F-Stops Vs. T-Stops Explained

A tried and true gag on any given film set is to send the new camera assistant looking for a T-Stop. This Snipe Hunt will end up with the cameraman pointing to the T-Stops on the lens and everyone having a good laugh.  There is a difference, if only slight. Here is a little Quick Take on this topic.

Zeiss has released the new Compact Prime CP.2 cinema lenses in Nikon, Canon EF and PL (Positive Lock) mounts. They are labeled in T-Stops and seemed to be a bit slower than their still camera predecessors. This is due to the difference between an F-Stop and a T-stop. I got the chance to ask Carl Zeiss, USA National Sales Manager Richard Schleuning to explain the difference between F-Stops and T-Stops. He does a great job of simplifying this concept. The f-number or focal ratio, better known as F-Stop, is a mathematical equation based on the physical size of the front element and the focal length of the lens. Here is the Wikipedia definition of F-Stop.

“In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture [1]) of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the “effective” aperture diameter. It is a dimensionless number that is a quantitative measure of lens speed, an important concept in photography.”

For example a 100mm lens with a 25mm front element will have a maximum aperture of F4. The same way a 200mm lens with a 50mm front element will have the same maximum aperture of f4. This should help to explain why faster lenses have a much larger front element than slower lenses.  The “Stop” part of this equation is just a way to fractionalize the values by either half or double.  In most cases the stop will represent a double in value as we move up or down the f-stop scale. That is why a full stop increment can be represented by the values of f1.4 and f2.0. According to the f-stop equation an f2.0 is actually double the amount or one full stop from an f1.4. This can be adjusted via the iris on the lens to increase or decrease the amount of light transmitted to the film plane or digital sensor. T-Stops are based on F-Stops with the light transmission loss taken into account. T-Stops were developed in the Motion Picture Industry to enable Cinematographers to swap back and forth between prime lenses and zoom lenses without having to adjust the aperture for each lens change. Here is the Wikipedia definition of T-Stop.

“Since all lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), T-stops are sometimes used instead of f-stops for exposure purposes, especially for motion picture camera lenses. The practice became popular in cinematographic usage before the advent of zoom lenses, where fixed focal length lenses were calibrated to T-stops: This allowed the turret-mounted lenses to be changed without affecting the overall scene brightness. Lenses were bench-tested individually for actual light transmission and assigned T stops accordingly (The T in T-stop stands for transmission), [3] but modern cinematographic lenses now usually tend to be factory-calibrated in T-stops. T-stops measure the amount of light transmitted through the lens in practice (actually on T-stops the amount of light is measured at the film plane), and is equivalent in light transmission to the f-stop of an ideal lens with 100% transmission. Since all lenses absorb some quantity of light, the T-number of any given aperture on a lens will always be greater than the f-number. In recent years, advances in lens technology and film exposure latitude have reduced the importance of t-stop values. So, F-stops are for focal ratio, T-stops are for transmission.”

(Source: Wikipedia)

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