#15. "RE: Metering with Lens Filter Oddity" In response to In response to 14 Mon 15-Oct-12 10:01 PM by MotoMannequin
Livermore, CA, US
>Excuse my lack of understanding. I don't know what T-stop >means, and I am only guessing at CPl lens. Would that include >a "G" rated lens?
The more commonly referred to "F stop" is a measure of the size of the entrance pupil, and controls DOF as well as providing a theoretical measure of how much light the lens will pass to the camera.
A "T stop" is the actual measure of how much light a lens passes to the camera. This is generally a little bit lower than the theoretical "F stop" because there is always some light lost due to scattering at every air-glass surface.
Generally we don't worry about T-stop because it's automatically accounted for by our use of through-the-lens (TTL) metering. If you use an external light meter, you might need to compensate for cases where the lens' T-stop doesn't match the F-stop setting you enter into the light meter.
In the case Stan is referring to, addition of a non-clear filter like your C-PL will reduce the amount of light the lens is actually transmitting (the T-stop) without changing the F-stop. If using an external meter you would need to account for this loss. The camera's internal TTL meter should account for it automatically, unless you do something to bypass or otherwise ignore the meter.
I think what Stan is saying is that the polarizer will cause glare to be cut at uneven levels depending on the angle of the reflecting surface, i.e. it will have dulled some areas of the car and had less effect on others, so if you expose to protect highlights, you end up with other areas underexposed.