SWITAR RX lens f stops

Post by **ECNtoo** » Wed May 03, 2006 1:58 pm

i understand that the switar RX lenses have the iris setting set to compensate for the prism viewfinder. hence the exposure would be overexposed if used on other cameras. this leads me to beleive the lenses are faster than what is indicated on the lens itself. anybody have any idea what switar rx lens true f stops are so i can use them on non bolex cameras?

Post by **Evan Kubota** » Wed May 03, 2006 3:45 pm

You shouldn't use them on cameras other than reflex Bolexes - the focal marks are different to compensate for the effect of the prism. If that prism is absent it's not going to focus accurately using the marks.

Post by **ECNtoo** » Wed May 03, 2006 5:22 pm

even if the lens is recollimated?

Post by **Lunar07** » Wed May 03, 2006 7:39 pm

ECNtoo wrote:even if the lens is recollimated?

Yes! The elements are arranged taking into account that medium of transmission is glass (of prism) not air. So, an RX lens is engineered to take into account the refractive qualities of the prism.

Anyway, the true f stops of an RX lens are 1/3 stop more what is on the lens.

Post by **ECNtoo** » Wed May 03, 2006 9:33 pm

well i guess its time to get rid of the ole preset lens then

Post by **clivetobin** » Thu May 04, 2006 1:24 am

Lunar07 wrote: Anyway, the true f stops of an RX lens are 1/3 stop more what is on the lens.

Sorry, but this is rubbish and has been demolished in prior postings on several groups.

It is clearly stated in all of the Bolex Reflex literature that the Bolex LIGHT METER has "adapted" settings to compensate for the prism. NOT the lens F stop calibrations.

The true situation is that the lens for RX mount has a different formula to prevent dispersion (color fringing) on account of the prism. This effect disappears generally if the aperture is set to f/2.8 or smaller. I doubt very much if it can be accomplished by ordinary collimation or moving individual elements around, though I am not an optical designer and cannot say.

Post by **CDI** » Thu May 04, 2006 4:30 am

I emailed Bolex in Switzerland, and they said that RX lenses should work stopped down below 2.8. At 2.8 or more open, I believe there are some issues based on the spherical correction and the prism. Basically, RX lenses are designed with the prism in mind and are slightly different in design than non reflex lenses.

I haven't shot tests with rx lenses, so I don't know what the results would be. Some people say they work fine, some don't.

Talk to someone (Erwin I think) at TCS in New York about this. They specialize in Bolex stuff.

Post by **Lunar07** » Thu May 04, 2006 4:54 pm

clivetobin wrote:
Lunar07 wrote: Anyway, the true f stops of an RX lens are 1/3 stop more what is on the lens.
Sorry, but this is rubbish and has been demolished in prior postings on several groups.

Let me rephrase:
Take an RX lens and use it on a NON-RX camera. Take an exposure reading. Close the RX lens aperture a 1/3 of a stop of the exposure reading (to compensate for the fact that an RX lens f stop calibration takes into account that a prism is behind it).

And that is, incidentally, the situation we get with the Bolex light meter when user is asked to adjust reading when a Non-RX camera/lens is used with the Bolex light meter.

ECNtoo wrote: well i guess its time to get rid of the ole preset lens then.

Lenses with a focal length of 75mm and higher can be used with both RX and non-RX cameras. If you are using a lens that is less than 75mm it is highly recommended that you get an RX lens for your RX camera and a non-RX lens for your non-RX camera.
The rule that you can use an RX lens at an f stop of 2.8 or higher with a non-rx camera, does not prevent from losing sharpness whatever you do and depending on your subject matter. This is an issue that can not be fixed by recollimation for reasons stated in the different posts.

Post by **ECNtoo** » Thu May 04, 2006 5:15 pm

my plan was to use them on an eclair and beaulieu. im not even sure if when people here are refering to an RX camera they mean a reflex bolex as opposed to a non reflex bolex or if they mean any other reflex camera other than a reflex bolex.

clive i hear you. i was in after my run the other day and like usual i wanted to pass out and felt somewhat delirieous (excercise and i are like the government and its citizens...nothing more than a necessasry relationship) when i had a thought. if the RX switar lenses were marked to include compensation for the prism, then in theory the acatual lens speed would be faster on another reflex camera since the prism wasnt there to take away light.

i just want to be able to use switar presets and other reflex lenses between reflex bolex and non bolex cameras without having to think about much beyond the normal shooting issues.

Post by **Evan Kubota** » Thu May 04, 2006 6:03 pm

"my plan was to use them on an eclair and beaulieu. im not even sure if when people here are refering to an RX camera they mean a reflex bolex as opposed to a non reflex bolex or if they mean any other reflex camera other than a reflex bolex."

As I said in my original post - no, only reflex Bolexes. Get some regular lenses for your Eclair or Beaulieu...

Post by **ECNtoo** » Sun May 07, 2006 3:54 am

im thinking mabye i should just dump the switars and get a set of early zeiss lenses and use adaptors where applicable. much easier than this. i never did like it when companies did the proprietory accesories bit. we got a ast pc in the early days that did that with the ram. the price was stupid at the time. bloody annoying.

Post by **audadvnc** » Wed May 10, 2006 1:02 am

Good idea. Just toss those dumb ol' switars in an envelope and send 'em to me, I'll make sure they're disposed of correctly :lol: