How subject distance and focal length affect depth of field and bokeh
Asked 2/8/2011
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I understand that aperture affects depth of field, but I’m unsure how subject distance and focal length change it.
For example, if I make two photos from the same camera position using the same lens settings except focal length — one at 24mm and one at 70mm — and then crop the 24mm image so both frames show the same field of view, will they have the same depth of field and bokeh? How does subject distance factor into this?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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In your example, assuming you kept the same distance to the subject, the 24mm picture would have more depth of field than the 70mm picture. To keep the same depth of field, you'd need to stop the 70mm lens down until it has the same absolute aperture diameter (so that 24/f-number = 70/other f-number).
In this picture, you can see that a 210mm f/11 lens has roughly the same depth of field as a 35mm f/1.8, when cropped to the same field of view and subject distance is kept constant.
Originally by user378. Source · Licensed CC BY-SA 4.0
user378
15y ago
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Generated from our catalog & community — verify before relying on it.
No. If you stay at the same camera-to-subject distance, a 24mm shot cropped to match a 70mm framing will still show more depth of field than the 70mm shot.
Depth of field depends on aperture, focal length, and focus distance. Subject distance matters a lot: the closer you focus, the shallower the depth of field becomes; farther focus distances increase it.
In your example, keeping distance constant but changing from 24mm to 70mm does not preserve depth of field, even if you crop the wider shot to match the narrower field of view. The longer focal length will give shallower depth of field and stronger background blur.
A useful way to compare them is by absolute aperture diameter: focal length ÷ f-number. To get similar depth of field at the same subject distance and final framing, the longer focal length generally needs to be stopped down until its physical aperture opening is similar to that of the shorter focal length.
If you want to explore combinations of focal length, aperture, and focus distance, a depth-of-field calculator is a good tool.
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