At 03:34 04.03.2003, Jan Steinman wrote:
>From: Thomas Bryhn <thomas.bryhn@xxxxxxxxxx>
>
>If you play with one of the DoF calculators and compare DoF for different
>formats you'll discover that for any angle of view DoF only depends on
>absolute aperture.
AND reproduction ratio. That's where people get tripped up. A dime that is
life-sized on a 4/3 sensor will have the same DOF as a dime that is
life-sized on 8"x10" large format.
Personally I would rather say "OR reproduction ratio", because reproduction
ratio makes more sense when you're changing lenses than when you're
changing sensor format. For instance, photographing a dime 1:1 with 8x10"
sounds like a waste of film to me, but I wouldn't hesitate to do it in
35mm. I would rather compare two pictures with the same angle of view,
taken on different film or sensor formats, i.e. you change format *and*
lens (and therefor r.r.), distance and framing stays the same.
>f/0.5 is the optical limit for lenses with air on both sides, and
>that would require an infinitly large opening...
I'm not aware of any such limitation. The front element diameter
approaches infinity as the focal ratio approaches zero, but that's a long
way from 0.5.
Normally we use the approximation f/# = f/D, more correct would be f/# =
n1/(n2*2*sin(t)), where n1 and n2 are indexes of refraction and t is the
angle spanned by the edge of the front element and the optical axis, seen
from the point where the optical axis intersects the film. n1=n2 for our
applications with air on both sides of the lens.
Sin(t)=tan(t)*cos(t), and for small t you can safely write sin(t)=tan(t)
because cos(t)=1. 2*tan(t) is nothing more than D/f, and inverting this
gives us our normal approximation.
Now assume a *very* large front element where the normal approximation is
no longer valid. Sin(t) will then approach 1 and we're left with another
approximation: f/# = 1/2 * n1/n2
Again n1=n2 for normal photography, so 0.5 is the limit.
Note: In microscopy special oils with high indexes of refraction (n2) are
used to push this limit, do a search for "oil immersion".
Thomas Bryhn
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