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Re: [OM] [OT] Questions for enlarging lens

Subject: Re: [OM] [OT] Questions for enlarging lens
From: Ken Norton <ken@xxxxxxxxxxx>
Date: Wed, 15 Apr 2009 09:00:15 -0500
There are four distinct ways of controlling the exposure of a sheet of
photographic paper:  Aperture, Time, Height and Development.

Aperture:  Controls the amount of light passing through the lens.
Time: Controls how long light will be passing through the lens.
Height:  Inverse-Square law.
Development:  Push/pull, etc.

Generally, you should have "Development" locked down to eliminate variables
there.  Inotherwords, in a calibrated darkroom system, you should be able to
develop the paper for exactly one or two minutes (Fiber is 2X the time of
RC) and only vary this for the rare problematic negative with a contrast
problem.

Height is a variable determined by aesthetics and print size. Rarely would
you adjust height after exposure determination. This is a "nailed-down"
aspect of the process. However....I'll get back to this in a moment.

Aperture is best determined by the enlarger height and the known highest
performance apertures. For example, when doing any form of dodging and
burning, I prefer to have my exposure times total around a minute. This give
me plenty of time to work the dodge/burn in a careful methodical manner. If
the print is a 5x7, I'll have to use a much smaller aperture than if I'm
making an 11x14.  The Aperture is just a means of getting myself "in the
neighborhood", but is not important for adjustment.  Once critical focus has
been attained, you stop down the aperture to the desired setting and verify
the focus again--most (all?) enlarger lenses will shift focus when stopped
down except when at the optimal magnification. (you may think that yours
does not shift focus, but when at the minimum/maximum magnifications there
is. I have yet to find an enlarger lens that doesn't--although most
focus-shift is extremely minor and corrected by the increase in DoF).

Time adjustment is the final control and must be the last adjustment made.
 You do your test strips, exposure readings and contrast corrections only
after Development, Height and Aperture are nailed down.  Once you have those
things nailed down you DO NOT touch them again--except under only under two
examples which I'll get to in a second.  For most timers, time is
essentially infinitely variable.  When working a 60 second exposure, if your
timer has 1 second granularity you won't get any closer than that. This also
brings up another point as to why my minimum exposure time is never less
than 20 seconds--enlarger light source variability. It takes no less than
five seconds for an enlarger bulb to stabilize at full brightness. If your
exposure time was short, you'll get large variabilities from one print to
another.

OK, so the point of all this is that Time is the "soft" variable in the
equation. This is the one variable which has essentially near-infinite
control.  If you have one soft variable, why have another?  Why is it
desirable to have an enlarger lens with clickless aperture settings?  Two
reasons:

1. Drydown compensation. If you are doing a complex exposure with multiple
timed dodges and burns, it is easier to make a minor exposure correction
with the lens than to adjust multiple exposure times.  This, however, is a
non-issue with a quality digital timer, like the RH Designs StopClock
Professional which allows you to make offset adjustments which correct all
child exposures.

2. An adjustment in Height.  The Ilford EM10 is a fantastic exposure meter
for this because you zero the meter on the starting height, raise/lower the
enlarger to the new height and then adjust the aperture to get the meter
zeroed again.  This is, by far, the quickest and easiest way to get multiple
size prints of the same negative.

As a general rule, I don't ever go mid-stop on an aperture. It's for too
difficult to achieve repeatability and makes for horrid note taking.  (F8
and a smudge?).   Frequently, I'll print a series of negatives from the same
roll where the exposure does not change at all. I will open up the lens so I
can see the framing better, then stop the lens back down x-number of clicks.
 One of my enlarger lenses doesn't click and I have to use my darkroom
flashlight (safelight red) to see how far I've gone. (Another one is backlit
when the enlarger bulb is on, so that's rather nice).

AG (in the dark) Schnozz
-- 
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