• Phil Davis has a really nice approach using an enlarger. I use a calibrated sensitiometer, so my approach is slightly different, but fundamentally they are the same. You need to determine much light is required to produce the required amount of exposure through a chosen step tablet density to produce a target density.

The speed equation is 0.8 / Hm (exposure in mcs at the point where the film has a density of 0.10 over Fb+f). For a 400 speed film, the aim exposure to produce a density of 0.10 over Fb+f is 0.8 / 400 or 0.0020 mcs. Generally you want to have this fall on the third or fourth step of the step tablet (0.15 step). The step tabet’s D-Max is 3.05 – 0.45 (3 steps) = 2.60 density. The equation to find Transmittance is:

Transmittance = Transmitted Light / Incident Light

Converting the equation to find Incident Light:

Incident Light = Transmitted / Transmittance

We already have the required transmitted light for a 400 speed film – 0.0020 mcs

Transmittance is the reciprocal of opacity or 1/ 10^density: 1/10^2.60 = 0.0025

0.002 / 0.0025 = 0.8 mcs

I like to use footcandles to measure incident light so for a shutter speed of 1/125 your will need:

(0.8 / 10.76) * 125 = 9.29 fc.

9.29 fc * 10.76 (convert to metercandles) * 1/125 (shutter speed) = 0.8 mcs

Transmitted light = Transmittance * Incident

.0025 * 0.8 = 0.0020

Equation for film speed 0.8 / Hm (mcs at 0.10 over Fb+f)

0.8 / 0.0020 = 400

For a 400 speed film exposing a step tablet with 9.29 footcandles for 1/125 second should produce a exposure through the 2.60 density step of 0.0020 producing a density of 0.10 over Fb+f.

You can use the reflected meter’s user manual to calculate how to determine footcandles or meter candles using the meter and a gray card.

I’m terrible with unit conversions, so I hope I got these right.