T-stops vs F-stops

Can someone please explain to me, in simple terms, what a T-stop is, and how it differs from an F-stop?

Thanks. Keep it simple, I'm not into techie stuff

I normally find it easier to watch a video than read LINK

Lenses lose light by internal reflections. The f/stop value is the physical size of the aperture. The T/stop is a measure of how much light actually gets through. So for depth of field purposes the f/stop is accurate, for exposure purposes the T/stop is closer.

However, T/stops haven't been used in stills photography since the early 20th century, and the f/stop is universally used. There will be minor discrepancies, but especially with modern coatings they don't add up to much.

T/stops in modern usage are used by the motion picture industry. Whether they still are Ken will tell us.

Thankyou both. The video is especially clear.

I saw some Samyang lenses with T-stops rather than F-stops, and was curious about them.

Just lately I've seen T/stops being referred to with the concept of the speedbooster on mirrorless cameras. Traditionally the only way to get more light in through a lens was to open the aperture, so you could honestly say that aperture related to the amount of light captured.

However with a speed booster, you were able to focus more light onto the sensor by use of some clever optics. Akin to burning paper with a magnifying glass.

In that case you were increasing the light received at the sensor by another means than aperture. For a fixed aperture, you got more light with a speed booster, than with the lens directly connected to the camera only. That's where the T/Stop measurement was more applicable.

T stops are used in cinematography where there is great need for accuracy, I imagine the Sanyang lenses are meant for video and have follow-focus gear rings on them. T is the actual transmission value and can differ greatly especially with zooms from the mathematically calculated F value. So we use T stops for setting exposure (remember that in film cinematography the shutter speed is generally fixed at 1/50 sec in Europe and 1/60 in USA) and F stops for depth of field.

F Stops quoted for 35mm camera lenses are usually very accurate
so T-Stops aren't needed.

Zooms are usually slightly worse than primes, but still good.
This one is fairly typical....

http://www.adaptall-2.org/lenses/01A.html

f/2.8 actual 2.82
f/3.8 actual 3.96 Probably didn't want to call it an f/4 lens

A Pentax 50mm f/1.4 lets in slightly more light at the
lower f stops than the f/1.7 does.

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