• Originally Posted by polyglot
I'm planning on doing a bit of sensitometry and am having trouble finding a formula that will tell me how much light a lens transmits, in compatible units at each side. The reason I want to know is that I want to calibrate my (flash) sensitometer using a flashmeter located where the film-under-test will be.

Consider a simple imaging chain: light source, subject, lens and sensor. I take an incident meter reading at the subject (Es) and an incident meter reading at the film plane (Ef). What is the ratio of Ef/Es as a function of the lens' aperture?

The only reference I could find is this one for CCTV sensitivity analysis, and it says:

Ef/Es = 0.2 * t * R / (N^2)

where t=lens transmission (any non-ideal filter-factor built-in to the lens due to imperfect coatings etc), R = scene reflectance and N = f-number. The presence of that 0.2 is at best highly unsatisfying. Given that their example has R=0.89, would I be right to suspect that the 0.2 is a poor approximation of the 18% reflectance standard therefore R=1 or R=0.9 for an 18% card? Or should I assume that if the subject is an 18% card and the lens is ideal, Ef/Es = 0.036/(N^2) ?

Any better references welcome...
I think the question comes down to how do you meter at the film plane in a camera with a meter designed to measure light falling on the subject. That question is answered with the light meter calibration formula that has these variables:
b = Constant with units of lx⋅cd–1⋅m2
θ = Angle between subject and lens axis
N = Relative aperture (f-number) of lens
F = Lens flare correction factor
f = Focal length of lens in m
V = Lens vignetting factor
Ls = Luminance of subject in cd⋅m–2
T = Lens transmittance factor
u = Subject distance in m

However, if your flash meter can read directly in Lux-second then it takes out all the above variables and that is all you need.

Personally, I 'calibrate' my sensitometer with some known film, like Ilford D100. There is a precedent in the literature on using film for this type of calibration.
Also, did you see the lengthy discussion about Bill's EG&G sensitometer and how he was able to check it with a flashmeter?