Most people don’t have a meter accurate enough to measure the light from and enlarger for sensitometric purposes. Also, no timer used for printing is consistent and repeatable enough. But you should be able to get an approximate value or at least a relative value.
How I determine the exposure for sensitometric testing is to first determine which step on the step tablet I want to produce a density that falls around the speed point. We know that the speed equation is 0.8 / Hm. For a 125 speed film, the exposure would need to be 0.0064 lxs.
Say I want to use the step that equals 2.60. Now all that is necessary is to use these three equations:
Transmittance = Transmitted light / Incident light
Opacity = 1 / Transmittance
Density = log Opacity
Opacity of a density of 2.60 = 0.0025
Required exposure at speed point for 125 speed film = 0.0064
Incident light = 0.0064 / 0.0025 = 2.56 mcs
Exposure meters want to produce and exposure at the film plane of 8 / ISO. For a 125 speed film that would be 0.064 mcs.
The difference between 0.064 and 2.56 is 40x or 1.60 logs or 5 1/3 stops. So, meter the enlarger light and open up 5 1/3 stops for this set of conditions.
I can’t tell you how well you will do working with an exposure meter and an enlarger because I use an EG&G Mark VI Sensitometer that was calibrated by EG&G (many years ago). But I can tell you from personal experience that the above approach calculating the exposure required to produce 0.10 density on a specific step on the step tablet works perfectly well.
Last edited by Stephen Benskin; 07-12-2012 at 10:00 PM. Click to view previous post history.