T2,9: how much correspond F?

[Asa Phelps]

Forgot the difference amount about iris value between cinematography and photography lenses:
anyone can help?

Thanks
ASa

[Emre Safak]

f/2.9, though f/2.8 is the closest standard aperture. From the Wikipedia:

Since all lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), for exposure purposes a t-stop is sometimes used instead of f-stop. The t-numbers are adjusted so that the amount of light transmitted through the lens at a given t-stop is equal to that going through an ideal non-absorbing lens set at that f-stop. (The t in t-stop stands for transmission.)