SUMMARY
F-stops are calculated based on a lens's focal length and the diameter of its entrance pupil, representing a theoretical aperture size. T-stops, however, measure the actual amount of light that passes through the lens to the sensor, accounting for light lost to reflection and absorption by glass elements. Because of this light loss, a lens's T-stop is always slightly higher than its f-stop. While still photographers often rely on f-stops as minor exposure differences are easily corrected, cinematographers depend on T-stops for precise and consistent exposure across different lenses and shots.
TAKEAWAYS
F-stops are geometric calculations of aperture size.
T-stops measure actual light transmission through a lens.
T-stops are always higher than f-stops due to light loss.
Cinematographers use T-stops for consistent exposure accuracy.