OddJobber Loc: Portland, OR

All lenses are not created equal (duh), that's just a fact of life.

Noticing that my 135mm prime lens seems to "grab" more light than my kit zoom at 135mm (1/2 or more EV) led to research as to why. Briefly, the answer is T stop or transmission stop.

Excerpt from Wikipedia: "A T-stop (for Transmission-stops) is an f-number adjusted to account for light transmission efficiency. A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmission and an f-number of N. For example, an f/2.0 lens with light transmission efficiency of 75% has a T-stop of 2.3. Since real lenses have transmission efficiencies of less than 100%, a lens's T-stop is always greater than its f-number." http://en.wikipedia.org/wiki/F-number

Makes sense. Assuming I want to shop for a 70 or 75mm prime lens, and now knowing that the maximum aperture is not the only factor in light gathering quality, where do you find information relating to the T stops for comparison? A search of Nikon's website found no info on this subject.

Frank T Loc: New York, NY

T stops were used extensively in cinematography but I haven't seen any dslr lenses that have those ratings. To arrive at a t/stop rating you would have to actually measure the light transmission of the lens in the lab, which is an expensive affair in itself and I don't know any lab that's doing that for the consumer.

OddJobber Loc: Portland, OR

Thanks. :(

elie Loc: France

jobber,don't you think the ttl reading provides correction for this? in case that's true, you've got no need to know the Tstop for your camera. best regards

hfb Loc: Northwestern Louisiana

I think Oddjobber would like to compare specs on some lens without mounting on a camera. I doubt if you would find much difference between comparable lens. Otherwise an external light meter would not work well.