Now we have to get the log exposure for each
step so that we can graph the characteristic curve for our
film sample. We can measure the amount of light that the
sensitometer is putting out by using a lux meter (a light
meter that measures lux. Lux and metre-candles are
the same thing, but lux is the preferred term). So we try
this and we get a reading of 100,000 millilux. The
instruction manual for the sensitometer tells us that the
exposure time is 1/5 of a second.
Exposure = Illuminance (in millilux) x Time (in sec)
Exposure = 100,000 x 1/5
Exposure = 20,000 millilux-seconds
Now we take the log of our exposure, 20,000
millilux-seconds, and get an answer of 4.3. Our log
exposure is 4.3.