Originally posted by kh1234567890 Hmm... No matter how much cropping and chair wheeling I do,
a shot like this (apologies for the bad composition) looks the same to this tired old brain and eyeballs. The man behind the coat and sunglasses girl refuses to pop into focus.
Originally posted by mohb All that cropping and resizing does is alter the visual signals we use to measure depth, the actual DOF will still remain what t was when the photo was taken.
Again, use equivalence to understand what is going on here ...
First, there is no such thing as "actual DOF".
Second, assume you crop the inner 50% x 50% part of the original image, i.e., you apply a crop factor of 2. And assume the original image was taken with 50mm F/4.
The cropping then means the cropped image has
equivalent shooting parameters of 100mm F/8 (equivalent wrt an uncropped image looking exactly the same on the same camera body).
Now, the DoF of 100mm F/8 is actually
shallower than 50mm F/4. And that's no surprise because you magnified the blur at the boundaries of the DoF region (the cropped image needs be magnified more).
This man will
never pop into focus, if anything, cropping will make things pop out of focus.
But the s
ame equivalence teaches us that we would have needed to take the original image before cropping at 50mm F/2 if we were after an image looking
like 100mm F/4 after cropping.
It may all sound confusing. But only if you didn't fully understand the concept of equivalence.