Orquid "Phoenix"

Orquid "Phoenix"

by ceinmart

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I'm curious about how my "area of acceptable focus" changes when the focal length of the lens I'm using changes as I zoom (or switch lenses). In particular, I'd like to know how the front & back focal planes change, thus changing the depth of field and the minimum focus distance.

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Is this related to hyperfocal distance? I think that might need to be a tag. –  reuscam Jul 15 '10 at 19:34
    
Some commentary on hyperfocal distance would be great. –  Craig Walker Jul 15 '10 at 19:37
    
this is also related to telephoto compression I believe. –  reuscam Jul 15 '10 at 19:46
    
@mattdm: Agreed. –  Craig Walker Oct 2 '11 at 17:34
    
Please specify: when you change focal length, do you (1) stay at the same place (thus changing subject magnification) or (2) keep the subject at the same size (thus moving farther when using a longer focal length). –  Edgar Bonet Oct 4 '11 at 18:48
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4 Answers

up vote 9 down vote accepted
+25

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.

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There's an interesting corollary to this then: the larger the subject is, the harder it is to get a shallow DoF (assuming aperture and frame size stay constant). –  Craig Walker Oct 4 '11 at 23:32
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If I understand you correctly, surely this can't be correct. I can take a headshot with a 200mm lens from a particular distance in which the background is out of focus. When at the same distance, using a 35mm lens, the subject is within the hyperfocal distance and the background is sharp. –  MikeW Mar 3 '12 at 23:12
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@MikeW — this answer assumes that the cropped 35mm image is enlarged to be viewed at the same size as the 200mm image. At the smaller size, both background and subject are acceptably sharp, but as you expand (again, given the condition that resolution is sufficient), the blur in the background becomes more obvious. –  mattdm Jul 2 '12 at 0:41
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Here are sample photos demonstrating your statement is true: luminous-landscape.com/tutorials/dof2.shtml –  dzieciou Jun 6 '13 at 6:22
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There is tool to calculate DOF by putting Focal length and f-stop here: http://www.dofmaster.com/dofjs.html
With result you can create an interesting graph

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If you stand still

The depth of field quickly gets narrower as you zoom in.

If you keep subject magnification constant

If the depth of field is large (comparable to the focusing distance), then it gets somewhat narrower when increasing focal length. If it is already narrow, then it is practically independent of focal length.

Front and back depth of field

When it's narrow, the depth of field is practically symmetric relative to the plane of best focus. As it gets wider, and specifically as it reaches the order of magnitude of the subject distance, it gets more and more asymmetric (more depth of field behind the subject than in front of it). At one point it reaches infinity, then things are sharp from half the focus distance up to infinity.

A simple rule that is probably more useful than my previous paragraph: the depth of field is always practically symmetric when read from the lens' focus scale.

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Depth of field is a function of the relationship between image magnification and diaphragm opening.

Lens focal length has nothing to do with depth of field.

The misconception arises because, from a given subject-camera distance, a short focal length lens gives a smaller degree of image magnification and consequently more depth at a given distance. The depth comes from the image size, and not from the shorter lens. If the images are of like magnification and the f-stop is identical, then the depth of field is identical, regardless of focal length.

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