Carl Looper Posted January 17, 2015 Posted January 17, 2015 Was having a discussion over in the lens section on the Optivaron - initially based on a misreading that the Optivaron was specifically "collimated" (or calibrated) for the Beaulieu, ie. suggesting it couldn't be used on a standard C-mount flange depth camera without re-collimation (or re-calibration) for that other camera. However it became clear that the Optivaron was actually intended to be a standard C-mouint flange depth lens, ie. that it could be used on any standard C-mount flange depth camera - not just the Beaulieu. This is because (as we're pretty well convinced) that although the Beaulieu camera body uses a gelatin filter it's not the lens that compensates for that. It's the Beaulieu camera body - it is manufactured to compensate for the gelatin filter. In other words the effective flange depth of the Beaulieu is actually a standard C-mount flange depth. The filter + physical flange depth together perform the complete correction. Which makes sense as this would allow any standard C-mount lens to be used on the Beaulieu - not just lenses specifically tweaked for the Beaulieu. In principle anyway. Or at least that's the conclusion we eventually reached. If there is any re-calibration required of the Optivaron, ie. for it to work on a standard C mount flange depth camera it will be purely to fix any deviation the lens has acquired away from any standard calibration for C-mount. The evidence for this conclusion was that two Optivaron lenses,made for the Beaulieu, when tested, were found to have precisely a standard C-mount flange depth. http://www.cinematography.com/index.php?showtopic=66106 C
Carl Looper Posted January 17, 2015 Posted January 17, 2015 With a Bolex reflex camera, and it's prism, for some reason they didn't go down that same sensible path. There its the lenses (RX lenses) that participate in the complete correction - and therefore RX lenses can't be used on other standard C-mount cameras. Not sure why Bolex did that. Perhaps in the day it was cheaper to manaufacture a new line of lenses then alter the camera body design. Or perhaps there are more complex optical reasons requiring a lens participate in the correction. C
Premium Member Dom Jaeger Posted January 17, 2015 Premium Member Posted January 17, 2015 Bolex did alter the camera body, the physical flange depth of a reflex Bolex is more than 3mm deeper than the C-mount standard to compensate for the effect of the prism. It's the same basic phenomenon as the Beaulieu, only instead of a tenth of a mm thick filter the Bolex has a 9.5mm thick piece of glass in the light path. The reason RX lenses were made was to correct for the aberrations that such a prism introduces, principally spherical aberration and astigmatism, but the back-focus setting of RX and non-RX C mount lenses for 16mm is the same. You can use RX lenses on other C mount cameras if you stop them down a bit. I did a comparison of RX and non-RX lenses on my cinetinker page, which may be of interest. Among other things it confirmed what Dennis Couzin was saying 40 years ago about the importance of rear exit pupil depth rather than focal length as an indication of how much effect the prism has on lens aberration. http://cinetinker.blogspot.com.au/2014/12/rx-vs-non-rx-lenses.html
Carl Looper Posted January 17, 2015 Posted January 17, 2015 Bolex did alter the camera body, the physical flange depth of a reflex Bolex is more than 3mm deeper than the C-mount standard to compensate for the effect of the prism. It's the same basic phenomenon as the Beaulieu, only instead of a tenth of a mm thick filter the Bolex has a 9.5mm thick piece of glass in the light path. The reason RX lenses were made was to correct for the aberrations that such a prism introduces, principally spherical aberration and astigmatism, but the back-focus setting of RX and non-RX C mount lenses for 16mm is the same. You can use RX lenses on other C mount cameras if you stop them down a bit. I did a comparison of RX and non-RX lenses on my cinetinker page, which may be of interest. Among other things it confirmed what Dennis Couzin was saying 40 years ago about the importance of rear exit pupil depth rather than focal length as an indication of how much effect the prism has on lens aberration. http://cinetinker.blogspot.com.au/2014/12/rx-vs-non-rx-lenses.html Thanks Dom - that's great information. So could use RX lenses on the Logmar as well - if they are stopped down enough. C
Carl Looper Posted January 17, 2015 Posted January 17, 2015 So that information throws up an interesting point. The Optivarons (and Cinegons) made specifically for the Leicina Special would also have been made to compensate for prisms, since the Leicina Special has a prism. So like an RX lens, they too, if used on another camera (assuming an M to C adapter) would also need to be stopped down somewhat if one were to mitigate their prism-correcting optics. C
Carl Looper Posted January 18, 2015 Posted January 18, 2015 For the M-mount Leicina version of the Optivaron (and Cinegon), an alternative to stopping the lens down might be to insert some glass (of the appropriate thickness/refractive index) between the lens and the film, ie. to reconstruct the conditions for which the lenses were originally designed (as I assume these lenses would have been). Would require some testing to determine what kind of glass and thickness were required. Of course, the other issue is whether there is enough room to do this sort of thing. Or even just to get a lens in without this sort of thing. As some have mentioned, a lens might have the correct mount/adapter but when threading it in, the architecture of the lens component might be such that it collides with some internal camera structure in front of the film plane, such as a mirror shutter. A follow on from this is regrding lenses made for digital sensors, and their use on a cine camera. Of particular interest would be the large range of C-mount lenses made for machine vision cameras. Now it's uncertain if such lenses actually do this, as many seem to be sensor agnostic in terms of what follows: digital sensors have a "sensor stack" (protective glass and various filters in front of them) so lenses specifically designed for such sensors, would do well to take into account such a stack. But if they did, then if redeployed on a cine camera, one would need to compensate for that, a solution being to insert some glass of the required thickness to emulate the sensor stack for which the lens was designed (if it were designed for such). Unfortunately there's no information in machine vision lens catalogues as to how or even if they do this sort of correction. There's no notes in the lenses as to which sensors (in terms of sensor stack depth) they work best with. Some lenses are made specifically for particular sensors, so I imagine these might very well take into account sensor stack depth. But if they do, sensor stack size doesn't appear to be something they publish in any specs. For the more promiscuous lenses I guess there could be just an averge stack depth to which they work. Or perhaps they don't bother at all. Some experiments with different thickness glass could be done to test for any improvements possible with a given MV lens. Carl Notes: http://www.lensrentals.com/blog/2014/06/the-glass-in-the-path-sensor-stacks-and-adapted-lenses It's mainly about "sensor stacks" and how they affect lens design and/or correction. When measuring the performance of a lens adapter the testers were intially perplexed by the results. It then occured to them that the testing equipment wasn't taking into account the "sensor stack" (glass and filters mounted in front of sensors) for which the adapter was actually created. They fixed this by inserting some glass of appropriate thickness into the optical path of the testing equipment and viola, the results went back towards the results they had been originally expecting or hoping. C
Carl Looper Posted January 18, 2015 Posted January 18, 2015 Of course, if one nominates to experiment with the insertion of glass to mitigate any prism correcting lenses, a reciprical adjustment in physical flange depth would also be required - further complicating the means by which one might hope to correct such lenses. Just stopping down the lens becomes a lot more attractive. And if it means working outdoors more, that's not such a bad constraint. C
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