It is actually "ƒ/2.8", not "F2.8" or "f/2.8". The symbol, 'ƒ' is Option + f on Mac keyboards. I have no idea what it is on Windows, probably Alt + [some arcane number].

'ƒ' (in photography and related optical fields) is a symbol meaning FOCAL LENGTH. The expression, 'ƒ/2.8', is read in English as "focal length divided by 2.8," as expressed in the same dimension as the lens (usually mm). So it is the CALCULATED diameter of the aperture in the diaphragm of the lens... i.e.; on a 100mm lens, the aperture ƒ/8 is 100/8, or 12.5mm. As it turns out, the same aperture number on different lengths of lenses represents the opening size that lets in "the same" amount of light.

In practice, there are variables such as manufacturing tolerances and the transmission of light through all those various glass elements, So ƒ/2.8 on one lens might be ƒ/3.2 on another, or ƒ/2.5 on yet another. In the motion picture and video (film) industry, we call those ACTUAL values, 't/stops'. A 't/stop' is the actual number on the lens that represents a truly standard amount of light transmission.

On multiple-camera movie set assignments, using matched lenses with calibrated 't/stop' scales is important, so that the color and brightness of each camera's output is matched to the others. That makes intercutting clips from several cameras much more seamless, and reduces or eliminates the need for separate color grading of each clip. It was/is a lot harder to get a match with film than with video. These days, downloading lookup tables into cameras can match them, and using a ColorChecker Passport at each camera can balance several cameras' color and exposure quite closely.

It is actually "ƒ/2.8", not "F2.8" or "f/2.8". The symbol, 'ƒ' is Option + f on Mac keyboards. I have no idea what it is on Windows, probably Alt + [some arcane number].

'ƒ' (in photography and related optical fields) is a symbol meaning FOCAL LENGTH. The expression, 'ƒ/2.8', is read in English as "focal length divided by 2.8," as expressed in the same dimension as the lens (usually mm). So it is the CALCULATED diameter of the aperture in the diaphragm of the lens... i.e.; on a 100mm lens, the aperture ƒ/8 is 100/8, or 12.5mm. As it turns out, the same aperture number on different lengths of lenses represents the opening size that lets in "the same" amount of light.

In practice, there are variables such as manufacturing tolerances and the transmission of light through all those various glass elements, So ƒ/2.8 on one lens might be ƒ/3.2 on another, or ƒ/2.5 on yet another. In the motion picture and video (film) industry, we call those ACTUAL values, 't/stops'. A 't/stop' is the actual number on the lens that represents a truly standard amount of light transmission.

On multiple-camera movie set assignments, using matched lenses with calibrated 't/stop' scales is important, so that the color and brightness of each camera's output is matched to the others. That makes intercutting clips from several cameras much more seamless, and reduces or eliminates the need for separate color grading of each clip. It was/is a lot harder to get a match with film than with video. These days, downloading lookup tables into cameras can match them, and using a ColorChecker Passport at each camera can balance several cameras' color and exposure quite closely.