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Flange focal distance


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#1 Paul Bruening

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Posted 04 July 2009 - 08:40 AM

Hey gang,

I scrounged a depth micrometer off of Fleabay for almost nothing. The seller grumbled but sent it anyway. If you're patient, you can snag some stuff for obscenely low prices off of Ebay. Has anyone here used one for setting the FFD of a lens board on a cine cam? If I use the lens flange I'll have to cut some of the T down off of the micrometer. I've thought about it and this seems to be the best way. This way I can measure both sides of the film frame to make sure the image doesn't keystone. Is this an OK approach or do you know a better way? Are there other factors I should consider?

Thanks in advance for any ideas and help.
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#2 Tom Jensen

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Posted 04 July 2009 - 06:44 PM

I'm not sure what you mean by the T but the end should be just a rounded ball type end. If there is a T it should be near the gauge and should cover the port. You have a flat plate that you place over the back of the aperture and move the gauge around to the corners if that makes sense.
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#3 Tim Carroll

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Posted 04 July 2009 - 07:32 PM

Paul,

The FFD gage I use is the one from ARRI. It's pictured below.

Posted Image

Posted Image

The dial indicator is spring loaded and reads down to .001 mm, which Axel Broda is always teasing me about being "too accurate". You place the dial indicator's shaft through the flange mount (a piece of precision steel that fits into the particular mount on your camera, in my case, ARRI Standard Mount, ARRI Bayonet Mount, and I also have a flange mount for ARRI PL Mount) and then you place the 52mm Standard (a precision ground metal tube) on a surface plate and gently set the dial indicator with the flange mount into the standard. Then you zero your dial indicator and carefully remove it (with the flange mount) from the standard and place it into the lens flange on the camera (after you have made sure the mirror is out of the way). You place a polished ground flat against the film side of the gate, and take your reading. The flange mount fits into the lens mount on the camera in such a way as you can move the dial indicator around in the lens mount, to check the different corners and sides of the gate, so you are sure the gate is flat and parallel to the lens mount.

Hope that makes some kind of sense. Never tried to explain that before.

Best,
-Tim
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#4 Paul Bruening

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Posted 05 July 2009 - 11:06 AM

Tom and Tim,

Thanks, Buds. I see a better way from what you've told me. I was going to shave the micrometer's base (the T that I was referring to) so that it would fit inside the latch tabs and flat enough on the flange to get an accurate reading. I would have to further shave the base to allow the unit to fit so that readings on both sides of the frame were possible. This was a problem as the base would be riding the edge of the flange and not fall dependably across the rather small ridge of the flange. Based upon Tim's info, I think I'll take a lock ring from a stripped down lens and put it in the board. If I get an accurate read of it with an outside micrometer I can simply subtract that number from the depth micrometer's reading. The lens ring stands up high enough that the micrometer's base will clear the board's ring and I won't have to shave anything off of it.
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#5 Tim Carroll

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Posted 05 July 2009 - 11:40 AM

Paul,

Just remember, when you are talking FFD on most cameras, the tolerance is usually plus or minus 5 thousandths of a millimeter or .005mm. That is plus or minus 2 ten thousandths of an inch, or .0002". Make sure your micrometer goes down that far and is accurate in that range.

You will also need some kind of a standard to check your micrometer against, a precision gage block will probably do okay.

Best,
-Tim
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#6 Paul Bruening

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Posted 05 July 2009 - 01:02 PM

Great info. Thanks.

What are your views on adjusting measurements and shimming for the center layer of the tri-pack instead of the film plane? Is that already a part of the specs, should it be factored in or should it be ignored?
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#7 Tim Carroll

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Posted 05 July 2009 - 01:32 PM

Paul,

Not sure what camera you are talking about. Remember you need to monitor the relationship of the lens mount, the gate, and the ground glass. The order in which I usually do it, is set the FFD first, then adjust the ground glass to match (using a collimator). If you then shim the lens mount, you need to go back in and re-do the ground glass. If you just shim the gate, and the lens mount to ground glass is already set proper, you don't need to re-do the ground glass.

Here's a kind of pictorial representation of what I am talking about.

Getting the most from your camera

Best,
-Tim
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#8 Stephen Williams

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Posted 05 July 2009 - 03:35 PM

Great info. Thanks.

What are your views on adjusting measurements and shimming for the center layer of the tri-pack instead of the film plane? Is that already a part of the specs, should it be factored in or should it be ignored?


Thats normal, from memory the Fries should be 51.97 - 51.98
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#9 Paul Bruening

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Posted 05 July 2009 - 04:55 PM

Hey Stephen,

Thanks for adding in. After reading your link, Tim, it occurred to me that I've just assumed the flange for my Nikon lens board was the ridge on which the lens's catches press against. From your link's diagram the measure point of the flange seems to be the outermost edge of the lens mount. So, my question to both of you is: Which surface on the Nikon lens board do I consider the measure point? I assumed I would find it during measurement as the one most closest to the specs, but have decided to just plain ask even at the risk of appearing dumber than usual.

