>but I wonder if Mike Ware's Euclidean geometry isn't a bit too simple. It
>assumes that a single point in the negative will have a very large circle
>of confusion caused by the dimensions of the area covered by his bank of
>tubes. But since the inverse square law kicks in here...
My admittedly simplistic calculation was only intended to highlight the
problem, and to get us to the starting line in discussing this issue
quantitatively. Certainly the inverse square law will diminish the
intensity of the 'unsharpening' light: a point on the print directly
opposite the centre of the tube will receive light from the ends of the
tube only 1/5 as intense as that from the centre - assuming my
'sake-of-the-argument' dimensions (32 cm long tube, 8 cm away) but this
still seems to me a significant contribution. However, as Philip goes on to
point out, there are many other potential causes of unsharpness:
>Is it possible there's some diffusion going on in the paper itself?
Certainly light-scattering will worsen the unsharp effect. It will depend
inversely on how strongly the sensitized paper absorbs the light in the
first place: I'm not sure how to make a meaningful calculation of this
contribution, and there's not much one can do about it anyway!
>Perhaps the light is even bouncing back up into the sensitized portion of the
>substrate after being reflected from the back of the printing frame.
Like halation effects in silver-gelatin emulsions? Agreed in principle; the
importance would again depend on the light absorbance of the sensitizer,
depending, in turn, on its chemical composition, concentration, thickness
and the wavelength of the light. Some calculations I've done suggest that
average alt-proc sensitizer coatings do absorb most (>90%) of the actinic
light falling on them in one pass through their thickness, so there's not a
lot left to be bounced back. I think in silver-gelatin emulsions the
absorbance is much less, so anti-halation backings are important there.
>Does the thickness of the paper or a dark backing make a difference?
It costs little to include a backing of black paper, plastic or rubber
sheet in the printing frame - I do for peace of mind, but I've never tested
if it has any visible effect. Wear a belt and braces!
>Or is it possible the lack of sharpness is caused by the paper fibres swelling
>>on wet processing and not necessarily drying down with a 0.15 mm point
>perfectly resolved in exactly the same position?
When you consider that the overall dimensional change of a paper after a
cycle of wet processing is usually less than 1% it doesn't seem that this
effect can make a significant contribution to a *local* loss of resolution.
(Even tho' it does cause a problem of re-registration in multiple
printing). A more likely cause, it seems to me, is the diffusion of image
substance during wet processing - especially with processes where there is
no binder: e.g. Prussian Blue in the cyanotype is quite mobile, because of
its very small particle size: you can even wash it out, as many of us know!
>It might be worth
>undertaking a test or two contact printing a negative of a lens test chart
>on conventional photographic paper or printing out paper. It shouldn't be
>too hard to see if there's that much difference between contacting
>printing normal photographic paper under a fluorescent fixture or a single
>incandescent bulb (the theory should still hold), even in those bleak
>sunless northern lattitudes you have to endure this time of year.
Thanks for the suggestion, Philip! I won't even have to black-out the
darkroom. But maybe someone on the List has already performed such tests? I
think the most important - and avoidable - cause of unsharpness is often
poor contact between neg and paper, and the bigger and closer the light
source, the more this will be magnified. N'est-ce pas... Judy?
Mike