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Transmission stop (T-stop)

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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Diglloyd DAP is DSLR-oriented, but also contains workflow and other topics. Much of the focus is on Canon and Nikon but also Pentax and Pentax medium format.

Special emphasis is placed on lens evaluation, focusing on Canon and Nikon and Sigma lenses, but with a few others like Rokinon/Samyang.

  • Make better images by learning how to get the best results right away.
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  • Real world examples with insights found nowhere else. Make sharper images just by understanding lens behavior you won’t read about elsewhere.
  • Aperture series from wide open through stopped down, showing the full range of lens performance and bokeh.
  • Optical quality analysis of field curvature, focus shift, sharpness, flare, distortion, and performance in the field.

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