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Bolex EBM - Focal Flange issues with new MFT c-mount lenses?


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#1 Matthew B Clark

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Posted 18 May 2017 - 09:59 AM

Hello.  I'm getting into a nice S16 Bolex EBM, and leaning into a bunch of potential lens options.  Trying to make the wisest decision for a good set of basic primes or possibly a good zoom paired with one wide angle prime to get very wide when needed.  I have a bayonet to c-mount adapter and a bayonet to M42 adapter (I have a lot of decent stills lenses in M42)...but...

 

I'm hitting a wall on two really concerning issues, no matter how much I ask around or research it --

 

1) Issues about focal flange distance are concerning to me, especially with the possible use of adapters, given the Bolex's notoriously weird "compensations" made with the Kern Switar lenses.  I know the Kern Switar H16 RX were designed to compensate for some kind of "odd" focal flange distance (is that right?) and prism compensation on the bolex EBM viewfinder.  The prism compensation doesn't bother me because I can just open up a third of a stop to do that myself, but the focal flange from back element of lens to film/sensor plane is very troubling.  I guess I just am worried that anything "other than Kern Switars" will be out of lens tolerance or something, and they may produce a slightly blurred or affected image, especially with adapters that may be a bit janky.  Is this true?  False?  Halfway true false true?  And does that mean if I slap some M42 mount lenses or newer glass on it via adapter, that it'll give me a blurrier image?  I'm really concerned that I'm going to be playing with fire by disregarding the Bolex specs/design tolerance by using new glass (say the Sigma 10-20 zoom, 18-35 zoom, or Rokinon primes, or even any of my olde pentax M42 lenses to be honest).

 

2) The other issue is the whole MFT sensor lenses on S16 camera issue.  In terms of coverage.  The thing that cracks me up is there are TONS of articles and posts online about using S16 glass on MFT sensor cameras, but almost NOTHING the other way 'round.  I guess I'm one of the few guys with a S16 set-up who really want to put some new Sigma or Rokinon lenses on it cheaply.  I kind of want to do the same thing people are doing with BMPCC, except using FILM stock instead of digits (beep boop). 

 

So far I'm looking at some combo of these lenses:

 

Camera: BOLEX EBM S16

 

Lens package option #1:

Sigma 10-30 zoom (or) Sigma 18-35 zoom

 

Lens package #2

Rokinon 14mm prime

Rokinon 35mm prime

 

Lens package #3

Kern Switar 10mm

Kern vario Switar 16-100

 

What I have now lens wise is from my K3 set-up as follows:

Peleng 8mm

Vivitar 28mm

Pentax SMC 50mm

Carl Zeiss Jena Sonnar 135mm

Meteor zoom 17-69

 

I guess I'm saying...."what's gonna be blurry....what's gonna be crappy....what's gonna be good...and what's gonna not be good on a BOLEX EBM S16??"

 

Many thanks.


Edited by Matthew B Clark, 18 May 2017 - 10:02 AM.

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#2 Roberto Pirodda

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Posted 18 May 2017 - 12:06 PM

I have used Peleng 8mm lens on a Bolex reflex and it delivers very good images for its price , but you can use only f 5,6 iris.  More open or closed iris will degrade quality very quickly


Edited by Roberto Pirodda, 18 May 2017 - 12:09 PM.

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#3 Doug Palmer

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Posted 19 May 2017 - 03:53 AM

Personally, I've found still 35mm lenses such as your Pentax 50mm work fine with the EBM.  My Nikkor 50mm appears sharp even at f1.2. Maybe their are issues if you go for shorter focal lengths.


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#4 Simon Wyss

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Posted 19 May 2017 - 05:17 AM

Correct. The double-prism cuboid glass body between lens and film introduces mainly longitudinal and some radial prismatic aberration. In other words, the focus of lenses is shifted down along the optical axis. To bring that back, DV and RX designated lenses have an additional positive element. A RX Switar 25 has seven elements whereas an AR Switar 25 has six (a Planar variant). Maths say that the prismatic error can be neglected above 50 mm focal length and beyond f/3.3.

 

Caution! Older lenses sometimes have a thread tube long enough to interfere with the reflex prism. The classic Kern-Paillard Switar 25-1.4 hits the glass, so can not be used on a RX camera. Same with Yvar 15-2.8, Switar 25-1.5, Yvar 25-2.5, Stereo-Yvar, and more lenses


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#5 Matthew B Clark

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Posted 19 May 2017 - 09:55 AM

Thank you Roberto, Doug and Simon, for your input on this issue.  It seems that Canon FD lenses work for this gentleman at 24mm and 50mm:

 

I think the 16-100 Vario-Switar RX zoom is my best bet. 

 

 

 

Caution! Older lenses sometimes have a thread tube long enough to interfere with the reflex prism. The classic Kern-Paillard Switar 25-1.4 hits the glass...

 

Simon, thank you for scaring me away from all but the most "RX" of RX lenses!  This is good information though, and it definitely eliminates using my K3 zoom, with its massively protruding rear element, I'm sure.  I am guessing the RX lenses are the safest lenses for the Bolex reflex for good reason...

 

Because this will be for personal projects, and I actually enjoy the look of what I've seen from the Kern Switars immensely, this shouldn't be a problem.  But I would simply like options, as we all do.  Especially since BMPCC users are trying to pretend they can make a digital camera "look filmic" by gobbling up my entire stock of lenses to pick from. 

 

So it would be a real blessing to know some sort of general rule of thumb regarding use of potential "other" primes for use with a Bolex EBM.  Say, how many elements it needs (7 always?)...or how deep the back threads can be in terms of exact measurement....any tricks to know that could help eliminate major problems at a glance so that I at least don't end up with what "won't work". 


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