Transmission (or T-stop) is a measurement indicating a lens’ capability to pass light onto the imaging plane (film or sensor).
While a lens’ ability to transmit light onto the imaging plane is strongly influenced by the used aperture (the visible size of the opening allowing light through), the aperture (or f-stop) is not the whole story. Instead, not only are the various lens elements of a lens not entirely transparent, also every air-matter transition (lens surface) loses some light.
As a result of this, two lenses of the same focal length and maximum aperture can transmit significantly different amounts of light, hence leading to quite different shutter speeds (at identical exposure) or different exposures (at identical shutter speeds).
Understand, that t-stops impact only the amount of light transmitted (exposure) and do not have an affect on a lens’ depth of field (which is defined by the f-stop).
Why have I not heard about T-stops before? Why are lenses sold with these misleading f-stops?
Firstly, f-stops are not misleading, because they define your depth-of field exactly. Also, while T-stops may vary between lenses with identical focal length and f-stop, the variance is typically not huge and more than compensated for by your body’s exposure meter. In effect, still photographers rarely need to know T-stops (in cinematography, it’s a different story)
Secondly, T-stops are not always reported, because whereas calculating an f-stop is relatively simple (it’s a mathematic formula, which only needs the focal length and the diameter of the visible aperture), calculating a t-stop is both difficult and time-consuming as well as not entirely straightforward.
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