Re: ATM Focus dept

[Anthony Stillman <atmer@flash.net>]

Marco,

I'm assuming you are asking about depth of focus.  To my knowledge there is
no simple formula relating reduction in resolution to defocus. Thanks to
the needs of micro-lithography however, this is a well studied field so,
should you want, you can saturate yourself with information. Several
articles in "Applied Optics" and "Optical Engineering" come to mind.

A simpler solution is to use a ray tracer that models the MTF of a system,
or the spot size.  Enter the model, then defocus it until you can't stand
what you see.

An even cruder method would be to think only of the greastest cross section
of the light cone.  An ideal 20mm f/5 lens focuses every thing to a point,
at the focal plane.  Zero distance from the element the light cone is 20mm
in diameter.  One hundard millimeters from the element, at the focal plane,
the light cone is 0mm in diameter.  One millimeter from the focal plane the
cone is (1/100 ) * 20 millimeters in diameter  (0.2mm).  That is, cone
diameter = defocus distance / f ratio.  The generally accepted tollerable
spot size for photography is 0.025mm.  Using this approach and assuming a
perfect f/5 system the range of defocus would be (defocus/f_ratio <= 0.025)
+-0.125mm (125um).  For an f/8 +- 0.2mm (200um).

In practice, the specifics of the system will either increase or decrease
the above numbers as residual abberations come into play.

Anthony