T-Stop

What this is about

On a film set exposure has to survive lens swaps. The DP meters the scene, calls a T-stop, and every lens in the set exposes identically at that number — that is the entire point of T calibration. The gaffer does not relight because the 35mm came off and the 85mm went on, and the colorist does not fight brightness jumps between coverage angles. Depth-of-field conversations, though, still run on the geometric f-stop; T only settles how bright the image is.

A term only helps when everyone on set means the same thing by it — so what follows is how it is used, not a dictionary definition.

What runs differently here

In planning terms, that means:

• T is always slightly slower than f on the same lens — the gap is the glass eating light.
• Two different lenses at T2.8 expose alike; at f2.8 they may not.
• Depth of field still follows the geometric f-stop, not the transmission number.

With TillyGen

In the production plan the DP's chosen working stop travels with the lighting notes, so every department preps for the same exposure.

Change one constraint and the consequences travel through the whole plan: affected shots are flagged, the call sheet is regenerated, and nobody keeps working from yesterday's version.