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I am using Nikon D7000 with 18-105. I have read people saying to get images with the minimum depth of field, the lens should be at its maximumum focal length, in this case at 105mm. However at 105mm the aperture of the lens drops to f/5.6, so I don't understand why f/3.5 at 18 mm won't give me a shallower depth of field than 105 mm at f/5.6.

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For what it's worth, you're misusing bokeh here: bokeh is a qualitative measure of the "niceness" of the out of focus regions of an image; what you're referring to is shallow depth of field. –  Philip Kendall Jan 24 '13 at 8:46
    
yes I have and its true. –  V.B Jan 24 '13 at 8:46

2 Answers 2

up vote 4 down vote accepted

Focal length affects depth of field as well as aperture.

At f 3.5/18mm the depth of field is over 20 feet deep. Focusing on a subject 10 feet away this means that everything up to 17 feet behind the subject will be in focus.

At f 5.6/105mm the depth of field is 0.6 feet deep. This means only 0.3 feet (10cm) will be in focus behind the subject; in other words, the background will be blurred, and you will get bokeh.

Check out http://www.dofmaster.com/dofjs.html

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thanks @Elendil. –  V.B Jan 24 '13 at 8:51
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how to calculate DOF? –  V.B Jan 24 '13 at 8:58
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The Bokeh is only caused by the circle of confusion, or the area in which a certain point is projected. You call something 'in focus' if the CoF is smaller than the pixel size. You can find information on how to calculate the size of CoF at en.wikipedia.org/wiki/Circle_of_confusion –  SinisterMJ Jan 24 '13 at 11:26
    
Should be CoC instead of CoF (Circle of Confusion). Mixed it up with DoF unfortunately. On the wiki page you also get calculations for the depth of field. –  SinisterMJ Jan 24 '13 at 11:34
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@v.b use the link in my answer, it's a DoF calculator! –  ElendilTheTall Jan 24 '13 at 12:44

Technically, at the same magnification of the subject and the same relative aperture (say, f/4) you have the same depth of field (as in depth of acceptable focus) although of course you will be much closer to the subject with the wider lens.

You'd have however different background blur due to the different magnification of the blurred background at different focal lengths. See here: http://toothwalker.org/optics/dof.html at the chapter titled "Background Blur".

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