How bright is a lens actually? The t-stop or “transmission stop” is a more useful measure of the actual brightness of a lens than f-stop, but it has all but disappeared except with cine lenses. The t-stop is directly relevant to the shutter speed that can be used, or the ISO that must be used to achieve a desired shutter speed.
The t-stop can vary by as much as 2/3 stop from the rated f-stop in this author’s experience—the f-stop as a measure of potential shutter speed is thus a lie with some lenses. It’s disappointing that manufacturers do not provide both f-stop and t-stop.
Some rounding is reasonable—lenses designated as f/2.8 might actually be f/2.7 or f/2.9 (f-stop = focal length / entrance pupil diameter). Significantly, the transmission of a lens can vary substantially. The ideal lens will have even spectral transmission (be completely neutral in color transmission like the Coastal Optics 60/4 APO) as well as transmitting all but a tiny fraction of the light.
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