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Flange depth question


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#1 Alex Corn

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Posted 24 January 2006 - 04:36 PM

OK, here goes. If the flange depth on a camera is off, will the viewfinder ever be able to resolve an image? For instance, say I test a lens and it is sharp at a distance marked on the barrel different than the actual distance. If i adjust the ground glass length, and it appears in focus in coincidence with the markings on the barrel and in real life, is it safe to assume that it will be in focus at the gate? I guess i'm wondering that if the flange depth is off, than the image will not be resolved at the gate in accordance with the barrel markings, and so I dont think you could adjust the groundglass distance and resolve an image that is soft at the gate. I feel like this is worded poorly, but hopefully someone can help with this little dilemna. Thanks.
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#2 Laurent Andrieux

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Posted 24 January 2006 - 04:51 PM

say I test a lens and it is sharp at a distance marked on the barrel different than the actual distance. If i adjust the ground glass length, and it appears in focus in coincidence with the markings on the barrel and in real life, is it safe to assume that it will be in focus at the gate?


No, it won't be sharp at the gate.

adjusting the ground glass has no effect on focus on the gate plane.

I guess i'm wondering that if the flange depth is off, than the image will not be resolved at the gate in accordance with the barrel markings, and so I dont think you could adjust the groundglass distance and resolve an image that is soft at the gate.


Yes you could, in a certain range, have a sharp image on the ground glass even though it's out of focus on the film plane, but who would like such a situation ?

The ground glass has to be adjusted so that it is virtually at the same distance from the lens as the gate is, so that you see what you will get on film.

The camera and lenses have to be adjusted so that the focus occures accordingly to the marks on the barrel and the ground glass has to be adjusted so that the image is sharp when focus is made with a lens that is correctly adjusted.
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#3 Tim Carroll

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Posted 24 January 2006 - 04:56 PM

The Flange Focal Distance can be set exactly accurate and your ground glass can be off, or your ground glass can be set exactly accurate and your Flange Focal Distance can be off, or they can both be set exactly accurate or they can both be off.

They are separate things and need to each be calibrated and set properly. In simplest version, the flange focal distance is the distance from your flange seat to your gate. Then your ground glass must be set properly from the flange seat to your ground glass.

As an example, I had a camera where the flange focal distance was right on, but the ground glass was off. So when I took my test lens (the one that had been recently calibrated) and framed up a focus chart at five feet away, and set the lens to five feet, the image on the film was dead sharp. But the image in my ground glass was blurry. If I focused using my ground glass, then the test lens would not say five feet and the image on my film would be soft.

They both need to be set independently. On some cameras, like an ARRI SR, if everything is set properly and then you need to remove the front of the camera (which holds the flange seat), when you put the front back on the camera, and set the flange focal distance, your ground glass may be fine (although I would still test it), because the mechanics that hold the ground glass in relationship to the gate, none of that has changed when you remove the front of the camera.

Well, that was a long winded explanation. Hope it made sense.

-Tim
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#4 Laurent Andrieux

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Posted 24 January 2006 - 05:15 PM

Usually, we do things in that order :

1) Have the focal flange of the camera accordingly to the camera's manufacturer

2) test your lenses. That means doing shots of a definition charts that you read afterwards with a microscope (a binocular magnifying glass, actually) at different lens settings, around the marked distance, this distance being close to 50 times the focal length, at wich you placed the charts, by steps of 1 mm + and minus 5 mm, wide open.

So you determine if the focus occures accordingly to the marks on the lens

3) Look in the ground glass, at the distance you're shoting your test, and focus by eye.

If the shot that is the best defined is the one that was done exactly at the distance marked on the lens, accordingly to the real distance of the shot, your lens is properly adjusted.

If it's not the case, have a technician calibrate your lens.

If the distance you find by eye is the same as your shot test, the ground glass is properly adjusted, even though the lens might have not been well calibrated.

If it's not the case, have a technician calibrate your groundglass.

Hope it makes sense etc.
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#5 Alex Corn

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Posted 24 January 2006 - 05:16 PM

"The ground glass has to be adjusted so that it is virtually at the same distance from the lens as the gate is, so that you see what you will get on film."

