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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

What I learned today getting my Scarlet and lenses checked by a lens tech.

You can rough in the backfocus the way broadcast cameras are usually done. Throw on the widest lens you have, wide open, set to infinity. Punch in at something at infinity and adjust backfocus until it's set. Keep in mind the backfocus adjust on some cameras has a "safety" screw protecting it you have to unscrew first.
 
sam karr said:
I'm totally ignorant about back focus and some other things. Is back focus off only affecting focus marks? Some people are telling that it should be sharp doing a rack focus even with back focus off. Is it true?
.
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Yes. for a prime lens But note if you have a zoom lenses and set focus then zoom your focus will be off too
 
Hey Brian as mentioned by an earlier poster, mass produced lenses are a lucky dip.

Your 24 - 70 could be just soft on the Scarlet not the 7D because of differences in the mount thickness etc. The only way to know is try out as many lenses in your quiver as possible if you don't have more lenses, beg, borrow , steal. Then if your stiil getting soft images , after some sharpening in post, then reset the back focus.

You've got a chart, it isn't black magic, it can be done pretty easily. After all shouldn't every camera owner know how to set BF on their own camera, particularly if swapping in & out of different mounts ?

P.S. it would be interesting to know B.F. drifts say during long travel periods etc.?
 
Now, I've seen some people worry that their camera isn't shooting in focus because the flange focal distance is off. This is most likely incorrect.

When you focus a lens, you are in effect changing the flange focal distance. Try it - grab that focus ring, turn it, and watch the lens move in and out in relation to the camera.

So you shoot something with your Red camera, and it doesn't seem sharp. You think, hey, I wonder if my flange focal distance is wrong and causing this. Do this test:

Set up your camera pointed to a sheet of newspaper, or Siemens Star chart. Focus, using the monitor on zoom in mode, by eye.

Were you able to focus before, and then after the chart? In other words, could you get the chart out of focus by turning the focus ring to the near end, then get it out of focus by turning the focus ring to the far end, passing a point of sharp focus in between?

If you can, then your flange focal distance might not be dead on, but it's close enough so that the lens is able to throw the sharpest image it can on the sensor. So if at this point you still feel that the image isn't sharp enough, then maybe you've discovered just how ultra sharp the Red camera sensor is and how it can turn up flaws in lense that you thought were super sharp. Or, maybe you need to do a better debay in Redcine. Or mabye you are looking at a lower res version of the file when you thought otherwise.

I just see a lot of misinformation here about how incorrect flange focal distance can make lenses soft, when in reality this would just shift the focus point, unless it's REALLY out and you can't focus near, then far, past your chart.
 
scott william said:
if i'm not mistaken you should be able to get a sharp picture even if your back focus is out by using the 'punch in button' to get focus, correct? just means your lens focus marks will be off and your infinity focus may not be precise. also means your zooms function more like variable primes.
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One note. The Canon 24-70 goes *BEYOND* infinity on its focus. That was an obnoxious discovery.
 
Brian, I believe on the mass-produced lenses it's so that when the collimation does drift on the lens, you have a but of leeway so that you can still hit infinity by focusing by eye. Kind of like a margin for error in the lens.
 
corrections

corrections

Wow the speculations are running rampant here:
Let me clear some things for you here:
Most, if not all true motion picture lenses pass infinity. Simply because the focus pullers want them to. This is how the focus pullers work.
also there is a good reason for the Canon lenses to pass infinity:
Canon traditionally uses fluoride glass elements in their optical design for low spherical aberration, good contrast and color rendition etc. The drawback of the fluoride glass substrate is low thermal stability. Hence the elaborate index mark "art work" on the focus window on the 24-70 lens. This is a top of the line lens for Canon and sloppy assembly simply doesn't apply.
BTW I checked one of my 24-70 in projection yesterday and it's a hell of a lens optically.....:wink5:
 
We found that both our Epic and the Scarlet needed a 1/2 turn counter-clockwise to adjust the BF to work with our 16-35, 8-16 and 24-70 lenses. Now they all hold focus throughout the zoom. Best to adjust with a 16-35 or wider.

That said... I bought a new 24-70mm in December and it took 3 to find a sharp one. One was really soft at F/2.8. The first time I bought a 24-70mm in 2006 it took 5 lenses to find a sharp one. Yes, Canon can adjust most lenses to be sharper but that is a pain and costs $$. I find it better to test when buying.

If you have enough Canon cameras and lenses to qualify you can join Canon Professional Services and you get two free clean / adjust coupons with the Gold membership for $100.00. And fast turn-around. Great deal. http://www.cps.usa.canon.com/about_cps/about.shtml
 
It's hard to tell Brian. I find sharpness to be a relative thing: you need to compare a well known sharp lens like the MP or S4 to your 24-70 to really tell.
Or try your lens on two different cameras like scarlet and 7d in still mode and see the results.
if you are able to go through focus in both directions the lens BF and the camera flange depth don't matter much....
 
Julien: The thing about changing mounts is, the mount is what's setting the flange distance for each particular type of lense. So, theoretically, if the BF is correct for one, it's going to be correct for the other.

Brian: you mentioned the focus marks being off a little (that, to me, means back focus is off for sure). Is it noticeably sharper when you set focus be eye (completely disregarding the focus marks)? I'm guessing you're setting it to where it's sharpest, and not using just the marks.

I've had this same thing happen with cheap-o F-to-EOS adapters. Focus marks are off (especially at infinity), but it can still get tack sharp when I disregard the marks. In either case, let us know what RED says. I guess that's one of the huge advantages of being in Cali.
 
