Someone mentioned maths and I just can't keep my mouth shut.
There were a number of points that were not raised in the discussion
that I think deserve mention:
1) because we're in the realm where mag factors need to be taken into
account, it is perhaps worth mentioning effective vs. relative aperture.
Even though you might (say) stop down 2 stops when going from an 8x10
to a 4x5 print, the act of focusing is going to move the lens further
from the neg which will result in a smaller effective aperture. This
effect is more pronounced with larger format negatives because we tend
to print things the same size regardless of format.
2) point light source stuff: there's 2 more complicating factors.
Firstly we *can* talk of point sources because we (usually) want each
point on the neg to be focused on a point on the paper. So there's no
question of the source not being a point. However the light source
(point, condenser, or diffuse), and the type of negative (silver or dye
image) will conspire to give you different patterns of light intensity
from the point source (on the neg). The latter issue may also render
your aperture less effective as a control (but you already knew that).
3) Reciprocity. Again 2 issues. The first is that you might require
longer exposure if your exposure is longer (you know what I mean).
However, highlights and shadows will be affected to differing degrees,
and the difference (you'll see it in the highlights probably) won't be
easy to fix with filtration. The best answer is to use a wider
aperture, but there are reasons why you might not want to do that too.
4) flare. It's possible that as you extend the bellows on your enlarger
to make a smaller print, that you get some stray non-image forming light
bouncing around in the bellows (My belief is that it's more likely with
the bellows extended further). I guess this is more theoretical than
any of the others as the effect of (3) is stronger, and this acts as a
slight mitigating factor, albeit one that is less predictable, and
variable across different images.
5) One other option available in some enlargers is to increase the
amount of light available. On mine I can have the light set to low or
high. It's rare to use low unless you're exposures on high are just too
short, however in theory, if you printed a small image using low, then
switched to high for a larger image, the effects of (3) may be
countered. In addition, if you're using an optimum aperture, you may
not have to change as far from it.
How's that, not one bit of maths! (Not one bit of an answer either)
Steve