So in the case of the Arri-S/SB, where the ground glass is at a much longer distance from the lens than the gate, what point would need to be the equivalent distance? The prism? If so, they are attached to the same unit, and cannot be adjusted independently of each other. Therefore, if I can resolve an image at the prism in accordance with the barrel markings of my recently calibrated lens, can it be assumed that, since the distances are inherently the same, that the image is being resolved at the gate as well?

In my head, the way I see it is this: If I were focusing at a screen that had a projection on it, and the porjection itself was out of focus, I could not adjust the lens to a point where I will resolve the image on the screen. This is kind of the same situation with the S. I feel like if I can resolve the image at all in the viewfinder, than it would have to be in focus at the gate, becuase the prism I am adjusting to is equidistant to the flange depth. Maybe my thinking is all wrong, though, and thats kind of what i'm trying to figure out.

Thanks for the help in any case, as usual you guys are on top of it.
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#6 Laurent Andrieux

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Posted 24 January 2006 - 05:26 PM

the ground glass is at a much longer distance from the lens than the gate,


I'm afraid you are mistaking the view finder and the ground glass.

But since Tim is a specialist for the arri 16 S, I'm pretty sure he can be helpfull for what is about adjusting the ground glass on this camera...

I only remember he said one should be aware that the door has the same serial number as the camera, because of the groung glass being part of the door...

Let's see what he says about that...

In my head, the way I see it is this: If I were focusing at a screen that had a projection on it, and the porjection itself was out of focus, I could not adjust the lens to a point where I will resolve the image on the screen. This is kind of the same situation with the S. I feel like if I can resolve the image at all in the viewfinder, than it would have to be in focus at the gate, becuase the prism I am adjusting to is equidistant to the flange depth. Maybe my thinking is all wrong


On this very point, I'm afraid it is.

You see, the ground glass is not providing an image from the film gate, like shooting a screen.

The shutter gives alternativly the image to the film plane during 1/48 s and to the ground glass (thtough the mirror and prism) during another 1/48s, so that the whole cycle repeats 24 times a second.

You once have the image on the ground glass, once on the film etc.

So the flange distance of both camera and lens make the image look sharp on the film and, for what is about the period of time during wich the light goes to the ground glass, it's the ground glass (and prism on the 16s) calibratingthat make the image sharp on the ground glass, and, therefore, in the viewfinder.

Am I etc. ?
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#7 Alex Corn

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Posted 24 January 2006 - 06:45 PM

Okay...Just to clarify, the ground glass in an Arri-S is at the end of the viewfinder, and as far the door goes, except on earlier models, the internal glass can be removed and exchanged, therefore enabling one to switch doors or viewfinders. Here is where my confusion arises: I understand how the mirrored shutter works, how the image I see through the viewfinder is not the image occuring at the gate and therefore is not the one going onto my film. But in the construction of the S, the prism that goes to the viewfinder and the gate are at 90 degrees to one another and on the same piece of metal, and they are completely symmetrically contsructed, so when an image is coming from the back of the lens, the flange depth (this length) is equivalent to the distance to the prism. Therefore, I assume (and this is the key word) that if the image that is on the prism is in focus in accordance with the barrel of the lens and real life, since that distance is equal to the flange depth, that the image on the gate will also be in focus at that mark, becuase I have focused the internal glass of the viewfinder onto an image being projected onto a prism from the mirror which in turn comes from the back of the lens which is the distance of the flange depth (the distance from the back of the lens to the gate). Here is where logic seems to fail me. The nominal flange depth of an Arri-S is 52mm. None of my 158 cameras have this flange depth. I cannot figure out how a lens could focus at the correct mark on the barrel, and still be in focus at the gate when the flange depth is off. But, I can manipulate the internal glass of the viewfinder so that my eye sees a sharp image at the correct mark. This could only occur, logically, if I was resolving an image appearing on the mirror out of focus. This is what I am asking, is that possible, or are my images going to gate ok. Sorry for being a pain in the ass, but I just cant get it into my head that I would be seeing an image that is out of focus being made into focus by a sceond lens. Thanks again.
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#8 Tim Carroll

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Posted 24 January 2006 - 07:01 PM

Okay, to put things in a bit of perspective. You are correct, the prism on the S/B is mounted to a piece of steel that is also mounted to the gate. But you must understand the type of tolerances we are talking here. We are not talking about thousandths of an inch, we are talking thousandths of a millimeter. I had an S that had it's flange focal distance (gate) out and I was trying to get it dead on. A friend was watching me and asked me how far off it was. I told him the numbers, but it did not register. Then I took a piece of my hair, and measured it with a micrometer and showed it to him and told him the FFD was off by one tenth of the diameter of that piece of hair. In other words, we are talking very small measurements here.

