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depth of field more apparent at long or short focal lenghts??


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#1 sam williams

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Posted 07 October 2008 - 11:52 PM

hi,

im not completely sure how depth of field is calibrated, but i would have thought that if the camera is zoomed in the objects visible in shot will inhabit a visually compressed space, so i would have thought then that area of focus would be quite large and it would all seem as though its in focus. But then i see zoomed in photos of people walking down the street and the people far behind are blurred, and then i see photos of close up stuff and there is a similar effect of shallow depth of field.

how does focal length relate to depth of field in this way???


thanks,

sam

Edited by sam williams, 07 October 2008 - 11:52 PM.

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#2 Chris Keth

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Posted 07 October 2008 - 11:55 PM

Shallow depth of field is much more pronounced when you're on long lenses for two reasons. One, there is actually less depth of field at a given subject distance and f/stop with a long lens than with a wide lens. The second reason is that ona long lens, the background is magnified more so that not only is it out of focus but it is also bigger. Bigger means easier to see detail, or lack thereof.
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#3 Dominic Case

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Posted 08 October 2008 - 02:00 AM

A longer focal length lens (zoomed in, as you put it) when set at the same aperture, has a much larger diameter than a shorter (wide angle) lens. In any part of the image that is out of focus, instead of sharp points of light, you get a tiny disc of light (where the cone of light rays hasn't quite come to a focussed point). The larger the diameter of the lens, the larger the corresponding discs of light (or, effectively, circles of confusion).

So an out-of-focus image made by a lens with a large diameter appears more out-of-focus than one with a small diameter.

Of course, the further away from the ideal focus distance that a part of the image is, the more out of focus it is. So with a longer lens, the image goes out of focus more rapidly, hence less apparent depth of field.
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