I think that Hollywood, whose exposures have to be correct, have T-stops incorporated into their lenses (taking into account all lens aberrations which compromise full accuracy). T- stops (transmission stops) are calibrated so as to give an actual doubling or having of light transmission with each step. F stops are supposed to also, but they are not perfectly matched to that precise requirement, although theoretically they are. Is there a reason why manufacturers opt for the somewhat less accurate f stop? - David Lyga
I own several cine lenses that are T-stop rated. T-Stop lenses have the aperture control on a ring on the lens barrel and it's "de-clicked", giving the glass a smooth step-less variable aperture (if you ever actually shoot with one, it's pretty awesome). From there, it's a simple calibration step at manufacturing time to measure the light transmission and put the markings on the lens barrel in the right places. The reason this is done is most cine lenses are primes (zooms exist, but are super expensive), and on set, once you have your lighting and exposure set up, changing lenses becomes super simple: make a note of the t-stop setting on the lens, put the new lens on and set it to the same t-stop setting. The exposure in the camera is exactly the same and you don't have to spend another block of time changing your lights to get the same exposure. Lots of lights used in cine land also come in standardized light outputs (e.g. a 1K light, etc.) it's simpler and easier to shoot with a matte box, take a light reading on the subject, then calculate how much ND you need to drop in your matte box to shoot at the T-Stop you want to shoot at. Again, when it comes time to change lenses, you don't have to go and re-figure the lighting back out.
It just makes it easier to set the lighting up where your ratios and dynamic range are what you want for the scene, and the overall brightness is a little on the bright side, and doesn't change from shot to shot and between lens changes, and you simply control your in-camera exposure and DOF with matte box ND and what you set your lens T-Stop to. Like lenses, good cine ND filters are calibrated, so 1 stop is 1 stop.
The reason for F-Stop ratings on most still cameras comes down to something very simple: electronic aperture control. It's reasonably accurate, and for a given lens and f-stop the DOF is accurate for that focal length. The downside, is the same f-stop on different focal length lenses don't transmit the same amount of light, though you can very accurately figure out what the DOF will be. On t-stop lenses they do transmit the same amount of light, but at the expense of not being able to calculate exactly what your DOF will be. For stills it's a big deal to have a razor thin DOF, and that has it's uses in motion picture land too, but the reality of the matter is, more often than not, the DOF is a lot larger than what you'd think it'd be simply because it's extremely difficult keep moving things in focus when your DOF is 1 inch. For motion picture work, you typically want to at least have all of a person's face in focus if not most of their head, but still shoot with a small enough DOF that you can do focus pulls between two people having a conversation, or pull focus to an object to emphasis it. It's not unusual to shoot a lot of stuff at T-4 to T-8 if you have enough light, and maybe get down to T-2.8 or T-2 for closeups and such (if the lens opens up that much).