So the prism and the gate are attached to the same piece of metal, but it can be off slightly one way or the other, and if it is off more than one tenth of the diameter of a human hair, it is going to effect your picture, especially with wider angle lenses. Also, you must remember the mirror effects the distance the light must travel from the flange seat to the prism and then on to the ground glass. And again, anything off by as much as one tenth the diameter of a human hair and you have problems.

Here's how I set up Arriflex 16S and S/B cameras. First I take them all apart and clean and lubricate everything with ARRI oil and grease. Then I reassemble the camera and set the Flange Focal Distance (with an ARRI gage) and check all three lens mounts on the turret to make sure none of them are off. After I know I have the FFD set correctly and the camera plate secured tightly, then I move on to the ground glass.

I take a lens that I have had calibrated by my lens tech and set up the camera five feet from a focus chart taped to the wall. I set my lens at five feet and I see how sharp the image is on the ground glass. If it is off, I go in and adjust the ground glass until the image is crystal clear and sharp as a tack (which is pretty impressive on some of the cameras I do which are fifty years old). Then I know the camera is set up right. And of course I do film tests to verify my work.

Hope that helps,
-Tim
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#9 Laurent Andrieux

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Posted 24 January 2006 - 07:23 PM

Yes, at 5 feet, I guess Tim is talking about a 25 mm. When you have a 1 mm offset on the lens barrel it means about a 1/100 of mm in the film or groundglass plane.

When you say you FFD is not 52, how do you measure it ? This distance should be measured from the mount to the film plane, it doesn't pass by any prism or anything. Gate open straight to the film. You need a special piece of metal you put in place of the film and a micro comparator.

Once you have a well calibrated camera FFD and lens, then you can adjust the ground glass.

I cannot figure out how a lens could focus at the correct mark on the barrel, and still be in focus at the gate when the flange depth is off.


If the lens is calibrated so that whole flange goes fine to the film, it's possible.

Imagine the camera flange distance is too short, and the lens flange focal is too long, that's possible. Imagine someone has calibrated the lens so that they look calibrated, even though the camera FFD is not the one that ARRI preconises...

But, I can manipulate the internal glass of the viewfinder so that my eye sees a sharp image at the correct mark.


Do you mean manipulating the diopter or calibrating the ground glass ?
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#10 Alex Corn

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Posted 24 January 2006 - 07:35 PM

You guys are awesome, these are the exact answers I was looking for. First, Tim, i too dissassamble the entire thing and lubricate with arri grease, but after that how does one adjust the FFD? At this point i just reassemble. Then, where does one get this piece of metal that simulates the film, becuase this could definitely account for why my depth gauge readings are so off. I have been told that the tolerances of the S's are about 5 microns, and if thats right than my cameras are waaaaayy off. The help so far is great, thanks again.

"Do you mean manipulating the diopter or calibrating the ground glass ?"

Calibrating the ground glass. This is something I do when all the lenses on a camera are off for no apparent reason. I take a lens from my tech in LA that has recently been calibrated and adjust the ground glass until sharp.
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#11 Tim Carroll

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Posted 24 January 2006 - 08:03 PM

You guys are awesome, these are the exact answers I was looking for. First, Tim, i too dissassamble the entire thing and lubricate with arri grease, but after that how does one adjust the FFD? At this point i just reassemble. Then, where does one get this piece of metal that simulates the film, becuase this could definitely account for why my depth gauge readings are so off. I have been told that the tolerances of the S's are about 5 microns, and if thats right than my cameras are waaaaayy off. The help so far is great, thanks again.


You need to get a Flange Focal Distance gage. You may be able to get one from ARRI in New York, but be prepared to pay about $1600 for it. It comes with the 52mm standard and the gage and the little metal block that fits in the gate. You can sometimes find them on eBay, but beware, some of the ones on eBay won't really do what you need. I got mine from a tech in Germany, it is the one sold by ARRI.

