P3D Optimum Sharpness

[John Toeppen <toeppen@xxxxxxxxxxx>]

Ooppsss. thanks for the reposting Bob....

The f# of a lens is the ratio of the the effective stop diameter to the
lens focal length.  The physical diameter of the f-stop is proportional
to the focal length of the lens by definition. Long lenses have larger
apertures for the same f#.

The 1.22 lambda (wavelength) formula is for the half angle (the angle
between the axial ray and the marginal ray). The f# is effectively is
the angle between the two marginal (outer edge) rays.   
2.44 x wavelength x f/D is effectively the same angle dependant
calculation.

Ultimately, it is the focusing cone angle which determines the focus
spot size.  Where you catch that cone determines how large of a spot you
get. Diffraction allows small focal spots only when the rays converge at
a high angle.  Such a cone is large on either side of that point. 
Critial focus is achieved at the price of shallow depth of field. Most
films barely resolve critical focus anyhow (Tech Pan will).

Film can't record resolution that a lens does not produce.  Nor can a
fine lens produce a fine image on large grain film.  Each stage in the
photographic process is a link in the chain.  The weak link is the limit
of the entire process. 

Current measurement techniques measure contrast and resolution factors
at the same time.  I use a Matrox Meteor frame grabber and Inspector
software in conjunction with a CCD to do this stuff these days.  Doing
this kind of work in film with a microdensometer to measure MTF was much
more difficult.  Why, now you just flip a dip switch to linearize your
gamma!

John Toeppen
http://home.pacbell.net/toeppen/


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