[pure-silver] Re: Magnification - Exposure compensation ruler
- From: "Nicholas O. Lindan" <nolindan@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Wed, 14 Nov 2007 09:57:29 -0500
The stops-ruler method
http://www.darkroomautomation.com/support/stopsruler1.pdf
can, of course, be used with other timers by
using an f-stop timing dial or stops table.
An example using a timing dial:
http://www.darkroomautomation.com/support/grastops1.pdf:
* Measure 3.6 as the image size using the stops-ruler
* Raise the head to the new magnification and focus
* Measure 2.0 as the new image size
* Correction is 3.6 - 2.0 = 1.6 stops increase to exposure
* GraLab pointer is at 3.2 (~9 seconds): 3.2 + 1.6 = 4.8 and
move pointer to 4.8 (~28 seconds)
An example using a stops table:
http://www.darkroomautomation.com/support/stopstable1.pdf:
* As above, measure original size as 3.6,
new size as 2.0, difference as 1.6.
* Timer is set to 9.2 seconds - read across on
table to 3.2 stops.
* 3.2 + 1.6 = 4.8. Look up time for 4.8 stops and
set timer to 27.9 seconds.
The above are more complicated than using an
f-stop timer but still easier than:
* Measure old size as 9 3/4" and new size as 16 7/8"
* The rest is left as an exercise for the reader... (try it)
Without a stops ruler, if you wanted the correction in
stops to open the lens, then the scientific calculator
comes out and log10 ((9 3/4 / 16 7/8) ^ 2) / log10 (2) ...
With a stops ruler the result is right there by subtracting
the two distances, 3.6 - 2.0, and opening the aperture
by 1.6 - 1 1/2 - stops.
Note the f-stop ruler's numbers decrease as the head
is raised, this is because the light, in stops, is
decreasing. As the light decreases by X stops the exposure
time is increased by the same value to compensate.
__________________
DARKROOM AUTOMATION
f-Stop timers - Enlarging Meters
http://www.darkroomautomation.com
Nicholas O. Lindan
Cleveland Engineering Design, LLC
Cleveland, Ohio 44121
----- Original Message -----
From: "Nicholas O. Lindan" <nolindan@xxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, November 12, 2007 10:25 AM
Subject: [pure-silver] Magnification - Exposure compensation ruler
Darkroom Automation's support web site has a file to
help determine the required exposure correction for
magnification changes.
The ruler measures square-law light fall-off in stops.
Simple subtraction of the new and old measurements
yields the required exposure correction - no dividing,
no squaring, no logarithms.
http://www.darkroomautomation.com/support/stopsruler.pdf
If great accuracy is needed then an exposure meter
should be used. Different enlargers require different
corrections for the non-square-law light fall off from the
condenser or diffuser to the lens - this is only critical
at small magnifications.
--
Nicholas O. Lindan
Cleveland Engineering Design, LLC
Cleveland, Ohio 44121
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