Posted Image

Also you really need to get a copy of the ARRI 16S service manual. Talk to George at ARRI in New York, he may be able to provide you one. Adjusting the FFD on an Arri 16S or S/B is almost an art form, so plan on lots of patience and take it slow. Took me a good few weeks of doing it over and over again till I got it down fairly well. Still can't set them as quickly as those guys in Germany, but what the heck.

And get yourself a dedicated lens and have your lens tech set it for you, and use it ONLY for setting your ground glass. Don't let any of the students borrow it for a shoot no matter how much the beg or you will be finding another dedicated lens.

-Tim
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#12 Laurent Andrieux

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Posted 24 January 2006 - 08:13 PM

I cannot figure out how a lens could focus at the correct mark on the barrel, and still be in focus at the gate when the flange depth is off. But, I can manipulate the internal glass of the viewfinder so that my eye sees a sharp image at the correct mark. This could only occur, logically, if I was resolving an image appearing on the mirror out of focus. This is what I am asking, is that possible, or are my images going to gate ok. Sorry for being a pain in the ass, but I just cant get it into my head that I would be seeing an image that is out of focus being made into focus by a sceond lens.


Neither the mirror nor the prism "form" an image, they are only optical ways, vehicles.

The image is formed on the ground glass only. (sorry for my bad english).

There is no image appearing on the mirror.

For the rest, read again what we said at the beginning of the topic, hope you'll get it.

And, you're not a pain in the ass, man ! if you were so, we wouldn't take time to answer ! :)
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#13 Alex Corn

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Posted 25 January 2006 - 09:46 AM

Okay, So i have all the neccesary tools for measuring the FFD (They were well hidden by the last tech) and have found that a lot of my cameras are fine. But some are off by one or two hundreths of a millimeter. My question now is, what is an acceptable tolerance? I've read in one place that one hunredth of a millimeter (10 micron) is the nominal tolerance for older arris (which i assume includes the S/SB), and about five thousandths (5 micron) for newer Arris (which i assume includes the SR/2/3). Is that about right? Thanks again, you've both ben a big help.
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#14 Tim Carroll

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Posted 25 January 2006 - 10:18 AM

Tolerance on the Arriflex 16S and S/B, flange to gate dimension, FFD, should be 51.970 to 51.980 mm. You may also want to send your 52mm standard over to ARRI in New York and have them gage your standard. I had them do that to mine and found it to be slightly off (mine read 51.995 instead of 52.000) You need to take that into consideration when using your gage to set the FFD. And the tolerance across the gate area should be within .005 mm. In other words, even though you have .010 mm to play with for the tolerance of the FFD, the variance across the gate from top to bottom, side to side, and corner to corner, should not vary more than .005 mm.

-Tim
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#15 Laurent Andrieux

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Posted 25 January 2006 - 01:18 PM

It would be good to have all your cameras at the same FFD, so that you basically can use lenses on whatever camera. Maybe you'better fix the ones to the same setting as you majority of cameras are.

If it's the same on ARRI16 S as it is on BL 35 and many cameras,

As to fix it, you need to unscrew the front lens mount of the camera. There you will see color fix rings. You need to have a set of them, they come in different thicknesses (1/2 100th, 1 1/100 th, 2 1/100 th etc.) If ARRi can't give you these fixes, you can buy sheets of them and cut them carefully at the proper shape.

You need a micrometric comparator, like a slide caliper, you see, to determine the actual thickness of the fixes you have in your mount, and then give it the proper fixes as to get the added required thikness, or change them for thinner ones if the FDD is too short.

Can you Tim, confirm it's like this on the 16 S or another way (my cameras are not in my house...) ?

After you have screwed back and are happy with the measurement, make shot tests, and finish with the groud glass as the last step.
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#16 Tim Carroll

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Posted 25 January 2006 - 01:52 PM

Can you Tim, confirm it's like this on the 16 S or another way (my cameras are not in my house...) ?


With the 16S and S/B there are no shims. You adjust the FFD by a number of different methods, but most of it is done by adjusting the plate or what the German techs call the "platina", which is the main metal plate that all the gears and sprockets are attached to and through. It is a bit tricky which is why I suggest getting a copy of the Arriflex 16S service manual or talking to someone at ARRI.

-Tim
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