Bob Talbot wrote:
>
> But for some reason the tale "wide angle lenses give you more depth of
> field" crops up once a month on every photo list. It is so stongly
> rooted I don't suppose it will ever go away. Maybe it is easier to
> understand than the highly mathematical concept of *magnification*.
>
> Bob
The reason may be, that it's plain simply true. DOF depends on focal length.
Pick your favorite DOF calculator and try it out. Or just do the following:
Take a full body shot of your favorite model, let's say at a magnification
of 1:50. Do it once with a 20mm (distance roughly 1m), then with a 200mm
(distance roughly 10m). Both times stop down to f/16, focus on the person.
The 20mm shot will exhibit a sharp background, because 1m is the hyperfocal
distance of a 20mm at f/16 (when calculated with my favorite circle of
confusion size of 0.025m). The 200mm background will be unsharp. You would
need f/160 to get the same circle of confusion size for the background at
infinity.
Usually at this point the second myth will come up. "Ok, the above is true,
but if I shoot from the same distance and then enlarge the prints to the
same subject magnification (and crop the wide angle print accordingly),
the DOF will be the same". It's not true either, every medium or large format
guy will know it better. You can NOT eliminate the focal length in the
formulae ruling DOF by introducing magnification, there's simply no way.
And yes, I DO understand the concept of magnification. And the formulae
are simple enough to understand even without a degree in maths. ;-)
For close ups you can forget about focal length. There the magnification
is all you need and the terms still containing focal length become too
small to have any significant influence on DOF. Then again, a full body
shot is not exacly a close up ;-)
Thomas Bantel
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