t-stops

• While f-stops are the theoretical value of aperture, the t-stops determine the actual tested value of light going in, since glass can absorb some light.
• Most photography lenses use f-stops and most cinema lenses use t-stops

Why do most photography manufacturers don't bother with the T-stops?

• Light metering will compensate for the difference in lenses with same f-stops but different t-stops
• Maximum light lost is around 1/3rd of an f-stop, which is not hard to fix in post-processing
• T-stop testing every device is expensive and time-consuming

In cinematography

• In cinematography, it's hard to post-process every scene, so correct exposure amounts are required there.