Quote Originally Posted by Worker 11811 View Post
I bet they are T-stops. The approximate reciprocal of F-stops which indicate the amount of light actually transmitted to the film. The slight difference between the scales accounts for light loss by the glass, aperture and other parts of the optical system.

T-stops are often used in cinematography but they can be used in still photography, too.

Tstops would be far, far closer together than f5.6 and T16. That would indicate a really awful, terrible, horrible lens. A realistic fstop/tstop combination is more like f5.4/T5.6 and even then, the lenses don't tell you the fstop. It's just labelled with the tstop and that's close enough to the fstop for calculations like depth of field that should be done with a geometric measure (the fstop) rather than a measured quantity of light (the tstop).