How can you practically measure a lens’s T-stop or light transmission?
Asked 8/9/2013
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T-stop describes the actual light transmitted by a lens, so two lenses set to the same f-stop can produce slightly different exposure. Cine lenses often list T-stops, but still-photo lenses usually do not. Is there a practical way to measure a photography lens’s transmission or estimate its T-stop without access to a professional optical lab?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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The only way to get an absolute measurement is to have a lightsource of known luminance (which requires use of a calibrated luminance meter) and then measure image of that lightsource as projected by the lens.
You can get a relative value for an unknown lens by shooting a uniform lightsource with two lenses, one of which with a known the T-stop (several websites provide this value e.g. DXO mark). You can then work out the T-stop by comparing brightness of these two images.
Both lenses should be set to the same F-stop and focused to infinity. The accuracy of your results will depend upon a) how accurately the F-stop is stated for the lenses (manufacturers round the values, usually in their favour) and how linear the sensor response is. If you avoid the top end of the exposure range, shoot RAW and use something like RawAnalyzer, the linearity should be good enough.
Finally I wouldn't place too much emphasis on T-stop performance unless you are severely light-limited; there are often other factors which are far more critical.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
13y ago
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Yes, but with limits. For a truly absolute T-stop measurement, you need a controlled setup: a stable light source and calibrated measurement equipment in a lab. That’s the precise way to measure actual transmission.
For practical use, you can estimate transmission relatively by comparing lenses. Photograph a uniform, constant light source with the unknown lens and with a reference lens, using the same camera, same ISO, same shutter speed, and the same marked f-stop. Focus both lenses at infinity to keep pupil effects consistent. Then compare the recorded image brightness from the raw files; because camera sensors respond linearly to light, this can give a good relative transmission difference.
Important caveats:
- Marked f-stops may not be exact; manufacturers often round them.
- Avoid clipping highlights and compare raw data, not JPEGs.
- Keep the light source stable and the framing as similar as possible.
If you want published measurements instead of testing yourself, lab-tested transmission data from sources such as DxO can be useful.
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