I have assumed that the ground glass is okay. The reason I am working out the FFD for the lens board is because, as Stephen knows, it is switchable on a Fries 35R3. It had the Nikon board on it when I bought it with the PL board in the case. But, it had a sheet of wratten cut to shape for the shim. This looked pretty dubious to me. So, I'm putting forth the effort to know how to set the FFD for both of the boards.
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#10 Tom Jensen

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Posted 05 July 2009 - 05:35 PM

If you suspect a flange is out and you're on location, stick to long lenses.
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#11 Tim Carroll

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Posted 05 July 2009 - 05:47 PM

But, it had a sheet of wratten cut to shape for the shim. This looked pretty dubious to me. So, I'm putting forth the effort to know how to set the FFD for both of the boards.


Paul,

I'll let Stephen give you the Fries specific details, because I don't know that camera.

But I wouldn't write off the sheet of wratten cut film. It may well be the shim used by the previous owner. Here in the States we use the ARGUS shim stock on different parts of the cameras, and some of it has the look of wratten film. When you are trying to shim .005mm or .010mm you got to realize you're looking at material that makes aluminum foil look like cardboard.

IIRC, once I did a comparison with the acceptable range of the FFD on a motion picture camera, the plus or minus five thousandths of a millimeter, and it came out to something like one tenth the diameter of a human hair. So we're talking real tiny stuff here.

And as Tom pointed out, it really comes into play when you are using wide angle lenses.

Best,
-Tim
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#12 Tom Jensen

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Posted 05 July 2009 - 06:17 PM

Paul,

But I wouldn't write off the sheet of wratten cut film. It may well be the shim used by the previous owner. Here in the States we use the ARGUS shim stock on different parts of the cameras, and some of it has the look of wratten film. When you are trying to shim .005mm or .010mm you got to realize you're looking at material that makes aluminum foil look like cardboard.

IIRC, once I did a comparison with the acceptable range of the FFD on a motion picture camera, the plus or minus five thousandths of a millimeter, and it came out to something like one tenth the diameter of a human hair. So we're talking real tiny stuff here.

Best,
-Tim


And, most shim stock is color coded which could give the appearance of a filter which it isn't. You may see several piecs of different colored shim stock.
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#13 Paul Bruening

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Posted 05 July 2009 - 07:14 PM

Then, it probably is shim stock. I didn't know shim stock was plastic or colored. I assumed shim stock would be metal. What about light leaking through that short length of plastic?
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#14 Tim Carroll

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Posted 05 July 2009 - 08:43 PM

Then, it probably is shim stock. I didn't know shim stock was plastic or colored. I assumed shim stock would be metal. What about light leaking through that short length of plastic?


Paul,

The Argus shim stock I was talking about is different colored, but it is all opaque. I was trying to say it is as thin as wratten film, not that it was transparent.

Now ARRI uses transparent colored shim stock on the 16SR and 16SRII gate assemblies, but those are well down into the camera and would not pose any kind of issue with light leaks.

Again, being ignorant of the Fries camera, I would not know how a transparent shim stock could or could not cause light leaks with that camera, sorry. I'm sure Stephen would know.

Best,
-Tim
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#15 Paul Bruening

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Posted 05 July 2009 - 10:35 PM

I found the shim stock you referred to on the web. Thanks for the lead, Tim. That looks easy to cut out to the lens board's shape.
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#16 Hal Smith

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Posted 06 July 2009 - 08:05 AM

http://www.practishi...ex.php?cPath=22
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#17 Paul Bruening

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Posted 06 July 2009 - 10:39 AM

Hey fellas,

Ya'll took me from a guy with just a notion to a guy armed with ability. That means a lot to me. Thank you all for the help.
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#18 Tim Carroll

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Posted 06 July 2009 - 12:57 PM

Or did we take you from a "guy with just a notion to a guy" who knows just enough to be dangerous. :D

Seriously, best of luck with it Paul. Let us know how it turns out.

Best,
-Tim
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#19 Tim Carroll

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Posted 12 July 2009 - 05:56 AM

Paul,

Gave you some bum information. It is ARTUS shim stock, not ARGUS shim stock. Sorry about that.

Best,
-Tim
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#20 Stephen Williams

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Posted 12 July 2009 - 06:11 AM

Hi Paul,

The measurment is to the front surface of the lens mount. The lens mount block can be shimmed differently side to side or top to bottom making adjustment fairly straightforward.

Bruce has a spare 2 perf gate on Ebay!

Stephen
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Glidecam

Metropolis Post

Aerial Filmworks

Ritter Battery

Wooden Camera

rebotnix Technologies