Why don’t 25mm at f/4 and 100mm at f/16 give the same depth of field at the same subject distance?
Asked 9/30/2023
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I expected a 25mm lens at f/4 and a 100mm lens at f/16 on the same camera, focused at the same 5 m subject distance, to give the same depth of field because the focal length and f-number scale by the same factor. But depth-of-field calculators show very different results. I am not trying to match framing; the camera position stays the same. I also assumed the same circle of confusion since the camera and final output are the same. Why doesn’t the depth of field match?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
2y ago
2 Answers
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Because magnification (focal length and subject distance) affects the Depth of Field more than aperture does (approx twice as much generally).
Start with 50mm @ f/8 focused at 10 ft. If you halve the aperture # (2 stops) it will result in ~ half the DOF (~3ft from ~6ft).
If instead you double the focal length (f/8/10ft) to 100mm the resulting DOF is ~ one quarter (~1.5 ft from ~6ft)... it would require 4 stops of aperture to counter this effect, not two.
Because magnification is constant for a given composition, focal length and subject distance tend to negate each other; leaving only aperture as an effective control... but that is only if you consider keeping the same composition. And similarly, cropping in post and outputting the same image (size/composition) is also a change of the final magnification and also affect the resulting DOF (as does viewing distance/visual acuity/etc).
And then there is the hyperfocal distance of a lens/aperture. Once you focus beyond that distance the DOF immediately jumps to "infinite." And the hyperfocal distance is much closer for a shorter FL lens. E.g. if you focused the 50mm f/4 at 6m instead. How close your settings and focus are to the hyperfocal distance will affect how much impact other changes make. E.g. focus a little long and the DOF remains infinite, but you lose a little from the near limit. But focus a little short and you lose a lot from the distant limit and the DOF is no longer infinite.
In your examples the physical size of the aperture openings are the same. So the depth of focus is the same.
The only thing that affects the depth of focus is the lens' physical aperture diameter. Which is probably where the confusion arises in focal length and depth of field. Depth of focus is how sharp details are recorded, and how unsharp other details are, at the image plane... it's also called "focus tolerance." Everything else about depth of field is based upon how the depth of focus appears to you based upon its' magnification/visibility (focal length, cropping/crop factor, display size, viewing distance, etc, etc).
If you were to equalize the two images so that the physical display size of the details are the same relative to you, you would then find the depth of field to be the same.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
2y ago
0
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Because focal length does affect depth of field when camera position stays fixed. Matching the f-number ratio alone is not enough.
At a given subject distance, a longer focal length gives higher magnification, and magnification has a stronger effect on DOF than aperture does. So going from 25mm to 100mm greatly reduces DOF; stopping down from f/4 to f/16 does not cancel that out.
The idea that focal length “doesn’t affect DOF” is only approximately true when you keep the same framing/composition by changing camera distance to compensate. In that case, focal length and subject distance largely offset each other, leaving aperture as the main control.
Your circle of confusion assumption is also part of the issue. CoC is an absolute blur size on the image. If you compare images made with 25mm and 100mm at the same distance, the 25mm image is much less magnified. If you enlarge or crop it to match the 100mm view, the acceptable CoC would need to be smaller, so the perceived DOF would not match.
So the calculator is not wrong: same camera position + different focal lengths = different magnification, and therefore different depth of field.
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