Editing F-number (section)

=== T-stop ===
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A '''T-stop''' (for transmission stops, by convention written with capital letter T) is an f-number adjusted to account for light transmission efficiency (''[[transmittance]]''). A lens with a T-stop of {{mvar|N}} projects an image of the same brightness as an ideal lens with 100% transmittance and an f-number of {{mvar|N}}. A particular lens's T-stop, {{mvar|T}}, is given by dividing the f-number by the square root of the transmittance of that lens:
<math display=block>T = \frac{N}{\sqrt{\text{transmittance}}}.</math>
For example, an {{f/|2.0}} lens with transmittance of 75% has a T-stop of 2.3:
<math display=block>T = \frac{2.0}{\sqrt{0.75}} = 2.309...</math>
Since real lenses have transmittances of less than 100%, a lens's T-stop number is always greater than its f-number.<ref>[https://www.dxomark.com/glossary/transmission-light-transmission/ Transmission, light transmission] {{Webarchive|url=https://web.archive.org/web/20210508111318/https://www.dxomark.com/glossary/transmission-light-transmission/ |date=2021-05-08  }}, DxOMark</ref>

With 8% loss per air-glass surface on lenses without coating, [[History of photographic lens design#Anti-reflection coating|multicoating]] of lenses is the key in lens design to decrease transmittance losses of lenses. Some reviews of lenses do measure the T-stop or transmission rate in their benchmarks.<ref>[https://www.dxomark.com/sigma-85mm-f1-4-art-lens-review-new-benchmark/ Sigma 85mm F1.4 Art lens review: New benchmark] {{Webarchive|url=https://web.archive.org/web/20180104073126/https://www.dxomark.com/sigma-85mm-f1-4-art-lens-review-new-benchmark/ |date=2018-01-04  }}, DxOMark</ref><ref>[https://www.lenstip.com/129.1-article-Colour_rendering_in_binoculars_and_lenses.html Colour rendering in binoculars and lenses - Colours and transmission] {{Webarchive|url=https://web.archive.org/web/20180104013937/https://www.lenstip.com/129.1-article-Colour_rendering_in_binoculars_and_lenses.html |date=2018-01-04  }}, LensTip.com</ref> T-stops are sometimes used instead of f-numbers to more accurately determine exposure, particularly when using external [[light meter]]s.<ref name=KMPCF>{{cite web |publisher=[[Eastman Kodak]] |url=http://www.kodak.com/US/en/motion/support/h2/intro01P.shtml |title=Kodak Motion Picture Camera Films |date= November 2000 |access-date=2 September 2007 |archive-url=https://web.archive.org/web/20021002095739/http://www.kodak.com/US/en/motion/support/h2/intro01P.shtml |archive-date=2002-10-02}}</ref> Lens transmittances of 60%–95% are typical.<ref>{{Cite web |url=http://forums.dpreview.com/forums/post/33785655 |title=Marianne Oelund, "Lens T-stops", dpreview.com, 2009 |access-date=2013-01-11  |archive-date=2012-11-10  |archive-url=https://web.archive.org/web/20121110221724/http://forums.dpreview.com/forums/post/33785655 |url-status=live }}</ref> T-stops are often used in cinematography, where many images are seen in rapid succession and even small changes in exposure will be noticeable. Cinema camera lenses are typically calibrated in T-stops instead of f-numbers.<ref name=KMPCF/> In still photography, without the need for rigorous consistency of all lenses and cameras used, slight differences in exposure are less important; however, T-stops are still used in some kinds of special-purpose lenses such as [[Smooth Trans Focus]] lenses by [[Minolta]] and [[Sony]].