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Topic: 35mm Analog Track to 70mm Magnetic Optical CL Difference

Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 12-14-2004 04:13 PM

Does anyone know if there is a standard for this(Theres gotta be one)? I have calculated it out and also measured some offset lens holders which confirmed a .130" calculated difference. Is this spec correct?

Thanks!

Mark

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Steve Kraus
Film God

Posts: 4094
From: Chicago, IL, USA
Registered: May 2000

posted 12-14-2004 09:16 PM

I'm confused about what you're asking. Isn't the difference the same as the 35mm optical sound offset (50% of the width of the track or 50 mils)?

Don't 35/70 machines run both gauges on the same centerline? If the 70mm image CL = the film CL and a 35mm optical sound print has image CL 50 mils off from film CL then the difference between 35mm and 70mm optical centers should be 50 mils or whatever that spec is.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 12-14-2004 10:03 PM

Mark, I'll have to check though John P will beat me to it. There is a spec somewhere in the SMPTE publications on the centerline of the film based on the reference edge of the film though I don't think there is an offset distance specifically stated.

As to 35/70 projectors. Most projectors required the lens to be shifted between the two formats. As such 70 and 35mm magnetic used the film centerline and the 35mm optical prints had the lens shifted to the optical centerline.

The current Kinoton FP75E has the 70mm and 35mm optical centerlines now in the same plane. A very smart move since 35mm magnetic is virtually non-existant except in collections. This will allow the vast majority of films to use the same centerline for maximum light efficentcy. With the old way, one had to choose which lateral centerline to use for the lamphouse. Now both gauges use the same lateral centerline.

Steve

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 12-14-2004 10:20 PM

quote: Steve K.
Don't 35/70 machines run both gauges on the same centerline?

No, This is the reason for the lens adjust knob at the top of the Century JJ lens holder and for the rotating lens holders in the DP-70. You have the offset due to the optical sound track while 70mm is on C/L. I measured the offset of a new Kinoton collar at 130 thousandths today right on the money.

quote: Steve G.
70 and 35mm magnetic used the film centerline and the 35mm optical prints had the lens shifted to the optical centerline.

I totally agree but whats interesting is that I have an original Todd A-O collar that has three different C/L markings. One for 35mm optical, 35mm mag, and 70mm mag... three different optical center lines marked on the back. I will try to find it and post a photo. Its the only one I've ever seen like it.

We're making our own lens collars since we were very unhappy with the recent Kinoton version. Its not split for clamping the 4" diameter holder in place and the three set screws to do the clamping make adjustment clumsy and not very definate. Ours will step directly down to 70.6mm diameter from the DP-70 collar O.D. I have all the blanks bored out and need to make an offset mandrel in the lathe and turn the O.D.'s to get the required offset. Our collars will be split lengthwise just like a Century or Simplex collar so there will be no set screws to fiddle with.

I would appreciate what ever you guys might be able to find out for me. I don't want to turn down 30 collars with the wrong offset [Eek!]

!!

Thanks,
Mark

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John Hawkinson
Film God

Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002

posted 12-15-2004 12:23 AM

It really sounds like this is something that is projector model -specific. SMPTE can specify the distance from the centerline to the reference edge (but we know its in the middle, except for 35mm's soundtrack .050" offset), but not the relationship between formats in a multiformat projector.

--jhawk

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 12-15-2004 08:40 AM

Mark,

In ANSI/SMPTE 59...35mm Camera apertures. They do list the official offset between the Centerline of the film versus the centerline of the image in Table 1 (Style A) item "G" as 0.050"

Steve

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Steve Kraus
Film God

Posts: 4094
From: Chicago, IL, USA
Registered: May 2000

posted 12-15-2004 08:51 AM

quote: Mark
No, This is the reason for the lens adjust knob at the top of the Century JJ lens holder and for the rotating lens holders in the DP-70. You have the offset due to the optical sound track while 70mm is on C/L.

You misread my question. I was talking about the film centerlines, not the image. Aside from the Kinoton Steve mentioned the typical 35/70 machine runs them both on the same centerline.

I guess I'm not understanding what the issue is. We know the 35mm optical sound image is offset by one-half the track width or 0.050." Since when is that news? Or is there a difference between this spec and real world practice?

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Mark Gulbrandsen
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Posts: 16657
From: Music City
Registered: Jun 99

posted 12-15-2004 10:09 AM

quote: Steve Kraus
Or is there a difference between this spec and real world practice?