Mike P. said:
Julien: The thing about changing mounts is, the mount is what's setting the flange distance for each particular type of lense. So, theoretically, if the BF is correct for one, it's going to be correct for the other.
Click to expand...

But if the mount is not seated exactly the same as the mount before the change, then the whole thing would be off... It would effect every lens you put on.
 
Gavin Greenwalt said:
One note. The Canon 24-70 goes *BEYOND* infinity on its focus. That was an obnoxious discovery.
Click to expand...

Most lenses do. Last thing you want is a hard stop at an infinity mark calibrated under certain conditions, only to find that when the temperature drops to well below freezing, that the infinity mark no longer lines up. As an AC or focus puller, a hard stop is the devil. You don't want to rack a focus from, say 35ft to infinity and end with a thud at the infinity mark. You'll be hard pressed to find cine lenses with a hard stop right at infinity, it's usually a bit after. SLR lenses are notorious for having markings that don't line up and they also include extra rotation at both ends of the focal range to compensate for differences. We don't set the back-focus or flange depth on SLR cameras -- they come from the factory to be built within a certain tolerance range. And the tolerances are surprisingly sloppy. The lenses and focus electronics compensate for this.... And SLR lens marks are not always accurate, even if a lens and camera are properly adjusted. So I wouldn't put too much stock in markings on an SLR lens... I've just never found them to be 100% reliable. Now, if they're off by a lot, like 1/4" off the mark, then something definitely is wrong somewhere.

Which leads to another interesting point within this thread. I haven't looked at any of the images available to download, just been reading. I'm not ready to rule out a problem with Brian's lens just yet, seems likely that it is the lens, however if he can't achieve proper focus at some point in the focus range, then his back focus must be quite a ways off. The lens could be off too, but not far enough off where the 7D can still make it work. Perhaps it's a combination of the camera and lens being off. If you have to set with a lens, the best way is to get the widest prime you can -- 14mm is a good place to start, set it to infinity. Stop down to at least F5.6, F8 or F11 are even better if it doesn't start affecting the lens performance. Adjust the back focus screw on the camera to focus on something at infinity distance -- trees, detailed building, etc... You might be able to pull it off, but trying to set the FFD with a Canon mount SLR lens is somewhat of a circle jerk though. One would do best to locate a lens tech with proper collimating equipment, or a diascope or other tools that can be used to check the FFD / back focus. That's really going to be somewhat difficult unless you happen to also have the PL mount for the camera. Some diascope systems (like the Optitek Optimator) have interchangeable mounts and can be used with a Canon mount.

I'd like to say I'm surprised that some cameras are coming from RED with improper back focus adjustments, but it's happened in the past. When my original RED One came back from getting an audio board upgrade, and some other fixes, they re-set my PL mount and it was way off.

As others have indicated, checking your back focus and getting it properly set are a normal part of prepping a camera for a shoot. Unfortunately, this is a real pain in the ass for those with only a Canon mount. Additionally, many in the Scarlet crowd are new to this, don't have access to the tools to check and set such things. And the tools are hard to come by that can connect to a Canon mount and do what is needed. Currently, the best way on the EPIC/Scarlet, unless you have an Optimator installed and calibrated/verified with a Canon mount, is to check and set the back focus with a PL mount installed, then swap mounts. The reinstall may leave the mount at just a hair off due to slight differences in manufacture of the mount plate or screw torque or whatnot. But should be well within limits of accommodating SLR glass, probably more accurate than a lot of the SLR cameras that glass is designed for.

Wish I had some better suggestions on how to set the back focus for the Canon mount crowd. If you're near RED, you can always bring the camera in and have them help you. For most of the rest of us, we have to find a tech with the right tools or come up with a solution on our own. I was using the IB/E Null system to set the back focus on my RED Ones and it works great. Unfortunately, the Null doesn't fit the EPIC. They're working on a fix, but it's been nearly a year now and I don't think they have come up with anything. Thankfully the TI mount on the EPIC has been solid and I have only had to adjust the back focus once to compensate for some serious heat last summer. But yeah, I had to track down a lens tech to do it since my kit doesn't fit... I'm not a fan of doing it with a lens, really hard to get it right that way.

Another thing to consider too is that RED images do have a soft look and feel to them. There is no in-camera sharpening or processing. Canon, on the other hand sharpens in camera, even their RAW is not truly RAW. There is some minimal noise handling and sharpening done at the RAW interpolation level.
 
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Hi Brian,

Do you have access to any other lenses? Its a good idea to try a few different lenses on there. Do they all have focus problems? Be aware that lenses are frequently not quite right--half of the Canon lenses I've mounted on the RED1 turned out to be incorrect, typically manifesting itself with incorrect focus distances. Also, backfocus on the camera body itself is something you'll need to check regularly (and tweak occasionally) -- so don't be intimidated by it. Embrace the process, learn how its done. Jay Kelly and Doug Underdahl both created helpful videos for the collimating camera bodies without a collimator...search for those videos online and watch them a few times. (oops! Missed Doug's link to them above!!) The first time I watched one of their videos and tried to follow it I had a very difficult time, until I realized some of the lenses I was trying to get to work were not properly set. If memory serves me correct (this is a few years ago now), Doug mentioned in his video that he keeps one relatively cheap, older lens specifically for checking backfocus--in other words, he does not use the lens to shoot anything, he keeps it safe in the shop as a standard to set the camera. I was eventually able to get things to work for myself when I worked out, via process of elimination, which of the lenses I had available that were correct and which were not. You might also just check to make sure your sensor is clean -- again, part of regular regime.
 
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