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Philip Forrest

Reflex lens T stops?

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With regard to lenses that have a stated T stop but also are fitted with a reflex viewfinder, what should be a corrective factor to account for the prism that is sitting within the optical path? I've seen lenses of the same type, one with, one without a reflex finder and haven't seen any mention of a T stop correction, ie Angenieux12-120.

Thanks for answering this curiosity of mine.

Phil Forrest

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I believe the reflex "dog leg" version of Angenieux's 12-120 is marked as T2.8 rather than the T2.5 of a standard version.

The iris scale ring on those zooms is usually marked in white f/stops on one side and red T stops on the other.  A normal 12-120 is f/2.2 and T2.5, so the lens itself loses a third of a stop in transmission. The reflex version loses another third of a stop to the finder, to become T2.8

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