View Full Version : Calculating new exposure for new print size

Don Wallace
29-Mar-2007, 08:50
I know that this topic has already been discussed, but I am hoping this time, someone will have a fairly clear answer, if in fact there is one.

How do you calculate the change in exposure for a change in print size? The inverse square method just doesn't work. I tried the formula in Tim Rudman's book, "The Photographer's Master Printing Course," and, nope, no go. It wasn't that it was a little off. It was WAY off.

I have been using the old Kodak exposure wheel and it actually seems to work but it is not great for fine tuning. I would like something just a little more accurate.

Just in case it matters, I am printing with a DeVere 504 dichroic head.

29-Mar-2007, 09:08
You can use a light meter or better darkroom analyzer.

domenico Foschi
29-Mar-2007, 10:14
I do use Tim Rudman's method, and I find it works.
Keep in mind that batches of paper are different one from the other and the formula Rudman's offer is not a magic bullet: at the end you will always have to make your own adjustments.

Silver is not a completely predictable medium as you know.

Don Wallace
29-Mar-2007, 10:55
I do use Tim Rudman's method, and I find it works.
Keep in mind that batches of paper are different one from the other and the formula Rudman's offer is not a magic bullet: at the end you will always have to make your own adjustments.

Silver is not a completely predictable medium as you know.

Domenico, this was paper from the same box. I checked and double-checked my numbers, but the enlargement was almost black. I will go back and check again, just in case I was having a bad day. It is good to hear from someone who has used this method that it does work. I also have an Ilford EM10 that I have never used. Maybe it is time to learn how.

Doug Herta
29-Mar-2007, 15:54
I use the EM10 for Ilfochrome as well as making enlarged negatives on lith film. It works within a small range of print sizes if you are changing the aperture on your enlarging lense to compensate for exposure.

If I know ahead of time that I will be making significantly larger prints, I dial in all three colors on the dicro head the same amount when making the smaller print as a neutral density filter. I incrementally open up the same amount on all three color channels when making a larger print and measure the light intensity with the EM10 to keep the exposure time the same. This maintains an optimal aperture for the lens and the same exposure time. It works well for me, but if others have tips and suggestions it would be good to hear.


Michael Gudzinowicz
29-Mar-2007, 17:21
This is one of my old rpd posts. It doesn't include reciprocity failure, but for
LF's somewhat limited print magnifications, that isn't a problem.

Suppose that your 15 sec, 8x10, was an 8X enlargement (old_M = 8) from a 35
mm negative, and the new magnification (new_M) would be 11X, for an 11x14
(printed so the 24mm dimension of the negative just fits the paper; the
long dimension is cropped). The exposure factors are equal to the
magnification plus one, squared, so:

new_time old_time
------------ = ------------
(new_M +1)^2 (old_M+1)^2


new_time = old_time x (new_M +1)^2 / (old_M+1)^2


new_time = 15 sec x (11+1)^2 / (8+1)^2 = 15 x 1.777 = 26.7 sec.

To simplfy matters, you can place a masking tape scale on the baseboard so
overall magnification can be read directly, even for cropped prints (use
overall magnification, not the cropped print dimension).

Using a spreadsheet, you can indicate new and old magnifications in a
lookup table, with the intersection formula equal to the ratio of
magnification plus one squared, as above. For magnifications from 4X to
12X, the table would be:

Old New Magnification
4 5 6 7 8 9 10 11 12

4 1.00 1.44 1.96 2.56 3.24 4.00 4.84 5.76 6.76
5 0.69 1.00 1.36 1.78 2.25 2.78 3.36 4.00 4.69
6 0.51 0.73 1.00 1.31 1.65 2.04 2.47 2.94 3.45
7 0.39 0.56 0.77 1.00 1.27 1.56 1.89 2.25 2.64
8 0.31 0.44 0.60 0.79 1.00 1.23 1.49 1.78 2.09
9 0.25 0.36 0.49 0.64 0.81 1.00 1.21 1.44 1.69
10 0.21 0.30 0.40 0.53 0.67 0.83 1.00 1.19 1.40
11 0.17 0.25 0.34 0.44 0.56 0.69 0.84 1.00 1.17
12 0.15 0.21 0.29 0.38 0.48 0.59 0.72 0.85 1.00

Bruce Osgood
29-Mar-2007, 18:01
Based on the Inverse Square Law as explained by NASA ..... http://quest.arc.nasa.gov/pioneer10/education/temp/

I was able to develop an Excel sheet that calculates the new exposure time based on the amount of change in the elevation.

Where NEW elevation is 30 and OLD elevation is 20 (with 12s exposure):

ELEVATION (units can be anything)
(new/(new-old))^2 X TIME = NEW TIME

30 10 2.250 X 12s = 27s

I use this when going from 8X10 to 11X14.

These lines appear to clog to the left when previewed, I don't know how to handle that, sorry.

29-Mar-2007, 18:24
I measure something on the first print that is 1 inch long. then measure that same thing at the new magnification.

old time = new time
-------- ---------
1 squared new measurement squared


old time X new measurement squared = new time

e.g. 4X5 to 8X10

15 sec = new time
------- -------
1 inch squared 2 inches squared

15sec X4=60sec
but different size papers are different speeds even for the same brand.
and reciprosity ( or the fog of war ) always make minor testing necessary.

29-Mar-2007, 18:28
my spacing got killed

old time divided by one squared equals new time divided by new measurment squared

Michael Gudzinowicz
29-Mar-2007, 18:40
An enlarger is nothing more than a macro camera that takes a "photo" of the negative or chrome in the carrier. The macro formulas for extension and effective aperture apply.

28-Jun-2007, 14:30
I thought you'd just take the area of the new size divided by the area of the old size to get an exposure factor.

E.g. going from 8x10 to 11x14:

11 * 14 = 154 square inches
8 * 10 = 80 square inches

154 / 80 = 1.925

So if your original exposure was 15 seconds for an 8x10, you'd have 15 * 1.925 = 28.875 seconds for the new exposure. Ideally you'd correct it for the aspect ratio differences and cropping (so for 8x10 you'd actually use 11.2 x 14 as your enlarged size to keep the ratio the same).

Greg Lockrey
28-Jun-2007, 15:24
:eek: I hate shortcuts, they never seem to work.:eek:

I call this the Calibrate Your Center Column Method.

If your negatives are "properly exposed" a quick trick would be to mark exposure times on the center column of your enlarger for each print size and/or crop position. Then in the future once you have the correct exposure/f-stop combonation for your given size, moving up or down the column would be a mere adjustment of the time holding the f-stop as the standard. Bare in mind that exposures are affected by lag time that the light comes on/off from the timer and the voltage of the light source. But usually it's close enough if you adjust your development times....and that's another issue too.

A good way to set your standard time is use your Zone II calibration negative and set the times for each print size from that negative developing them the same amount of time to match the Dmax. Don't have a meter? Use a bright light.