Well, since we're being pedantic, there are only *two* things that
affect DOF:
1) focal ratio
2) reproduction ratio
#1 is what everyone calls "aperture." Not saying anyone is wrong for
using the latter term, just that "focal ratio" is more precise and
descriptive in this context.
#2 is the ratio between the real-life size of something and its
eventual rendering. This is often expressed as the ratio to film-size
for macro work, but as Chuck points out, is generally expressed as the
ratio to an 8"x12" print size in DOF tables and markings on lenses.
It's a technical fallacy that focal length has anything to do with it:
crop and magnify a portion of a wide-angle shot so that it is
identical to a telephoto shot *taken from the same location*, and they
will have the same DOF, if they were shot at the same aperture.
But *practically*, wide-angle has a well-deserved (if false)
reputation for "having more DOF" simply because it tends to increase
the reproduction ratio, just as telephoto has a well-deserved (if
false) reputation for "having less DOF" simply because it tends to
reduce the reproduction ratio.
Another reason focal length has a reputation for affecting DOF is that
one often changes perspective when changing focal length. Move back
and forth until the subject is the same size, and take pictures with
both a wide angle and a telephoto, and the DOF of a shallow subject
will be about the same.
But by moving back and forth and changing perspective, YOU have
altered the relative reproduction ratios of the foreground-background
objects in your scene, and so the wide angle lens WILL "have greater
depth of field," but because you have moved, NOT because it's a wide
angle lens. You could construct a mosaic wide-angle with the telephoto
from that perspective and have identical DOF.
Clear as mud yet? :-)
> From: Chuck Norcutt <puhpxabephgg@xxxxxxxxxxxxxxxx>
>
> And in the real world of prints (as opposed to the viewfinder) the
> size
> of the print. Standard DOF tables assume about an 8X print
> magnification. Larger prints result in less DOF and smaller prints
> result in greater DOF.
>
> NSURIT@xxxxxxx wrote:
>> Three things will affect
>> your depth of field.
>>
>> Focal length of the lens.
>>
>> Aperture you set on the lens.
>>
>> The distance between you and your subject.
>>
>> Bill Barber
>
:::: They that can give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety. -- Benjamin
Franklin ::::
:::: Jan Steinman <http://www.Bytesmiths.com> ::::
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