Jeff Moghadam

The fact that a lens is measured with a T-stop as opposed to an F-stop doesn't imply a different design. A T-stop is just a more accurate measurement. An F-stop is theoretical and simply represents the size of the aperture where a T-stop is a measurement that takes the F-stop and factors in the amount of light loss due to reflections. This is why T-stops are slightly less than F-stops; because the transmission of modern lenses lens (Transmission stops hence T-stops) is about 95-98% of incoming light.. meaning generally 2-5% of light is lost due to reflections. In older lenses this would be more like 20-50% lost, however coatings have significantly improved the figure.

This all means that a T2.9 lens would be *approximately* the same as a f/2.8 lens. I say approximately as the transmission of the f/2.8 lens might differ.

Also, it's necessary as it allows you to match exposures among multiple lenses with differing transmissions.

Again, transmision is % of incoming light that hits cmos