


Hello,
I use the enlarging dial in my Kodak Darkroom Data Guide. There is no math calculations involved. A ruler is the only other tool involved. Very simple. There is also a exp correction for using polycontrast filters. Since Kodak seems to be turing it's back on us, they probably don't publish them now(would be nice to be wrong). If you can at least look at a copy, you could make a chart for future reference. If I didn't have a Kodak Data Guide, I'd do the math and make a chart.
Cecil

This being one of the more thorough discussions of this topic, let's make it a sticky thread.
My personal take on it:
Recognize these numbers4, 5, 8, 11, 16? Both the standard sizes (4x5, 5x7, 8x10, 11x14, 16x20) and approximately the f:stop series. One stop more exposure for every standard increase in size.

CORRECTION:
It's been so long since I have written about the ruler, I forgot how to use it.
You use it to measure lenstoimage distance, not the size of the image on the baseboard.
The following are experimental results, Beseler 45 series enlarger, condenser lamphouse, 150mm lens.
Exposure adjustments as measured with a Darkroom Automation meter, the DA exposure compensation ruler and by applying the "inverse square law" on the image size.
11 mm base image size
New size.Metered.......DA ruler......Image size square law
21 mm  1.11 stops  1.30 stops  1.8 stops
31 mm  1.97 stops  2.00 stops  3.0 stops
45 mm  2.75 stops  2.81 stops  3.9 stops
45 mm base image size
New size.Metered.......DA ruler......Image size square law
31 mm  0.78 stops  0.81 stops  0.95 stops
21 mm  1.64 stops  1.51 stops  2.15 stops
11 mm  2.75 stops  2.81 stops  3.9 stops
Obviously, the most accurate method is a meter. The stops ruler will get you within better than 0.1 stop for magnifications of 2x or more and within 0.2 stops down to 1:1.
Gadzooks, just looked at the ruler pdf, and the instructions on it are wrong  cross out 'image size'  it only works for lens height.
Last edited by Nicholas Lindan; 12122008 at 05:47 PM. Click to view previous post history.

There was a thread about this a while back. Now that this new thread is a 'sticky' it needs to come to the correct conclusion.
The equation I use is:
new_time = old_time x (new_M +1)^2 / (old_M+1)^2
where M = new magnification (print/neg) and m = old magnification (print/neg)
The exposure time factor would be:
Factor = (M + 1)^2 / (m + 1)^2
I wonder if Nicholas could supply the magnification in the above examples. I would like to see how this equation (converted from 'factor' to stops) compares to the metered values measured by Nicholas.

Originally Posted by icracer
I wonder if Nicholas could supply the magnification in the above examples.
The negative carrier size is 9.2mm. The projection of the carrier outline was measured, this contributes somewhat to the error as the carrier is 3 dimensional and the lens was focused on the negative in the middle of the carrier.
Going from a negative size of 9.2mm to the following sizes, the exposure factor correction in stops is:
Size......Magnification...Metered...Formula...DA Ruler
11mm ...1.2
21mm ...2.3...............1.1..........1.1.........1.3
32mm ...3.4...............2.0..........2.0.........2.0
45mm ...4.9...............2.8..........2.9.........2.8
The ruler and the formula should provide identical results as the ruler is based on the same optical formula  I am not sure where the discrepancy arrises at the 21mm image size. Apart from that they all seem to agree within experimental and rounding error.

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Just another thought that came to me. There are rulers printed on clear plastic (CThru makes nice ones in the US) that could be substituted for the negative at both degrees of enlargement, then magnification changes could be easily calculated against a ruler on the easel. Might be quick enough for those without an enlarging meter.
Lee
I may have preanswered Dan's simultaneous post.

Originally Posted by icracer
I wonder if Nicholas could supply the
magnification in the above examples.
M&m; magnification. And how are the values for
the two established? I see that a ratio of squares
is involved. I'm suspicious. A "...correct conclusion."
is at stake. Dan

OMG! :o
Make another test strip ...

Originally Posted by David Brown
Make another test strip.
I just made and evaluated a test strip and it took 4 minutes, 45 seconds, 2 minutes of that time were in the developer.
Using the ruler took 30 seconds. Used no chemicals, paper or electricity.
Add 15 seconds if you don't have an fstop timer and need to use a chart.
Saves yer money  saves yer time  takes yer choice.
A note on using Darkroom Automation's stops<>seconds chart: If you are only applying a stops correction to a time you can simply multiply your present exposure time by the number in the chart.
As an example:
 your present exposure is 5 seconds and you want to add 1.4 stops
 look up 1.4 stops in the chart and find 2.6
 multiply 5 seconds by 2.6 => 13 seconds is the new time

Originally Posted by Nicholas Lindan
The negative carrier size is 9.2mm. The projection of the carrier outline was measured, this contributes somewhat to the error as the carrier is 3 dimensional and the lens was focused on the negative in the middle of the carrier.
Going from a negative size of 9.2mm to the following sizes, the exposure factor correction in stops is:
Size......Magnification...Metered...Formula...DA Ruler
11mm ...1.2
21mm ...2.3...............1.1..........1.1.........1.3
32mm ...3.4...............2.0..........2.0.........2.0
45mm ...4.9...............2.8..........2.9.........2.8
The ruler and the formula should provide identical results as the ruler is based on the same optical formula  I am not sure where the discrepancy arrises at the 21mm image size. Apart from that they all seem to agree within experimental and rounding error.
I have been following this thread with interest and thanks for posting.
I may have missed something but you are talking here of images in millimetre sizes and this seems really small, can you elaborate please?
Regards
John

