Do all lenses transmit the same amount of light at the same f-number?
Asked 2/7/2013
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If two different lenses are both set to the same f-number, such as f/1.8, will they always deliver the same exposure to the sensor? Or can actual light transmission vary between lenses? If exposure can differ, what measurement describes the real transmitted light across different lenses?
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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No, this is not the case. Aperture F stops are calculated on pupil size and focal length of the lens.
From wikipedia
In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture1) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil.2 It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography.
Whereas a T-Stop is a measured unit and lenses set at the same T-Stops will make the same exposure.
A T-stop (for Transmission-stops) is an f-number adjusted to account for light transmission efficiency. A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmission and an f-number of N. For example, an f/2.0 lens with light transmission efficiency of 75% has a T-stop of 2.3. Since real lenses have transmission efficiencies of less than 100%, a lens's T-stop is always greater than its f-number.[6] Lens light transmission efficiencies of 60%-90% are typical,[7] so T-stops are sometimes used instead of f-numbers to more accurately determine exposure.[8] 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. In still photography, without the need for rigorous consistency of all lenses and cameras used, slight differences in exposure are less important.
Originally by user11295. Source · Licensed CC BY-SA 4.0
user11295
13y ago
0
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Not exactly. An f-number describes the ratio of focal length to entrance pupil diameter, so it is a geometric value, not a direct measurement of transmitted light. In practice, two lenses set to the same f-number usually give very similar exposure, which is why f-stops are used for photography.
However, real lenses do not transmit 100% of the light that enters them. Glass design, coatings, and the number of elements can cause small transmission losses, so two f/1.8 lenses may not pass exactly the same amount of light.
The term for actual measured light transmission is T-stop. A lens set to the same T-stop as another lens will produce the same image brightness/exposure, because T-stops account for transmission efficiency as well as aperture size.
So:
- f-stop = theoretical/geometric aperture
- t-stop = actual transmitted light
For still photography, f-stops are usually close enough. T-stops matter more when exact exposure matching is important, such as in cinema.
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