The T-stop or “transmission stop” is an idea similar to the F-stop except that it denote the actual light transmission. The T-stop is critically important for professional film-making, where lenses must be matched to a small fraction of a stop, where even 1/10 stop is perhaps too much variance.
With still images, T-stop is generally of little importance, except that if you’re buying an f/2.8 lens with the goal of faster shutter speeds under dim lighting, then getting a lens that has a T-stop of f/3.2 or f/3.5 is self-defeating.
The original Nikon 70-200/2.8 VR was a rip-off in this regard, showing at least a 1/2 stop light loss as compared to a prime lens, documented here. That means shooting at 1/60 second instead of 1/90 second, which can be a big issue in some scenarios, such as sports. For an interesting list of t-stops, see this thread (I don’t vouch for the reliability of the figures there, and they were all done with green light).
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