A ratio of amount of light where one stop represents a x2 change – doubling or halving of the amount of light. The operating range of film and electronic light sensors, such as CCDs and CMOS, are quoted in stops. Typically, a camera’s shutter speed and the lens’s aperture setting restrict the light arriving at the sensors/film so the mid brightness of the required scene corresponds to the middle of the sensor’s or film’s sensitivity range.

Stops are simply the expression of a ratio, not absolute values. As they represent doubling or halving of light, they are actually powers of 2. So

1 stop = x 2
2 stops = x 4
3 stops = x 8
4 stops = x 16 etc.

F stops are the simple calculation taking focal length and aperture into account. This does not fully solve the problem of how much light gets to the sensor/film when lenses are not 100% clear. The T stop takes the transmissive quality of the lens into account. The transmissive quality is affected by the glass used to make the lens and any additional anti-reflective and conditioning coatings applied to the lens elements.

For example:

Use the formula:-  T-stop = F-stop/transmission_fraction. Where transmission ranges from 0 (opaque) to 1 (perfectly clear)

For a perfect lens the calculation is T = F/1 ->  T == F

For a lens with 80% transmission (0.8) then T = F/0.8

So for an F stop setting of 4 the real measure of light getting to the sensor is equivalent to 4/0.8 = 5.0

Note that the depth of field will still be calculated from the F stop setting – only the exposure is set using the T stop.