Flange distance question

Tyler Purcell · February 21, 2016

This may seem rudimentary, but I've stumbled upon a question that I haven't been able to answer.

So I've got 35mm, 16mm and digital cinema cameras.

The flange distance on all the cameras is different, the 35mm camera of course has the longest distance.

I can put 16mm glass on my digital camera no problem, it vignettes of course, but it works fine.

Yet, when I put the same glass on my 35mm camera, the back focus is way off... so the image is never in focus.

What I don't understand is how you can put standard PL mount 35mm glass on a 16mm camera and get it to work if the flange distance is so different between the two formats?

Dom Jaeger · February 21, 2016

Flange distance is not format specific, 16mm Arris have the same 52.00 mm flange depth as 35mm Arris for example. That's the PL standard.

The flange depth of an Aaton with an Aaton mount will be 40.00 mm, but if it has a PL mount it will be 52.00 mm.

A digital camera with a PL mount will also have a flange depth of 52.00 mm, regardless of the sensor size. If it has a Micro 4/3 mount it will have a flange depth of 19.25 mm.

Lenses are set up so that their back focal distance matches the flange depth of the mount system.

You use adapters to change the back focal distance of a lens to match a camera's flange depth, though not all adapters are accurately set up to within 0.01mm. A lens with a short back focus (like C-mount or M4/3) can't be used on a camera with a longer flange depth (like a PL mount Arri) unless it sits far inside the mount or you only want to shoot macro.

If you were more specific about the lenses and cameras you're having trouble with I could offer you less generic advice.

Tyler Purcell · February 22, 2016

Yea, that's what I had always figured that PL mount was PL mount. But I was guessing that maybe there was some format difference, but I guess not.

So I have a set of Optar primes in PL mount for 16mm, none of them work on my moviecam which has a standard PL mount. It appears to be a back focus issue, but maybe I'm wrong.

I can focus on a very close subject to the camera, but they won't get near to focusing at infinity... same goes for my Zeiss zoom... but that has a B to PL adaptor, so I wouldn't expect it to work properly.

Dom Jaeger · February 22, 2016

Hmm, would this be the Moviecam you pulled apart and "lubricated"?

If none of the Optars or the zoom are focussing to infinity then its a pretty sure bet the camera is the culprit and something is out of tolerance somewhere. For the record, the B to PL adapter on the zoom should work fine, it just might not be accurate to within 0.01mm, which is what a technician would set the flange depth to. But your camera must be out of tolerance by a lot more than that.

The first thing to check is that there are no issues with the mount, like burrs or an oversized locating pin that would stop the lenses from properly seating. The lenses are not hitting anything inside the mount before seating? The PL lock ring slides over the lens mount flanges and locks them in, right?

So now the issue at this stage isn't really the flange depth but the groundglass depth, which is what determines the viewfinder accuracy. Until you shoot a film test (or have a technician check the FFD with a depth gauge and backing plate), you can't really check if the flange depth is out too. It could be that the groundglass position is way off, but the flange depth is OK. It could be the mirror height that is off, causing the optical distance to the groundglass to have changed. Or it could be that the lens mount is sitting forward, either from being incorrectly fitted, or because the floating rib that holds the innards is out of adjustment, which means both flange depth and groundglass are out.

When you pull a complex film camera like a Moviecam apart, there are multiple crucial settings that need to be either left alone, or reset with jigs, gauges or a collimator. I don't know what assemblies you might have removed, but if it was the mirror or the groundglass holder you may need to send the camera to a technician to be correctly reset.

By the way, be careful fitting 16mm format lenses to a 35mm reflex camera, occasionally the wider focal lengths can protrude enough to hit the mirror. There is more clearance space between mount and mirror in a 16mm camera.

