DOF and Cropping take 2

Started 8 months ago | Discussions thread
James O'Neill
Veteran MemberPosts: 3,939
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Re: Cropping from within a format (101)
In reply to Ian Stuart Forsyth, 8 months ago

Ian Stuart Forsyth wrote:

moving_comfort wrote:

James, I don't want to speak for Ian but I think you're misinterpreting what he was trying to show.

Basically, I think he was just trying to show how if you crop an image, and then display the crop magnified (ie, at the same or 'regular' display sizes) the DOF changes - more of the image is 'in focus' after the crop and magnification. It's the act of magnification that does this, nothing changes internally in the image.

It's exactly the same reason why a native FF shot has a different DOF (for the same FOV, aperture and distance) as a native aps-c shot taken from the same position - but if you crop the FF shot to match the aps-c FOV and then follow through by magnifying that crop to the same display size, the DOF is now the same as the aps-c shot - when before it was different.

Yes thank you

This is what (I think) Ian was tying to show in order to convince awaldram, who's now claiming he never denied it. I don't think Ian's crop examples there were the best idea, because awaldram thought they were meant to show him that the "DOF is always the same"

Ian - what you were trying to show was absolutely correct. But what you used to demonstrate it was suspect, and some of the other things that came in just added to the confusion.

  • The things that determine circle size are square of focal length, aperture and distances where the lens is focused and where the subject is.
  • What determines whether something appears in focus depends on the size of the circle relative to the part of the image you looking at. This is the second place in a week where I have seen some contributors erroneously equating circle size alone with depth of field.
  • Therefore changing from viewing the whole image to view part of it, magnified, changes the d.o.f. even though the circle of confusion says remains the same.
  • It doesn't matter if you get to "part-of it, magnified" by cropping down an image from a big sensor or putting a smaller sensor behind the lens.

On equivalence

If to use different cameras at the same place to get the same field of view the ratio of their focal lengths must be the same as the ratio of their sensor sizes. So must the circle size you use to determine in-focus/out-of-focus. Because the circle size increases with the square of focal length, you if you want the same apparent depth of field you must also use a smaller aperture with the longer focal length, so for equivalent field of view and d.o.f you have to increase the f/ number in the ratio of the sensor size and focal length. Since the light getting to the camera is the same, if you increase the f/ number you need to change the shutter speed or the ISO to get the same exposure.
There's never 100% equivalence because when one parameter changes you have to change a second parameter to get other things the same.
To get the D.o.F I get with my phone at a hand holdable shutter speed a plate camera user needs to put his camera on tripod - but he can isolate things from the background in a way I never can

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