Quote Originally Posted by Fast Glass


Actually it's a150mm f/1.3


Only the "T-stop" is 1.3; the "F-stop" is 1.2.


A T-stop tells you the transmissivity of the lens. Some light is lost as it reflects off the lens elements, so a 20-element f/2.8 lens is much dimmer than a 7 element f/2.8 lens. If there were no loss in transmissivity, the t-stop and f-stop would be the same, but in practice they are slightly different.