Correct Aperture of RX Switars on a Digital (or non-Bolex) Camera

[Christian Flemm]

Hi all,

I've seen in written that the RX Kerns are in fact 1/3 a stop faster than the aperture markings on the barrel. ("The RX lens is calibrated to pass 1/2 to 1/3 more light than the aperture markings on the barrel"). This, because the lenses were manufactured with a light loss of 1/3 a stop from the reflex prism in mind. In other words, due to the light loss, I'm effectively at f1.4 on the 25mm, despite the lens actually being 1/3 a stop faster than that.

Ignoring the issues that come alongside using those lenses on any other camera, does it follow that the 25mm f1.4 would, on a non-reflex camera, come out to f1.2? Now, at f1.1, the 26mm is quite fast; so, imagining I adapted mine to the BMPCC4K as many others have (again, ignoring the focusing and sharpness issues that come along with doing so), and imagining that I'm shooting wide open, with no reflex prism diverting 1/3 a stop of light, am I not truly at f1.0 (or f.95, if it's passing 1/2 instead of 1/3 more light than the number on the barrel)?

Anxious to hear what folks have to say about this...

Christian

[Dom Jaeger]

Hi Christian, 

Thanks for bringing this up. I know this misunderstanding is floating around the net, sometimes on otherwise good sources of information, so it's good to address it. 

RX lenses do not let in more light than normal lenses. The f stops are exactly the same. I've personally compared RX and AR lenses on an oscilloscope to double check.

There is no official Bolex literature that says RX lenses are calibrated to let in more light, and in fact it goes against the accepted definition of f stops.

All Bolex RX primes are marked in f stops. An f stop is a mathematical relationship between focal length and aperture size, it's a fixed geometric calculation, you can't change it to let more light in.  When lens aperture rings are marked to compensate for light loss (as more professional cinematography lenses are), they are marked in T stops, but that only ever refers to internal light loss, not an external factor like a reflex prism.

RX lenses only correct for the optical aberrations introduced by the reflex prism. That's why Reflex Bolex manuals have tables of "adjusted" exposure times to compensate for the prism light loss. They never say that an RX lens already compensates for it. On top of this, Bolex only commissioned RX lenses in focal lengths up to 50mm, because the reflex prism doesn't introduce aberrations in longer focal lengths.  But the light loss affects any lens fitted to a reflex Bolex.  So imagine the confusion if some lenses on a Bolex turret camera needed to have the aperture compensated for while others didn't - it just doesn't make any sense.

I've been trying to dispel this misinformation for years, but it keeps popping up. It doesn't help that certain online Bolex guides and websites keep spreading it. I've written to several authors asking them to correct their misinformation but so far no response. Perhaps they don't believe me. But I challenge anyone to find actual Bolex literature that says an RX lens lets in more light. 

[Simon Wyss]

Thanks, Dom, for the explanation which I second completely. My English will never be so fluent and pinpointed that I could make myself understood the way you do. T stands for transmission, if I may complete something.