Quote Originally Posted by polyglot View Post
However I recall concluding from that discussion that I needed a flashmeter reading of about f/16 at the film-plane.
I think I wrote that... That's what the EG&G does bare bulb at the film plane. Then you are supposed to add lots of Neutral Density because that's too much light for a 400 speed film. Rafal reduced the light 8 stops to f/1.0 and then a bit lower.

Calculating Neutral Density doesn't involve difficult math. Because the numbers are logarithms, you add or subtract them. Every 0.3 density is a whole stop. Step wedges with 0.1 density and 0.15 density intervals have steps that are 1/3 stop and 1/2 stop respectively.

You don't necessarily have to take the antilog to find the actual light in meter candle seconds - you are welcome to do that ... it helps to mentally see the whole picture ... but for practical purposes just count steps and work in stops.

All that heavy math of meter calibration and optics comes when you try to relate the film tests to light meters and cameras calibrations... If you just want an exposure index and curves, you are going to get that without any of those calculations.

For the film tests, the only math you need is addition and subtraction, multiplication and division. Make a wild guess and expose a sheet of film through a step wedge and then develop it.

If you did not give enough light, the high numbered steps will all be clear film. Count how many wasted steps you have (how many clear steps). Multiply by 0.1 or 0.15 (depending on your step wedge steps) and divide by 0.3 - That's about how many more stops of light you need to hit the film with. Same goes for overexposure, try to guess how many steps down you would have to go to hit clear film if every step is too dark. You could graph it or compare it with other tests, and get pretty close. Ideal is when you have one, two or three clear steps in the high numbers 21, 20, 19 - and then you hit 0.1 density on your film test with a step near 19. You don't aim for exact - you aim to get it on there.