Thats what I am trying to determine. The 130 thousandths offset on the new Kinoton lens collar I measured doesn't seme right to me. I do know my measurement is correct and was measured out to 5 decimal places!! The offset in my mind should be half the width of the optical sound track like you said. Am beginning to wonder if the main DP-70 lens mount also has some offset built in that the collar is compensating for.

Mark

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Steve Kraus
Film God

Posts: 4094
From: Chicago, IL, USA
Registered: May 2000

posted 12-15-2004 02:14 PM

Of course the perfect projector would keep the lens mount where it is (in perfect alighnment with the lamp) and move the gate and trap side to side. The variation ought to be small enough that the rest of the mechanism can stay put; otherwise move it all.

BTW, what is the image CL of 35mm 4-track mag-only? It can't align with film CL what with narrow surround channel track and larger center channel track.

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Mark Gulbrandsen
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Posts: 16657
From: Music City
Registered: Jun 99

posted 12-15-2004 03:40 PM

What had me confused was that the Kinoton adaptors have a 100 thousandths offset while the original Phillips have 50 thousandths offset. The latter is obviously correct.

Thanks everyone!!

Mark

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 12-17-2004 07:55 PM

Here are some photos to show you why I was wondering what the heck was going on with the offset.....

Note: The lens bore was gauged in to .0001 to be perfectly concentric in the four jaw chuck in all photos.

The first photo is an original DP-70 lens collar made by Phillips.

Here is a brand new Kinoton DP-70 Lens bushing... Defective no less..... And we all thought they never made anything wrong.....

A better idea... make it all one piece! And with the proper offset. These were machined from spin castings to eliminate any possibility of air bubbles or sand pockets that could mar the O.D. or the bore. They were heat treated before machining.

Mark @ CLACO

[ 12-17-2004, 09:23 PM: Message edited by: Mark Gulbrandsen ]

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Steve Kraus
Film God

Posts: 4094
From: Chicago, IL, USA
Registered: May 2000

posted 12-18-2004 12:26 AM

I asked James Bond about combining the Norelco adaptor with the 4"-to-70.6mm collar when he had some made and he pointed out that he likes to use PC collars for perfect image positioning.

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

posted 12-18-2004 04:07 AM

You shouldn't talk to Bond about stuff like that. He just uses the equipment and messes it up in the process. Talk to Q instead.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 12-18-2004 09:11 AM

quote: Steve Kraus
I asked James Bond about combining the Norelco adaptor with the 4"-to-70.6mm collar when he had some made and he pointed out that he likes to use PC collars for perfect image positioning.

There should be no real need for PC correctors in installations that are designed to SMPTE guidelines. I guess if Jim is running his projectors off position as might happen in a two machine install then PC correctors may be handy. But in a new install with single projectors on screen center they are un-necessary. They are sometimes nice for improperly designed installations that are far off axis or for installations that use very short focal length lenses and end up with alot of keystoning. These new lens collars will increase accuracy of the lens position over using two seperate lens adaptors and especially over the Kinoton made collars. Running your optics as far off optical center as the PC's are able to place them is not always a good thing either. Optics are generally designed to be used on axis, not off axis. If you look at any good quality cinema lens with a lens test projector in a darkroom and view the entire image circle it can project you will see that they tend to loose some resolving power and have a slight gain in chromatic aberation when you get out towards the edges of the glass.

BTW, the new 4" Black Schneider(made by Century Precision) PC corrector is a piece of crap... The first really awful product that company has made that I can think of. You cannot adjust it without the image shifting all over and the adjustment eccentric they provide is a waste of time. If you have a scope lens in one of them the weight of the lens causes the lens to droop because of the lock system the corrector has. It doesn't function as to allow you to exactly position the lens because it is too touchy. Remove it and throw it out, just turn the lens by hand to position it.... then try to lock it down. You will fight with it for hours, its a joke! The older design gold PC correctors worked fine though they had a bit more limited range.

There are always plenty of old style Phillips collars in case we need to use a PC.

Mark

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 12-19-2004 12:01 AM

First...even in a single projector installation...a PC adapter can be quite handy. That is, if have more than just the basic "FLAT/SCOPE" lenses...once you get into the 3rd and 4th format...they become essential for precise cenering unless you projector is truely on the screen centerline (in both planes). The different magnifications of the various lenses will cause one to change their centering.

As to the Schneider PC adapters...I've become quite adept at preceisely setting them rather quickley. I totally dispise their lens loocking mechanism since it barely grips the lens. It also is way over priced.

Steve

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