P3D depth of focus

[Peter Abrahams <telscope@xxxxxxxxxx>]

Depth of focus is the distance the film can be moved (fore & aft) before
the image of a point exceeds the circle of confusion (Sidney Ray, The
Photographic Lens).  It is dependent on focal ratio of the lens (decreases
with fast lenses) and other optical qualities, and  is effected by lens
resolution, film grain, the arbitrary choice of acceptable circle of
confusion, and other factors.  It's measured in millimeters or less.  It
doesn't have an exact correspondence with depth of field, because it only
applies to a point, but in general factors that increase depth of field
tend to decrease depth of focus.
Equations for depth of field include distance to near and far point.
Ray gives the following equation for depth of focus; it doesn't seem right
to me, aperture should be in the denominator and f.l. in the numerator; but
it's a safe assumption that he's right:
depth of focus equals: [ 2 x (diameter of circle of confusion) x (lens to
image distance) x (aperture of lens) ] / (focal length of lens)

A large depth of focus gives you more slop room in your focusing, but also
makes it harder to focus.  A small depth of focus makes it more critical,
but gives you some snap when you reach focus.
It isn't thought of much in normal photography, because the film isn't
generally adjustable fore and aft, though when a lens focuses an image onto
the film, it is a similar situation (not identical, I don't think).  I
suppose you could swap lenses if you're having difficulty in focusing, to
using a short depth of focus lens.  Whether the Realist focusing system (by
moving the film plane) has any effect on lens design compared to moving
lens focusing is an interesting question.  

_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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