An exposure calculator is photographic film with a dense black image so dense
that no light will pass through the image. The emulsion companies offer
these exposure calculators with letters and numbers of different sizes.
The exposure calculator can be used two ways. One is to lay it over the emulsion
or capillary film, and then cover all of the exposure calculator, except a
narrow area like 1”, with something that absolutely no light can pass
through. This thin strip is then exposed for a limited time like 30
seconds. The light is turned off, and the cover is pulled back exposing
another strip. That is expose the same time which means the first strip
now has the exposure time from two light exposures. This process is repeated
until the last strip. If you believe the optimal exposure time is longer
than the exposure time of the first strip, then a time is selected for the
last strip that will result in the longest exposure being too long and the
shortest exposure time being too little exposure. When the screen is
washed out, you select the time in between that produces the optimal image
quality with the least time of exposure.