Here's my problem. I have an enlarging wheel, came in a Kodak photography manual, which is just fine except that I now have some 16x20 paper and the wheel only goes up to 10x enlargments. Is there an enlarging wheel for big prints or a formula I can use to calculate the correct exposure for giant prints once I know the exposure for smaller prints?

-- Jeremy illingworth (zoltan@yield.com), January 13, 1998

Answers

Enlargers follow what is known as the "inverse square law". It works like this:

Measure something on the old enlargement. This could be the length of the longest side, or the shortest side, or the magnification.

Mesure the same thing on the new enlargement.

Divide the new by the old.

Square it (i.e. multiply it by itself).

Multiply the exposure by the result.

For example: Old print longest side is 8 inches. New print longest side is 20 inches. 20/8 = 2.5. 2.5 squared is 6.25

If the old exposure was 10 seconds, the new one would be 62.5 seconds.

Hovever, photographic materials suffer from "reciprocity failure", which basically means that as exposures get longer, they need even more. It's best to use the new exposure as a starting point for a test strip. In this case, I would try 60s, 90s, 120s, and 180s, and compare the test strip with the original print.

-- Alan Gibson (gibson.al@mail.dec.com), January 14, 1998.