Hi Rob
I don't know why my response went to you off-list,
and I got your name wrong (sorry about that),
Repeating the parts that may be interesting to
others -
I said :
I was suggesting a possible connection between
Ctein's reports on his
attempts to get sharp focus on vc paper under an
enlarging lens and Bob's
plans to use enlarging paper as a negative in his
view camera. Ctein may be
out on his own, I don't know (interestingly
his book describes and allows
for the "Gainer effect" noted by Don
Bryant). But surely the
common
experience of pinhole camera users using paper negatives can give us
no
guidance at all on this or any other question about
focus.
You said:
The key is that when selecting pinhole size, people
typically use Rayleigh's equation which is optimized for a single
wavelength. When using photo paper then l = 550nm is pretty
common. No serious problems with "focus shift" if you are designing for a
single wavelength, especially if it is near the peak of the paper's spectral
response.
I think the focus shift people are referring to is a function of
the glass, coatings and mirrors used in the enlarging lens and focusing
tools. No mirror in the lens, just the tool, obviously. The
formation of the areal image seems to be at issue.
To a first approximation,
the smallest detail you can resolve is something like 1/2 your pinhole diameter
(working from memory so could be wrong), neglecting fringing effects. In
practice I've found it to be somewhat "worse" but a well made hole in the
thinnest stuff you can find works VERY, VERY well.
To which I say:
Ctein reported the peak spectral
response of the Kodak vc paper he used was about 420nm. Would the difference
between 420nm and 550nm make much difference to the conventional values
derived from Rayleigh's equation?
On the original topic of effective speed, I
stumbled upon this link suggesting pre-flashing (in the darkroom) to improve
shadow detail
Regards
Don Sweet