t vs f stop

Elie Kamal · August 20, 2007

hey there,

my name is elie and i'm a student filmmaker. this is my first post so take it easy on me...hehehe

well this is a very classical and FAQ for which i haven't got any convencing answer yet.

if T stop is more accurate than F stop then why use a light meter and not a spot meter all the time?

and if my question is not very accurate would you please explain to me "in a convincing way" the difference between Mrs T and Mrs F?...

thanks a lot

regards,

Elie Kamal.

David Mullen ASC · August 21, 2007

A spot meter is a light meter -- you mean why use an incident meter instead of a spot meter. Which just depends on whether you want to measure the amount of light falling on an object (incident) or the amount of reflectance/luminence the object has (spot).

An f-stop is a physical size, the number determined by a ratio between the length of the lens and the size of the aperture. A T-stop is just an f-stop mark that has been shifted over on the barrel to compensate for any light lost as the light passes through the lens. In other words, for example, they measure the amount of light coming through the aperture that should be f/2.8 on their meter, but find it is one-third of a stop less than that, so they move the marking for "f/2.8" on the barrel so that you actually get f/2.8 out of the back end of the lens. But having adjusted the position of the f-stop mark, they can't really call it an f-stop anymore, so they call it a T-stop ("transmission" stop.)

So if your meter tells you to shoot at f/2.8, if you set the lens to T/2.8, then you'll get f/2.8 in terms of exposure. If your lens did not have T-stops, then f/2.8 on the lens may actually be slightly less in terms of exposure.

Elie Kamal · August 25, 2007

thank you David for the clarification!

Adam Frisch FSF · August 26, 2007

Also, if you're new to exposure I would strongly avoid using spot meters from the beginning. They induce such varied readings (that have no real bearing) that they can confuse rather than help. Get a good incident meter and start with that - that will cover you for 99% of all shooting situations.