Tyler Purcell · February 22, 2016

For the record, I only touched the movement which is not part of the front of the camera where the mirror shutter is. I simply removed it from the body which is a standard practice, took the cover off the back (which is held in with little pins) and dabbed some lubricant in the appropriate positions. I wanted to see if the movement had any signs of it being able to be converted to 3 perf. The moviecam has alternative offset pin holes for the main part of the movement, which interested me. I thought about how it worked though and realized those holes were only there to offset the balance of that assembly. I did't examine further because there was no reason since I couldn't find a way to change the movement speed differently then the shutter. None of the movement was disassembled (it really doesn't come apart very easily). Like the Arri's, it's one solid lump that's keyed and simply pressed back into the camera. There is NO way to mess it up and it has zero relationship to the mirror/shutter or anything that could effect this discussion.

Anyway, my camera had been used for a shoot before I bought it and visual products stamped it's seal all over it, so the likelihood it's damaged is pretty much slim to none. The ground glass only goes in one way and the optics of the viewfinder/mirror shutter are all flawless. I have two viewfinder tubes and tried them both. Also, I took one of the lenses and removed the mount so I could manually adjust it's location. No matter what I did, it wasn't even remotely close to being focused. I of course ignored the ground glass and tried to focus just the viewfinder with zero success. So the camera body flange distance doesn't appear to be the issue.

My theory is that the 16mm glass I happen to own, simply won't work due to it's design which is very old school. It's absolutely related to the rear element location (back focus) which needs to get closer to the film plane in order to focus. The larger mirror on the 35mm camera, pushes it too far away from the ground glass. I haven't yet taken my calipers out to measure these things, but I can do that today when I have time just to verify. I was thinking someone would simply say, yea you can't do it and then I'd be home free. Ohh well, I guess I need to figure out myself! Thanks!

J. Winfield Heckert · February 23, 2016

Is it that only the 16mm glass won't focus to infinity?

I had an issue where my lenses wouldn't focus to infinity. Visual Products who sold me the camera claimed it was the lenses at fault. I sent the camera to Bernie and he found the ground glass to be out of alignment. After that every pl mount lens I've mounted on it focuses great.

Illya Friedman · February 23, 2016

As was already stated 16/35 PL mount lenses are supposed to be the same FFD, but it's of course possible that someone shimmed glass to a particular camera, rather than the proper way (the other way around.)

Any qualified lens tech can tell you if you lenses have a FFD problem in a few minutes. Since you are in L.A. Tyler, Hot Rod Cameras can of course do a quick check for you in a few minutes for a small fee.

All the best,

I.

Tyler Purcell · February 23, 2016

Well, shims would mean you're using the camera mount. I bypassed the camera mount entirely and was able to move the lens all the way into the camera until it hit the mirror, still it was out of focus. So it's absolutely NOT a mount issue. I assume the camera is fine because it worked when I got it. I KNOW the lenses are fine because I've used them on 3 shoots and they work great with my 16mm camera.

I just feel stupid because nobody in their right mind would TRY what I accidentally stumbled upon. I don't know what a camera shop would say if I handed them my poop and said "what's the problem". First thing they'd probably do is laugh and then probably charge me an hourly rate to figure out there is nothing wrong and this is totally normal.

Anyway, I'm trying to sell my 35mm camera anyway, so the whole thing is moot. I was just interested to see if anyone else had a similar issue and it turns out nobody has! Pretty strange considering I've tried two brands of lenses and two different style of mounts (PL and Arri B. So I'm pretty darn confused!

Tyler Purcell · February 23, 2016

Is it that only the 16mm glass won't focus to infinity?

Yep. It focuses perfectly fine close, but can't focus to infinity.

The ground glass position is literally impossible to change on the Moviecam. So either the camera is designed for a special type of glass (doubtful, but possible) or my lenses are very unusual for some reason. I'm guessing my lenses are unusual and that's the problem. I don't have access to real 35mm glass to test.

Simon Wyss · February 23, 2016

FFD becomes crucial only for infinity. At shorter distances man decides by the image he sees on the GG. If you can’t have the horizon sharp at a lens’ infinity stop, the distance ring is not properly set. Of course, its scale is also not useable precisely.

The infinity stop can be set for infinitely many distances. Is the sun to be sharp at the ∞ mark, the moon, mountains four miles away or what? Not simple to know. The collimator-calibration lens process is based on some ∞ presumption, too. One has to know the focal length of the calibration lens rather precisely.

Dom Jaeger · February 23, 2016

The first thing to check is that there are no issues with the mount, like burrs or an oversized locating pin that would stop the lenses from properly seating. The lenses are not hitting anything inside the mount before seating? The PL lock ring slides over the lens mount flanges and locks them in, right?

..

By the way, be careful fitting 16mm format lenses to a 35mm reflex camera, occasionally the wider focal lengths can protrude enough to hit the mirror. There is more clearance space between mount and mirror in a 16mm camera.

So do the lenses seat properly? No need to remove mounts and whatnot, either they seat and the PL lock ring turns over the lens flanges, or they are hitting something first and the lock ring won't lock. The Optars from memory are like Super Speeds in that the rear element recedes towards the film plane as you focus to infinity. So here's a simple test: set a lens to close focus (with the rear element at its furthest from the mirror) and see if it mounts OK. If it does, are the marks lining up? If so then the camera ground glass depth is OK and probably the flange depth. If the lens mounted OK but the focus ring simply stopped turning before it reached infinity, the rear element was hitting something, probably the mirror. I wouldn't recommend doing that again however.

Like I said before, some 16mm PL mount lenses may hit a 35mm PL mount camera mirror. Usually the wider focal lengths, and usually only approaching infinity. I'd be surprised if every Optar went in too deep but I don't have a set to look at, and mounting them on 35mm cameras is not usually something people do.

A Moviecam is not designed for "special lenses", it takes normal 35mm PL lenses.

Dom Jaeger · February 23, 2016

FFD becomes crucial only for infinity. At shorter distances man decides by the image he sees on the GG. If you can’t have the horizon sharp at a lens’ infinity stop, the distance ring is not properly set. Of course, its scale is also not useable precisely.

The infinity stop can be set for infinitely many distances. Is the sun to be sharp at the ∞ mark, the moon, mountains four miles away or what? Not simple to know. The collimator-calibration lens process is based on some ∞ presumption, too. One has to know the focal length of the calibration lens rather precisely.

Flange depth is important for a number of reasons other than infinity. The ability to change lenses and have them all line up accurately at their focus marks is vital in the world of professional cinematography. Zoom lenses won't hold focus through their range if the flange depth is off. Anamorphic lenses will be astigmatic and increasingly unuseable the more the flange depth changes. Certain modern lens designs with fixed rear groups and internal focussing need to be set to specific back focus distances or they won't work optimally.

On anything but extreme telephotos the difference between even one mile away and infinity is essentially zero. On a bench collimator, infinity is infinity, parallel rays. What can change the callibration sometimes though, is the wavelength of the light.

Tyler Purcell · February 23, 2016

So do the lenses seat properly?

Actually, a few of them don't. As you said, the longer focal distance ones do, but the shorter/wider don't.

The reason I removed the mount on one, is because I wanted to move the lens in and out to verify it was actually hitting the mirror vs something else.

I did try adjusting the focus of the lens, but didn't really help. The ring turns freely, it just doesn't do much to help the problem.

What's funny is that my Zeiss 10-110 MKI zoom lens does exactly the same thing.

Also, when I mean out of focus, I mean you can't identify anything when the lens is installed. It's not like the focus is "off" that's just spacing. It's like the lens needs to move into the camera body a good 10mm+.

J. Winfield Heckert · February 23, 2016

Do you have any lenses that do focus on the 35mm moviecam? From what I've been told a PL mount is 52.00mm no matter if it's 16mm, 35mm or Digital. It should be the same.

If you can borrow a made for 35mm prime lens I'd see if you can get it to focus to infinity. If it does that would confirm the theory that there's some difference between 16mm & 35mm PL.

Dom Jaeger · February 23, 2016

Actually, a few of them don't. As you said, the longer focal distance ones do, but the shorter/wider don't.

The reason I removed the mount on one, is because I wanted to move the lens in and out to verify it was actually hitting the mirror vs something else.

I did try adjusting the focus of the lens, but didn't really help. The ring turns freely, it just doesn't do much to help the problem.

What's funny is that my Zeiss 10-110 MKI zoom lens does exactly the same thing.

Also, when I mean out of focus, I mean you can't identify anything when the lens is installed. It's not like the focus is "off" that's just spacing. It's like the lens needs to move into the camera body a good 10mm+.

OK well if some lenses are seating properly but you still can't focus or the focus marks are just way out, then there is an issue with the camera (beyond some of the 16mm rear elements hitting the mirror). Specifically the viewfinder system, since that's what you're judging focus by. It's linked to the flange depth but is actually a completely different pathway, reflecting off the mirror onto the ground glass, which forms the image that the eyepiece and viewfinder optics focus on.

The only other option is that all your lenses have been drastically altered from their original 52.00mm Arri standard back-focus setting. But if they work on other cameras with standard mounts and adapters they should still be correctly set. To re-iterate: Arri PL, Bayonet and Standard mounts are all set to 52.00mm, no matter the format. I've worked on these things for 20 years, you can trust me on that! B)

There can be a number of things causing the viewfinder image to be out. Firstly, you need to be focussed on the ground glass, the framelines should be sharp. Secondly, the ground glass needs to be correctly fitted and accurate. On the Compact and SL that I'm familiar with you can only fit the ground glass in one way (with the etched side down), so unless it's been incorrectly set in its frame (not unheard of by the way) it should be good. After that it has to be a mechanical issue, either the mount, the mirror or the ground glass holder are incorrectly set.

I would take it to a rental house to have the settings checked, won't take more than 15 minutes.

Robin R Probyn · February 24, 2016

Thanks Dom.. its been interesting reading your comments.. learnt a few things ..

Tyler Purcell · February 24, 2016

I would take it to a rental house to have the settings checked, won't take more than 15 minutes.

Yea, I'm just gonna bring it over to my friends place and test some standard 35mm PL glass. I'll bring BOTH cameras and I should be able to figure out the problem in a jiff. It's just extremely embarrassing to show up with a camera and be like "hey dude, wtf".

I'm just glad that what I'm doing SHOULD WORK! Because I honestly I was always told it WOULD, just never had the camera bodies and lenses sitting around to try the theory.

Thanks for the assistance, I will report back soon!

Illya Friedman · February 24, 2016

Flange depth is important for a number of reasons other than infinity. The ability to change lenses and have them all line up accurately at their focus marks is vital in the world of professional cinematography. Zoom lenses won't hold focus through their range if the flange depth is off. Anamorphic lenses will be astigmatic and increasingly unuseable the more the flange depth changes. Certain modern lens designs with fixed rear groups and internal focussing need to be set to specific back focus distances or they won't work optimally.

On anything but extreme telephotos the difference between even one mile away and infinity is essentially zero. On a bench collimator, infinity is infinity, parallel rays. What can change the callibration sometimes though, is the wavelength of the light.

Well said.

Illya Friedman · February 24, 2016

Yea, I'm just gonna bring it over to my friends place and test some standard 35mm PL glass. I'll bring BOTH cameras and I should be able to figure out the problem in a jiff. It's just extremely embarrassing to show up with a camera and be like "hey dude, wtf".

I'm just glad that what I'm doing SHOULD WORK! Because I honestly I was always told it WOULD, just never had the camera bodies and lenses sitting around to try the theory.

Thanks for the assistance, I will report back soon!

Perhaps you'd be surprised how many people really don't know what's going on and need help.

Also, I've learned the hard way that "SHOULD" has nothing to do with a functioning piece of motion picture equipment, especially when you are losing the light!

My two cents... there's several potential problems going on, and if you want this figured out it's better to swallow some pride and ask a professional for help.

I.