RED TECH - DSMC2 Back Focus

[Phil Holland]

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.

 

[Michael Tiemann]

[...]
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.

 

[Phil Holland]

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.

 

[Jon Dishler]

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...

 

[Phil Holland]

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.

 

[Michael Tiemann]

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.

 

[Alan Greene]

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.

 

[Phil Holland]

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.

 

[Jacek Zakowicz]

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.

 

[Jon Dishler]

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.

 

[Jacek Zakowicz]

That is correct if all your lenses are the same back focus wise. BTW we should not use the term"back focus" in relation to the camera adjustment. Back focus is the term that applies to lenses. The camera equivalent is the FD (Flange Depth). If you start exchanging them it gets confusing on what you want to do. Adjusting "back focus" should only apply to lenses. Adjusting the "flange depth" only applies to the camera. But what do I know- only have done this for 32 years haha ;-)

 

[Michael Tiemann]

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.

Phil, you are often right about a great many things. But i beg you: DON'T PULL OUT YOUR SHARPIE AND TRY TO CORRECT THE RAY TRACES OF TELEPHOTO vs. WIDE-ANGLE LENSES!

This article from Schneider-Kreuznach explains telecentricity (and the bilaterally telecentric--but also afocal) Kepler Telescope.

 

[Mike P.]

DSMC1 doesn't have the storage/lock bolt, right? It's just the cap and adjustment bolt underneath?

 

[Blair S. Paulsen]

The advice I got from Blair Paulsen was that with lenses much longer than 20mm, backfocus has less of an impact. (snip)

First off, Jacek is correct, it's really FFD that we are talking about.
Secondly, getting the FFD spot on is an important part of a proper prep no matter what lenses you are using.
Thirdly, Alan's reference from me is a practical one. Assuming your FFD is not radically off, with 24-85mm focals you can usually sort the issue with a slight adjustment to the focus mark of the objective lens. OTOH, with ultra wide angle lenses, even a tiny FFD error can make critical focus impossible.

Cheers - #19

 

[Michael Lindsay]

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.

I would agree with you on this ... apart from the ‘nothing to do with telecentric design’ part....now if there was zero glass between the back of the lens and the photo site then I would even agree on that... but there is with most digital movie cameras;-)... wide not very telicentric lenses seem to come up with bigger errors, and wide zooms tend to be pretty good in this regard... now please don’t misinterpret me the telecentric thing is not a big deal at all but it can be a small factor (especially with some older 16mm Lenses)

with ultra wide angle lenses, even a tiny FFD error can make critical focus impossible.

Absolutely! I once grabbed a camera that someone had fiddled (supposedly correcting ....to a badly shimmed longish lens) with and couldn’t get a shot cause my 16mm couldn’t get past about 8ft when wide open...

Ps Made an edit to change ‘non retrofocal lenses‘ to ‘not very telicentric‘

 

[Jacek Zakowicz]

I would agree with you on this ... apart from the ‘nothing to do with telecentric design’ part....now if there was zero glass between the back of the lens and the photo site then I would even agree on that... but there is with most digital movie cameras;-)... wide non retrofocal lenses seem to come up with bigger errors, and wide zooms tend to be pretty good in this regard... now please don’t misinterpret me the telecentric thing is not a big deal at all but it can be a small factor (especially with some older 16mm Lenses)

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...

 

[Dustin Hoye]

Happy to see the video get put up. I had requested it (as I am sure many others did) to their channel, after endless searching of the existing (and great) information already here on RedUser.

I have a quick question- my Sigma ART 18-35 is spot on the tested distance marks from 18 to 35. However my Canon 24-70 (USM II) is only good from 28-70. At 24mm mark, I find I have to re-adjust the focus, at that point, from the same set of tested distances. This seems strange, right?

Is this something that can/should be fixed with the lens itself? Or is this just expected with stills glass, like it?


Thanks in advance.

 

[Blair S. Paulsen]

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

 

[Jacek Zakowicz]

Happy to see the video get put up. I had requested it (as I am sure many others did) to their channel, after endless searching of the existing (and great) information already here on RedUser.

I have a quick question- my Sigma ART 18-35 is spot on the tested distance marks from 18 to 35. However my Canon 24-70 (USM II) is only good from 28-70. At 24mm mark, I find I have to re-adjust the focus, at that point, from the same set of tested distances. This seems strange, right?

Is this something that can/should be fixed with the lens itself? Or is this just expected with stills glass, like it?


Thanks in advance.

here are some photo zoom testing videos. you should get an idea how to check yours from them
part1 https://www.youtube.com/watch?v=3pKFkXfOiog
part2 https://www.youtube.com/watch?v=ki-31j51krE
hope that helps

 

[Phil Holland]

Well if I am contradicting myself, I'll just say this:

Ensuring your Backfocus is correct on your camera can improve your image quality and ensure your marks are lined up.

Checking to make sure your marks are correct across your full set of glass is also important.