Re: Trying to understand depth of field and f stop
- From: "dadiOH" <dadiOH@xxxxxxxxxxx>
- Date: Fri, 22 Aug 2008 07:50:15 -0400
Michael wrote:
A smaller absolute aperture gives you better depth of field.
(To the OP)
And that smaller aperture size gives increased depth of field because the
angle of the light rays at the focal plane is smaller. To help you
visualize this, draw a big "X". Now draw a horizontal line across the two
top points of the "X". The top line represents your lens; the legs of the
"X" are light rays coming thru and being refracted by the lens; the place
where the legs of the "X" intersect are the focal plane. (In reality, the
rays coming thru the lens form a cone [because the lens is round] but this
two dimensional representation is good enough for this explanation).
The lens can only focus one subject distance perfectly (or as close as
perfect as the lens can get). That subject distance is represented by the
intersection of the legs of the "X"; all other distances will be represented
as if you moved the focal plane up or down so that they intersect the "X" at
other than the cross point...the farther away from the cross point the more
out of focus because the distance from leg to leg is greater.
Now take your big "X" and mark a small aperture on the top line which
represents the lens; next, draw another "X" from those aperture marks so
that the lines intersect at the same place as did those in the original "X".
As you can see, the angles made in the new "X" are less than those in the
original; consequently areas of the scene that are out of focus will be
*less* out of focus than with the larger aperture and the depth of field
will be greater.
--
dadiOH
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- Trying to understand depth of field and f stop
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