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non rx lense adjustable for rx?


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#1 john smith

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Posted 13 March 2006 - 09:49 PM

Hello,
I recently purchased a set of Switar primes from a dealer in Ca. I specifically asked if these were RX lenses as I have a Bolex Reflex 1 camera. I was told yes and when recieved them noted that none of the lenses had the "RX" designation on the lense. I was told that they had their back focus adjusted so they would work properly with the rex camera.
The focul lengths purchased were 10,25, and 75.
I paid pretty much retail for these.
I have heard positive comments about the seller. He repairs lenses, and modifies cameras for S-16.

The question(s): Is this a viable way to make a non-rx lense work with a reflex system and will I take a hit when I go to sell these after the project is complete i.e. is it good enough to tell somebody that they had the back focus adjusted and should work properly.
I have read +/- about adjusting lenses. I just don't know how it all comes out in the real world of shooting.
I appreciate your replies, Thankyou.
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#2 Clive Tobin

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Posted 13 March 2006 - 10:13 PM

I recently purchased a set of Switar primes from a dealer in Ca. I specifically asked if these were RX lenses as I have a Bolex Reflex 1 camera. I was told yes and when recieved them noted that none of the lenses had the "RX" designation on the lense.


(Can't anybody on this forum spell the word "lens"?)

The RX lenses of 50mm and less are specially computed to correct for dispersion (color fringing) caused by the glass reflex prism.

No adjustment of the lenses is going to re-design them for a correction that is lacking from the lens design formula. I think the seller has done a number on you.

Some lenses will work on both RX and non-RX cameras, mostly those over 50mm and for many zooms, as I understand it.
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#3 Tim Carroll

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Posted 13 March 2006 - 10:15 PM

I am certainly no Bolex expert, but from my recollection, the difference between an RX lens and a non-RX lens had nothing to do with back focus. It had to do with the aperture scale. The Reflex Bolex cameras steal some of the light coming in the lens, before it gets to the film. This is how you can see an image in the viewfinder. So the RX lenses are set up to compensate for that. When you use a non-RX lens on a Relfex Bolex, if your light meter tells you to set the lens at f5.6, and you do that, you will be underexposing the film, because of the light taken by the RX prism(the light sent to the viewfinder), never gets to the film. The RX lenses compensate for this by letting in more light when set to a particular aperture, say f5.6, than a normal lens would.

Never heard of needing a different back focus for a Reflex Bolex compared to a non-Relfex one, but I guess that is possible.

-Tim Carroll
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#4 Mike Rizos

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Posted 13 March 2006 - 11:07 PM

Hi
I agree with both Clive and Tim. The Rx lenses correct for sperical abberations introduced by the prism, and compansate for the light loss, which is 25%.
The lenses are still usable though above about f2.8. The sperical abberations are only apparent at large apertures, and they are gone by f2.8.
I would try to get my money back in your situation, since the non Rx lenses go for about 1/2 of Rx.
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#5 Michael Carter

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Posted 14 March 2006 - 12:57 PM

I would try to get my money back in your situation, since the non Rx lenses go for about 1/2 of Rx.

I've heard that it could be done but it would not be as good as if one had a RX lens. However, it would help.
The f stop was f5.6 and above for non corrected lenses. Perhaps that is where it improves, down to the lower setting spoken of.
My camera is a RX-1 also and it has been much too expensive to get perfect RX lenses.
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#6 Clive Tobin

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Posted 14 March 2006 - 08:39 PM

... So the RX lenses are set up to compensate for that. ...


I have never heard that one before! This is a possible answer for another dimension, but it is flat out wrong for the one we are living in.

On the contrary, the Bolex *light meter* has the adapted shutter speeds to compensate for light loss, *not* the lens scale. The old Bolex literature is very specific on this point.

Edited by clivetobin, 14 March 2006 - 08:48 PM.

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#7 Sam Wells

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Posted 14 March 2006 - 09:56 PM

The most visible problem (to my eye) with the 10 & 25mm non-RX Switars on Rex camera is astigmatism.

-Sam
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#8 Tim Carroll

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Posted 14 March 2006 - 10:10 PM

I have never heard that one before! This is a possible answer for another dimension, but it is flat out wrong for the one we are living in.

On the contrary, the Bolex *light meter* has the adapted shutter speeds to compensate for light loss, *not* the lens scale. The old Bolex literature is very specific on this point.


Hey Clive, chill out buddy. I mentioned I was not a Bolex expert, just passing on what I remembered, and maybe I got it wrong. And I was not referring to a "Bolex *light meter*" I was referring to my trusty old Sekonic.

So you are saying that the shutter speeds are somehow slowed down to compensate for the light loss? How is that done, obviously they are not running the camera slower, so is the shutter angle not what they say it is? How are they leaving the shutter open longer if they are not slowing the camera down and the shutter angle is what they say it is? Cause that is how I have always calculated shutter speed, by taking the camera speed and shutter angle. Then I plug that number into my light meter, and take a light reading, and set the aperture accordingly. Where in that process is Bolex compensating for the light lost by the prism?

-Tim Carroll

PS: Okay, I just looked at the Bolex EBM manual Tim has posted in the manual section. I can see that Bolex lists the "photometric" shutter speeds, which are not the actual shutter speeds, but the "prism compensating" shutter speeds that you enter into your light meter. So I was obviously wrong on my earlier post. I hope Fliersmith accepts my sincere apology.
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