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A few RX Lens Questions


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#1 nstout

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Posted 30 May 2005 - 05:44 PM

I read on the internet that Switar RX lenses are callibrated to give you an extra 1/3 to 1/2 stop exposure than what the actual reading on the lens says (to make up for the light that is lost from the bolex beam splitter). So if I'm shooting with an RX lens I don't need to factor the effective 133 shutter angle in when using my external meter (in other words, the RX balances out the 133 shutter). So if I have a 10mm RX, 26mm RX, and 75mm (non-RX) on a Bolex, I would have to compensate by 1/3 to 1/2 a stop for the 75mm but not the others. Is there no 75mm RX so they are all the same and easier?

This is basically the same if I use an RX lens on a Beaulieu right (which I believe has 144 degree shutter angle)?
I've been told that I don't need to get a 26mm RX lens specially collimated for a bolex or beaulieu, whereas if it was under 25mm I would. Is that right?

Any help would be much appreciated, thanks! :D
ns
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#2 Robert Hughes

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Posted 30 May 2005 - 10:47 PM

The reflex viewing system on the Bolex H16RX has a prism in the optical path. If regular lenses were used on a reflex Bolex the image would suffer from spherical abberation, so the lenses (signified "RX" ) are designed to compensate for the extra glass of the prism. Short focal lengths are more susceptible to the optical distortion, whereas longer lenses ( >50mm ) are less so.

I don't know if the RX Switars have exposure calibration to compensate for the reflex system. Check with Paillard Bolex.
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#3 robtags

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Posted 31 May 2005 - 02:03 AM

I don't know if the RX Switars have exposure calibration to compensate for the reflex system. Check with Paillard Bolex.

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They do let in more light than normal at each f/stop to compensate for the reflex light loss.

http://www.sheepwash...olex_Manual.pdf page 3 explains this
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#4 Sam Wells

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Posted 31 May 2005 - 09:48 AM

The newer RX Switars have a T stop marking supposedly compensating for the light loss in the prism system (but not considering the shutter angle).

I don't use it, I prefer f stops and calculate exposure allowing for (with respect to a 172.5/180 degree shutter & no prism) 2/3 stop (it's really between 2/3 & 1/2) "loss" i.e. rate 100 speed film 64. Works for me. (partly I guess because I have some older RX Switars, so I'm not using two different systems.

The exception would be for something like time exposures where shutter angle is not a factor, only the 'open time'

-Sam
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