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M Joel W

RX Lens T Stops

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I am curious about Bolex lenses for RX cameras such as the REX-5.

I was looking at a range of lenses that has both t stops and f stops marked. Unsurprisingly, 10mm and 25mm focal lengths are marked as RX. 75mm is not. All have t stops and f-stops.

Do the 10mm and 25mm t-stops factor in the 1/3 stop light loss from the prism, while the 75mm does not? Thanks.

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Hi Matt,

Lenses marked RX do not compensate for the prism light loss, they only correct for the aberrations it introduces.

If you think about it, the idea of needing to compensate exposure for some lenses on your Bolex turret but not others would just be a recipe for confusion. Most C mount primes are marked in f stops, which directly relate to the size of the aperture (or entrance pupil), so they can’t be “adjusted”, they are a fixed mathematical relationship.

Lenses marked with T stops are adjusting the stop number to account for the light loss that occurs within the lens, never for an external factor like a camera viewfinder. Which C mounts have you seen with T stops?

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I while back I found this document which mentions that "The RX lens is calibrated to pass 1/2 to 1/3 more light than the aperture markings on the barrel indicates. When using RX lenses with the H-16 camera, the effective shutter speed is back to 1/65 of a second."

Dom, have I been led astray?

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Yes you have. That document is incorrect.

F stops are f stops, mathematical standards. 

Besides which, why would Bolex only commission RX lenses up to 50mm if they compensated for the light loss as well as the aberrations, it doesn’t make sense.

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Thanks, Dom, it seems you are right as usual. I found some nice looking Schneider Cinegon/Cine-Xenon and Tele-Cine-Xenar (lots of names) with both f-stop and t-stop markings on the barrel.

The difference between the two is consistent with what's normal with older coatings, but I was confused by the same document Jordan linked to.

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