The F number is a ratio calculated purely from the focal length of the lens and the size of the aperture. Any light meter talks in terms of F, Shutter, and ISO.

Different glass, or just different number of elements, is going to change the number of photons that get though the lens; we compensate for ND filters by adjusting F. What am I missing? Why aren't t stops part of exposure calculations?

In other words, I know what T numbers are, and how they differ from F-stops. What I don't know is why handheld light meters and related tools have no provision to take T into account, but talk only in terms of F.


It IS part of the exposure calculation in fully professional cinematography; these folks prefer lenses marked in T-stops.

A full F-stop is actually a factor of two (or one half) in light transmitted, while the typical transmission losses of a lens are in the range of an eight to a quarter stop. This makes transmission nearly irrelevant to correct exposure, especially given that cameras well suited for exposure-critical media (digital sensors and slide film) are usually of the TTL-metering variety that will automatically compensate. The difference between two lenses DOES become important if exposures from two different lenses will be made part of one visual work (eg a cinematic film).


Sometimes it is. But for most photographic purposes, it doesn't make that much of a difference. In special cases when it does, it usually is part of the calculation.

Even a highly complex zoom lens with many elements, such as the Canon EF 24-70mm f/2.8L II with 18 elements in 13 groups is T2.8 at 24mm, T2.9 at 35mm and 50mm, and T3.1 at 70mm. At worst, the T-stop is within 1/3 stop of the f-number. The smallest exposure increment most modern cameras use is 1/3 stop, so the difference between the f-number and the T-stop is no greater than the difference between using 1/100 sec or 1/125 sec, between ISO 200 or ISO 250, or between f/2.8 or f/3.2. No matter how accurately we can measure for exposure, the smallest increment of exposure we can actually set on our cameras is 1/3 stop. Even that is an approximation.


Besides the fact that lenses are not perfectly transparent, there are umpteen more negative factors that impede any instrument from proclaiming the “correct” exposure. Naming a just a few: activity of the developing process, color of the ambient light, medium sensitivity to ambient light, variability of the ISO of the medium, inaccuracy of shutter speed, inaccuracy of iris diaphragm diameter (f-number), and more. The bottom line, for absolute accuracy, one must run tests specific to the camera and the medium under controlled conditions. In other words, likely your exposure will only be an approximation. When the chips are down, we bracket our exposure and hope for the best.

Not the answer you're looking for? Browse other questions tagged or ask your own question.