Thread: RED TECH - DSMC2 Back Focus

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  1. #21  
    Senior Member Michael Lindsay's Avatar
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    Quote Originally Posted by Jacek Zakowicz View Post
    I think you are mixing up telecentric and retrofocal- they are not the same. Also I don't understand what "bigger errors" mean but I think you are contradicting yourself and Phil as well...
    No I not mixing them up at all. One is a lens design approach (various evolutions of the reverse telephoto lens) and one a property (that we should actually be more specific in naming as ‘tending towards image space telecentric’) ..and yes you can have a retrofocal lens that is far from image space side telecentric.... however it is I believe tricky to have a wide angle PL lens tending towards image space telicentric lens that is not retrofocal..
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  2. #22  
    Senior Member Jacek Zakowicz's Avatar
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    Quote Originally Posted by Michael Lindsay View Post
    No I not mixing them up at all. One is a lens design approach (various evolutions of the reverse telephoto lens) and one a property (that we should actually be more specific in naming as ‘tending towards image space telecentric’) ..and yes you can have a retrofocal lens that is far from image space side telecentric.... however it is I believe tricky to have a wide angle PL lens tending towards image space telicentric lens that is not retrofocal..
    It is not tricky, maybe not practical because of the size of the rear optics but they are not that big for s35 anyway. As the matter of fact true telecentric machine vision application lenses are not retrofocal at all- they don't need to be. Tt complicates things unneccesarily.
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  3. #23  
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    Quote Originally Posted by Michael Tiemann View Post
    The wider the lens, the more it will show errors in backfocus. If you have any experience working with extension tubes you'll know that a 15mm tube on a 200mm lens has only a small effect, whereas on a 25mm lens it's way into super-macro mode. I would argue that you have a slightly long backfocus, which your shorter lenses are telling you and which your longer lenses are hiding.
    Yes, this was the answer, I adjusted the back focus with a 20 at 3 ft then moved up to 24, 35, 50, 85, 135 and they were all good. Really happy with doing this, thanks!

    Also wanted to mention that besides zooming the image, using the edge focus tool was very helpful for critical focus.
    Last edited by Jon Dishler; 09-06-2019 at 09:25 AM.
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  4. #24  
    Senior Member Michael Lindsay's Avatar
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    Quote Originally Posted by Jacek Zakowicz View Post
    It is not tricky, maybe not practical because of the size of the rear optics but they are not that big for s35 anyway. As the matter of fact true telecentric machine vision application lenses are not retrofocal at all- they don't need to be. Tt complicates things unneccesarily.
    You are clearly confusing (and I worry on purpose) machine vision object space telicentric lenses with anything being discussed here. (Basically bit more parrellel at the back).... all the wide b4 zooms we used to use attempted to be image space telicentric..remember how they did it?

    I tried to clarify with the clarifying use of ‘tending’ and ‘image space’ and I obviously realises that no cinema lenses are object space telicentric..

    If any one is vaguely interested in why a lens may want to be image space teleicentric I would even recommend the little write up on Wikipedia...
    https://en.m.wikipedia.org/wiki/Telecentric_lens

    Little tip for others someone from ARRI showed me ...If you look in the back of the lens the image of the iris distance from the back of the lens is a indicator how image space telicentric the lens is..
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  5. #25  
    Senior Member Michael Lindsay's Avatar
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    Ps great video...really really needed.. well done,!

    1) would love to reiterate the need to double check the locking screw is not in position (the red grub screw is a good idea but don’t count on it)

    2) If we have the time I prefer to not use the focus tool but rather just go 1:1 and use a loupe on a 17inch monitor ... last time I checked the red focus tool was using just the green pixels. (Makes sense as there is more of them and our senses of sharpness is really informed by green) ... many lenses are fine using the focus tool to set but not all..

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  6. #26  
    Senior Member Jacek Zakowicz's Avatar
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    Quote Originally Posted by Michael Lindsay View Post
    You are clearly confusing (and I worry on purpose) machine vision object space telicentric lenses with anything being discussed here. (Basically bit more parrellel at the back).... all the wide b4 zooms we used to use attempted to be image space telicentric..remember how they did it?

    I tried to clarify with the clarifying use of ‘tending’ and ‘image space’ and I obviously realises that no cinema lenses are object space telicentric..

    If any one is vaguely interested in why a lens may want to be image space teleicentric I would even recommend the little write up on Wikipedia...
    https://en.m.wikipedia.org/wiki/Telecentric_lens

    Little tip for others someone from ARRI showed me ...If you look in the back of the lens the image of the iris distance from the back of the lens is a indicator how image space telicentric the lens is..
    Again, there is no difference between focus error showing for retrofocal and non retrofocal lenses. No matter what they say on the internet... but then again- I've only done it for 32 years...
    That iris location in the rear is called exit pupil and we use it to design and verify adapters among other things.
    Also, since the dawn of the reflex cameras all wide and normal lenses are retrofocal to make space for the rotating mirror. So why are we talking about non retrofocal lenses= we don't use them....
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  7. #27  
    Senior Member Dustin Hoye's Avatar
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    Quote Originally Posted by Blair S. Paulsen View Post
    Dustin, now we are talking about shifting back focus and how "parfocal" your optic is. A truly parfocal zoom will hold focus from all the way in to all the way out. Even the top tier zooms have some variance, but when in perfect condition, it's extremely well controlled. OTOH, zooms designed for stills shooting may have dramatic focus variation from tight to wide and no amount of tweaking will change that.

    Cheers - #19
    Makes sense- thanks!
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  8. #28  
    Senior Member Jacek Zakowicz's Avatar
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    I've got to say however that most of the quality photo zooms hold focus quite well and so far the ones I tested were fixed by correcting the back focus. Yours may be the same from what you wrote.
    Here are couple more videos
    part1 https://www.youtube.com/watch?v=1uQP1Sbigkw
    part2 https://www.youtube.com/watch?v=xtBJbawy1ls
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  9. #29  
    Senior Member Dustin Hoye's Avatar
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    Quote Originally Posted by Jacek Zakowicz View Post
    I've got to say however that most of the quality photo zooms hold focus quite well and so far the ones I tested were fixed by correcting the back focus. Yours may be the same from what you wrote.
    Here are couple more videos
    part1 https://www.youtube.com/watch?v=1uQP1Sbigkw
    part2 https://www.youtube.com/watch?v=xtBJbawy1ls

    Thanks Jacek. I probably should have been more clear that the Canon 24-70 does focus at 24mm, just not with a single focus mark at both ends of the zoom. It's likely I'm just lucky with the Sigma, as neither are listed as parafocal on paper, to my knowledge.

    After my first round of FF adjustments on my Helium at 3 and 6 ft between the two lenses, I am still suspicious that the 24-70 (my copy anyhow) will always require some focus ring adjustment at either end of the zoom, no matter the focal flange adjustment between them, but I will of course double check everything. Cheers!
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  10. #30  
    Senior Member Jacek Zakowicz's Avatar
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    Quote Originally Posted by Dustin Hoye View Post
    Thanks Jacek. I probably should have been more clear that the Canon 24-70 does focus at 24mm, just not with a single focus mark at both ends of the zoom. It's likely I'm just lucky with the Sigma, as neither are listed as parafocal on paper, to my knowledge.

    After my first round of FF adjustments on my Helium at 3 and 6 ft between the two lenses, I am still suspicious that the 24-70 (my copy anyhow) will always require some focus ring adjustment at either end of the zoom, no matter the focal flange adjustment between them, but I will of course double check everything. Cheers!
    Sounds good- the second video deals with this issue where you have to refocus at the wide end
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