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

Subject: Re: [OM] [OT] Questions for enlarging lens
From: Michael Wong <mialop@xxxxxxxxxxxx>
Date: Wed, 15 Apr 2009 23:37:35 +0800
AG,
Thank you very much! Very details. I have learned very much :-)


---
Michael



2009/4/15 Ken Norton <ken@xxxxxxxxxxx>

> 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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