Calculating Exposure Time for New Enlargement Size

Exposure time is, of course, dependent on magnification.  Very often I make 8 x 10 work prints until I decide the image deserves to be printed in a larger size.  Rather than start over completely, I try to use information gained from making a smaller print to narrow down exposure and then make the necessary refinements.   However, I think it is almost impossible to go directly from one print size to another without some trial and error.  For example, I almost always make small adjustments in contrast, increasing it as I increase print size and vice versa.

The first step is to estimate the amount of magnification in the original print size and the new print size.  Unless considerable cropping was involved, one can make a reasonable guess by dividing a film dimension, or better yet, a negative carrier dimension,  into a print dimension.   You  could measure the dimensions of your negative carrier or use the rough dimensions of some common film formats shown in the table below:
 

Rough Dimensions of Common Film Formats

Format	Short	Long
35mm	24mm	36mm
645	43mm	56mm
6 x 6	56mm	56mm
6 x 7	56mm	67mm
4 x 5	90mm	120mm

The javascript calculators below will provide a decent estimate of magnification if you can enter an uncropped film dimension along with its corresponding print dimension.  The first table is for those that measure prints in inches and the second for those that use centimeters.

As an example, let us take a 35mm negative that has been enlarged full frame  to make a 8 x 12 print.   If we enter 24mm as the film measurement and 8 inches as the print measurement the magnification will be 8.5.  The same answer would be obtained if we enter 36mm and 12 inches.  However, if we made a 8 x 10 print by cropping the long dimension, we would want to use only the short dimension to calculate magnification.

Exposure Change
Original Time
Original Magnification
New Magnification
New Time

As an example of calculating a new time,  let us assume that we have made an 8 x 10 enlargement from a 35mm negative and that the short dimension was not cropped and, thus had a magnification of 8.5.  Also, assume the original printing time was 16 seconds.  We now want to make a 11 x 14 print.

If we enter 24 and 11 into the top table, we will get a (New) magnification estimate of  11.6.  Now we are ready to calculate a new exposure time.  In the bottom table enter 16 as the original time, 8.5 as the original magnification and 11.6 as the new magnification.   An extimate of the new time would be 28.4 seconds.

Remember, this is only a starting point.  Hopefully, you will be able to get to a final print a little more quickly or with less paper waste, but this will only provide you with a rough estimate.  If you increase print size, as is the usual case, the printing time will most likely be longer.  Photographic paper decreases in sensitivity at lower levels of illumination.  Also, I use a color head with variable contrast  black and white paper and generally add a little more magenta to increase contrast which wil , in turn, also increase exposure time.

All Text and Images © Joe Miller, 2004