[Leica] Re: Darkroom Focusing

[kanner at acm.org (Herbert Kanner)]

This is, in part, a retraction of my previous post. It is also a 
correction of an error. In describing the central portion of the 
lens, I named the wrong F stop.

First, I want to say that my advanced physics degree is vintage 1951 
and that I gave up physics for computer biz in 1958. So, regarding 
geometric optics, it took a while before old memories filtered back 
into consciousness.

So, to simplify the complicated way I stated things earlier, trying 
to be accurate: a focus shift implies that the outer portions of a 
lens have a slightly different focal length than the central portion. 
OK, the correction for spherical aberration is supposed to prevent 
this. But, it doesn't. In a simple, one-element lens, the focal 
length changes monotonically as one goes from the center to the 
periphery. Now, aberration corrections are always a compromise. If 
there were no spherical aberration, a plot of focal length vs. 
distance from the lens center would be a horizontal straight line. 
The best correction that can be made is a curve that wiggles on both 
sides of this line. So, the average focal length of the central 
portion may differ from that of an outer portion.

Now, why do we stop down when enlarging? The main reason is to 
minimize the effect of the various aberrations. A secondary reason is 
to lengthen the exposure for a) accuracy and b) to facilitate 
manipulation such as dodging and burning in. Now, having expounded on 
the nature of the spherical aberration correction, I can state a 
theorem: the loss in sharpness caused by focusing at maximum aperture 
and then stopping down will be less than the loss caused by making 
the enlargement at full aperture.
-- 
Herbert Kanner
kanner@acm.org
650-326-8204

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