How does changing focal length affect depth of field when framing stays the same?
Asked 7/15/2010
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How does focal length affect depth of field as you zoom or switch lenses? I'm especially trying to understand what happens to the near and far limits of acceptable focus, and how this relates to minimum focus distance. If I keep the subject framed the same by moving or cropping, does a longer focal length still give shallower depth of field?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
16y ago
2 Answers
15
While it is a fact that changing focal length from shorter to longer reduces DOF and using a smaller (less light) aperture in will increase DOF (providing format is identical) however there is a simpler way to think of it.
DOF decreases the larger the subject is in the frame regardless of the lens and increases with smaller apertures.
Example: If you shoot the same photo, say a headshot, with a 200mm lens and, at the same distance, with a 35mm lens. Then take the image from the 35mm and crop it to match the image from the 200mm you will find the DOF/image identical.
Of course this is an example assuming that the resolution would not be factor. Which is WHY we change lenses and don't just crop.
Originally by user6815. Source · Licensed CC BY-SA 4.0
user6815
14y ago
0
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Depth of field depends mainly on subject magnification and aperture, not focal length by itself. A longer focal length seems to give shallower depth of field because, from the same camera position, it magnifies the subject more.
If you keep the camera in the same place, zooming longer will usually reduce depth of field. If you keep the subject framed the same by moving farther away with the longer lens, or by cropping the wider shot to match, depth of field is effectively the same at the same f-stop (assuming the same format and ignoring resolution limits).
So the practical rule is: depth of field gets shallower when the subject appears larger in the frame, and gets deeper with smaller apertures (higher f-numbers).
Minimum focus distance is a separate lens design specification. Changing focal length does not create a universal minimum-focus rule; each lens or zoom setting has its own close-focus limit.
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