"T" stop and "F" stop
Posted 09 September 2013 - 07:48 AM
Posted 09 September 2013 - 08:17 AM
T stops are a measure of the light transmitted (I think of them as Transmission stops) which is used to set exposure.
F stops are a mathematical ratio which is used to determined Depth of Field (which is why I think of them as focus stops).
99.9% of the time they are interchangeable and so close to the same it is of no difference. The time they start to very is as you start to add in more and more glass in the lens (such as in a zoom lens) where you may have a F 2.8 lens which only has a T of say an F4.
Posted 09 September 2013 - 08:33 AM
To add to that, think of a T-stop as a "corrected" f-stop mark on the lens, corrected for the actual amount of light that gets through, which is often slightly less than the theoretical amount based on f-stop, which is based on a physical dimension. So typically a T-stop may be a 1/6th or a 1/3rd at most "stopped" down from the f-stop, i.e. a lens that opens to f/2.8 only transmits a T/2.9 amount of light.