TStop

T-stop, sometimes written as T-stop or t-stop, is a measure of a lens’s true light transmission. It represents the effective amount of light that reaches the image plane, accounting for losses from absorption, reflections, coatings, and internal scatter. While the f-number (f/N) describes the geometric aperture, the T-stop reflects actual exposure by incorporating the lens’s transmittance.

The relation between T-stop and f-number is given by t = N / sqrt(η), where N is the f-number and η is the lens transmittance expressed as a decimal fraction (0 to 1). If a lens has a transmittance of, say, 0.92, an f/2.8 setting corresponds to a T-stop of about 2.92. Because transmittance can vary with wavelength and across different elements, T-stops can differ slightly from their corresponding f-stops, and between different lenses.

Transmittance, and thus T-stops, are typically determined under controlled lab conditions using instruments such as an integrating sphere and spectrometric sensors. Because transmittance can vary with wavelength, T-stop values may be wavelength-dependent; cinema lenses often specify T-stops to reflect luminous performance more accurately than f-stops alone.

Usage and significance: T-stops are standard in cinematography and other professional video work to ensure consistent exposure across lenses and when swapping gear. Lenses are labeled with T-stops (for example, T2.0, T2.8, T4.0) to aid lighting and exposure planning. In still photography, f-stops are more common, but some lenses and workflows also reference T-stops for precise light transmission understanding.

See also: lens, f-number, transmittance, cine lenses, exposure.