrated (hopefully) well

moving_comfort wrote:

awaldram wrote:

moving_comfort wrote:

Q: Does cropping FF (to ap-sc, for example) change DOF when displayed at the same sizes? Because I think you used to hold the position that it does not.

Simple question how can it.?

It absolutely does. Remember, we're displaying the image at the same size after the crop - we're not doing something like cropping an 8x10 to 5x7 and then just looking at the 5x7! Cropping the FF image to aps-c size changes the DOF of the original FF image to match what an aps-c shot would have had when the crop is displayed at the same size.

You are stuck on this issue, and I suspect you're thinking of the image staying in the same dimensions as you cropped it to, which isn't the case when you either view or print the crop. None of the 'good' information you've managed to reference supports your error, if you pay attention.

Here - please read carefully through what I wrote in this thread , which I'll past below - please take the time to read, understand and answer it paying attention to the last paragraph:

. . . pasted below ...

...Actually, I don't think you do agree with that point, unless I'm misreading you. Let's try a simple test to see where you stand:

We know that a 300mm f/4 shot on FF and then shot on aps-c from the same position would have different DOF in the final image, from:

• Using the same lens on a APS-C crop sensor camera and a 35mm full frame body, the APS-C crop sensor camera image has 1.5x LESS depth of field than the 35mm image would have

Now, take that 300mm f/4 FF shot, and crop it so that it has the same FOV as the aps-c shot, and display both images (aps-c uncropped and FF cropped) at, say, 8x10. Do the images show the same DOF now?

If the answer is yes**, how could that be true if cropping does not change DOF? Remember, focal length and distance to subject didn't change, the only thing that changed was that the FF image was cropped.

** spoiler alert - the answer is yes


An illustration of the above that works better than words....  Imagine "8x10" across each side of these crops to remind oneself that they're presented at the same display sizes as the image before.

(Image from Lee Jay)


Now, we know nothing can change in  the image itself simply because of a crop (which awaldram was spending a lot of time, I think, trying to say,) but as the we move up those examples and the crop % grows, the % of image area in acceptable focus in each image grows, the image DOF changes... which was what Ian was trying to show, and I describe in the post above.

Cropping (and enlarging the crop to the same display/print sizes) also affects image DR and visible noise in the image - even though the original file itself, of course, didn't physically change.  People can get all hung up over that, too


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