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A T-stop is the same kind of measure as an f-stop; that is, they both measure the openness of your aperture.

The difference between T-stops and f-stops is that while f-stops are calculated theoretically, T-stops are specifically calibrated for their particular lens, making them more accurate.

The T-stop numbre is the true f-stop number of a lens if it is completely free from all reflection and absorption losses. The T transmission number respresents the f-stop number of an open circular hole or of a perfect lens having 100% axial transmission. The T-stop can be considered as the "effective" f-stop. T-stops are calibrated by measuring the light intensity electronically at the focal plane, whereas f-stops are calculated geometrically. So, f-stops are based on light that enters a lens then t-stops are based on the intensity of the light that emerges from the rear of the lens and forms the image. However, there is no fixed ratio between t and f stops, between all lenses. The difference represents light losses within elements of a given lens due to reflection from the glass-air surfaces and from absorption within the glass itself. Therefore there is no t-stop exposure meters.

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