My guess is that the DOF will depend only on the outer diameter and have
nothing to do with the central obstruction. Remember that DOF is based
on the "circle of confusion" which is created by a point source of light
being formed from a cone whose diameter is decreasing to a minimum at
the point of focus and then increasing afterwards. Imagine two dunce
caps placed point to point with the points at the precised plane of
focus. The circle of consusion is the diameter of those cones at any
point away from the focal plane. If the circle of confusion on the
final print is less than the visual acuity of your eye the print looks
sharp.
If those cones are formed by a small aperture lens (small outer
diameter) the angle of the cones will be shallow and the circle of
confusion will be smaller leading to greater DOF.
Because of the central obstruction the readings of f/8, f/11, f/16 on
the mirror lens are really light transmission values and are not based
on clear aperture as would be an unobstructed lens. Therefore, the
actual diamter of the diaphragm is not changing as much as would be
expected in a conventional lens and is having a lesser effect on the DOF
than the aperture reading would indicate.
Chuck Norcutt
tOM Trottier wrote:
> Seems to me that Depth Of Field depends on the width of the front lens, so
> for the same light
> gathering ability, is less with a mirror lens because of the middle blockage.
>
> Even with a diaphragm, the DOF could not be less than what you would
> calculate for the width of
> the internal blockage, the secondary mirror.
>
> tOM
>
> --
> tOM Trottier, Ottawa, Canada
> 758 Albert St, Ottawa ON Canada K1R 7V8
> +1 613 231-6115 N45.41235 W75.71345
> http://groups.yahoo.com/group/ottawa-photo-clubs
> "The moment one gives close attention to anything,
> even a blade of grass, it becomes a mysterious,
> awesome, indescribably magnificent world in itself "
> -- Henry Miller, 1891-1980
>
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