Thread: RED TECH - DSMC2 Back Focus

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  1. #1 RED TECH - DSMC2 Back Focus 
    Moderator Phil Holland's Avatar
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    I haven't shared a RED TECH vid in a minute, but this one is super damn important and Nate did a great job here. It's one of the regular questions I get from newer filmmakers and this covers it pretty well for DSMC2 and Ranger cameras.


    https://www.youtube.com/watch?v=VAhQtjNKlj8

    One caveat I'll add. Some of the lower end optics do not hold consistent focus when going from close focus to mark, then infinity to mark. These are not the ideal optics to use for checking and adjusting your back focus, but if that's the case and they are your only glass, try to keep some consistency by going from close focus to mark and repeating that action to get a predictable result. A good lens mechanically hits the mark no matter what direction you are coming from.

    Extremely happy to see the Critical Focus terminology getting out there. This is one of the major procedures in ensuring you are getting the most of your camera and lens combination. If backfocus is off, beyond missing marks, this can effect your image quality and lens performance if it's way off. Prep it and don't regret it. The time and effort spent here can save a lot of headaches and heartbreak when filming.

    One other hot tip, during prep clean the PL Mount on the camera and even PL mount on the lenses. A little dust layer, grease, or gremlin sneaking in there makes a difference when you're talking thousandths of an inch, especially on longer glass.
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  2. #2  
    Quote Originally Posted by Phil Holland View Post
    [...]
    One other hot tip, during prep clean the PL Mount on the camera and even PL mount on the lenses. A little dust layer, grease, or gremlin sneaking in there makes a difference when you're talking thousandths of an inch, especially on longer glass.
    By longer glass, I think you mean shorter glass, like the Tokina 11-20 zoom.
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  3. #3  
    Moderator Phil Holland's Avatar
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    Quote Originally Posted by Michael Tiemann View Post
    By longer glass, I think you mean shorter glass, like the Tokina 11-20 zoom.
    If it's off it's off, but yes, also apparent on ultrawides as well.
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  4. #4  
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    Advice? I watched the video, and tested at 4' with a fairly new prime set of 7 Sigma PL lenses. All the long ones were exactly on the money (85/105/135). But the 20/35/50 came into critical focus at 4.5 feet. It was a little hard to tell on the 14 since so wide but also looked to be 4.5-5.0 feet on the scale. I did not test closer nor further away, but does this mean these lenses need to be shimmed? I don't really use a tape so probably will just live with this, but kind of surprising to find...
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  5. #5  
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    Quote Originally Posted by Jon Dishler View Post
    Advice? I watched the video, and tested at 4' with a fairly new prime set of 7 Sigma PL lenses. All the long ones were exactly on the money (85/105/135). But the 20/35/50 came into critical focus at 4.5 feet. It was a little hard to tell on the 14 since so wide but also looked to be 4.5-5.0 feet on the scale. I did not test closer nor further away, but does this mean these lenses need to be shimmed? I don't really use a tape so probably will just live with this, but kind of surprising to find...
    Likely and worth investigating. You can shim them and I know Duclos Lenses offers this service.
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  6. #6  
    Quote Originally Posted by Jon Dishler View Post
    Advice? I watched the video, and tested at 4' with a fairly new prime set of 7 Sigma PL lenses. All the long ones were exactly on the money (85/105/135). But the 20/35/50 came into critical focus at 4.5 feet. It was a little hard to tell on the 14 since so wide but also looked to be 4.5-5.0 feet on the scale. I did not test closer nor further away, but does this mean these lenses need to be shimmed? I don't really use a tape so probably will just live with this, but kind of surprising to find...
    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.
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  7. #7  
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    The advice I got from Blair Paulsen was that with lenses much longer than 20mm, backfocus has less of an impact. Of course, the tricky part is that it's much easier to judge focus on a longer lens. FWIW, I have the same set of Sigma Cine primes with PL mounts, and haven't needed to shim them.
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  8. #8  
    Moderator Phil Holland's Avatar
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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.
    It actually depends on the lens design itself. Typically yes, the wider will show issues as well, but the more telecentric the design, less so.

    I'm not saying don't test your wides, I'm saying verify back focus with your entire set of glass for each project. It will tell you about both the sensor positioning as well as a potential issue with the glass itself.
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  9. #9  
    Senior Member Jacek Zakowicz's Avatar
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    Quote Originally Posted by Phil Holland View Post
    It actually depends on the lens design itself. Typically yes, the wider will show issues as well, but the more telecentric the design, less so.

    I'm not saying don't test your wides, I'm saying verify back focus with your entire set of glass for each project. It will tell you about both the sensor positioning as well as a potential issue with the glass itself.
    This has nothing to do with telecentric design and everything to do with depth of focus which is a function of magnification and f stop. The wider the lens the more back focus sensitive. The closer you get to the subject /chart, the more critical/sensitive/visible the errors is , if any. So you want to use fast, wide lens at short distance for back focus checking. Wide zooms are even better for that.
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  10. #10  
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    Thank you so much for this advice! It might be possible then to adjust the back focus on the camera for the wider lenses and when they are correct, then the longer lenses would still be pretty close? It does seem the wider I test, the more it is off. this would be a lot easier than sending in the lenses for shimming and might be the correct approach anyway.
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