Re: STARTING POINT
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.
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
----- Original Message -----
From: "Robert Young" <email@example.com>
Sent: Wednesday, April 23, 2008 2:33 AM
Subject: RE: STARTING POINT
> > Hi Bob
> > Excuse me joining in late from the antipodes.
> > You may perhaps have a focus problem using variable contrast enlarging paper
> > in a large format camera. Ctein reports in Post Exposure that some vc
> > emulsions are extraordinarily sensitive to the uv end of the spectrum which
> > the human eye can't see to focus. Of course he was testing with enlarger
> > lenses, which may perform differently from camera lenses, but he found the
> > plane of focus for uv was as much as 12mm away from the visible plane of
> > focus at about 10x enlargement for the Kodak paper he was using.
> > If this is a problem then I guess a uv filter would solve it, but at the
> > expense of a good deal of speed because it seems the paper has little
> > sensitivity above green.
> If this was a SERIOUS problem, then most of the images found in the B&W gallery over at www.f295.org wouldn't exist and the general consensus as to paper "speed" as a film wouldn't exist.
> Ilford MG IV RC works fine. With or without a 0 or 00 printing filter to lower contrast or a #15 camera filter but at the expense of "speed". You can also pre-flash the paper as 0 to help with contrast control. Also used lots of Arista EDU.ULTRA VC RC (because it was cheap to play with) and it worked fine, it is one of the Foma papers, can't remember which exactly.
> Graded paper works too. Lately been using the Arista.Ultra VC Graded #2 (again, cheap and made by Foma) and that works very well with a preflash.
> Also used lots of HP5+ in Rodinal and Pyrocat for when I wanted both sky and shadow (obviously ot photo paper so off-topic, but it works great).
> The papers are quite UV sensitive and so sky tend to blow out if you are exposing for good shadow detail. Likewise, urban scenes with lots of concrete (reflected UV) tend to burn out for the same reason. Or you can expose for sky and get good detail there at the expense of shadow detail elsewhere. Same old story for ortho emulsions since before Brady was sniffing the ether.
> Short answer, just go forth and experiment. Know that your contrast range will be compressed relative to B&W film. Know that you will have a bit of a soft focus (but you can control that a bit with good pinholes and careful camera contruction) and that there will be some chromatic distortion. It is tough to make an APO pinhole after all...