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Topic: curved gates - why?

Scott Norwood
Film God

Posts: 8146
From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99

posted 10-24-2002 05:57 AM

With respect to the article on aperture plate distances: what is the supposed advantage of curved-gate projectors? Cameras have straight gates, why shouldn't projectors? I've run quite a bit of film through straight-gate Century C's and never had any sort of problem with them; I've only had one experience with straight-gate Simplex X-L's, but they seem fine, too. I'm not a Cinemeccanica fan, but the straight-gate V5 seems to be very gentle on film. So why are nearly all modern projectors using curved gates?

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Pat Moore
Master Film Handler

Posts: 363

Registered: Mar 2000

posted 10-24-2002 08:07 AM

Scott;

The curved trap and gate was implemented because film, being the plastic medium it is, loves to flex and bend. Theory is that if you curve it into a pre-defined shape along one axis, it minimizes other flexing the film can do as it passes through the trap. In general the theory works really well.

Pat

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 10-24-2002 08:40 AM

Curved gates really help maintain focus stability as power levels increase. Several technical papers published in the SMPTE Journal years ago by Willy Borberg of General Precision Laboratory, and E. K. Carver, Fred Kolb and Paul Preo of Kodak, led to the development of curved projector gates.

By gently curving the film, the print is more resistant to flexing and changing focus position as it is heated by the intense focused energy of the lamphouse.

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
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e-mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

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Jack Ondracek
Film God

Posts: 2348
From: Port Orchard, WA, USA
Registered: Oct 2002

posted 10-24-2002 12:49 PM

Is there any "standard", suggesting at what wattage a curved gate would be adviseable over straight... or do the variables make this less a science than art? Considering different lamphouse characteristics, glass/metal reflectors, and the varied approaches to heat baffling behind the aperture, this seems about as subjective a subject as when to use water cooling.

I use both straight and curved. The most notable result we saw was at the drive-in, where we had been fighting the focus problem with our old Brenkerts. Switching to Simplexes with curved gates made that problem a thing of the past.

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Pat Moore
Master Film Handler

Posts: 363

Registered: Mar 2000

posted 10-24-2002 01:49 PM

It is a variable thing, Jack, with all that mentioned in the mix and more. We built 870 projectors with air stabilization on that very large frame of film. The trap was compound -- curved above and below the aperture, but flat in the image area because the lens needed a flat plane to focus on.

In general, though, I think the curved gate and trap is the simplest and most effective method of controlling the tendency of film to flex in normal theatre operations. Modern lenses are design with that field curvature in mind. Heat is probably the second biggest influence affecting focus quality.

Pat

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Randy Stankey
Film God

Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99

posted 10-24-2002 02:39 PM

Correct me if I'm wrong, but I was under the impression that the reason gates are curved is to keep the film within the "plane of focus" of the lens.

Since the plane of focus isn't really a flat plane, but more of a parabola, curving the gate helps the entire frame of the film stay closer to the "focal parabola" ( ) of the lens, thus minimizing the phenomena where the center of the picture is in focus but not the edges, or vice-versa.

This is not to say that putting a curve into the film doesn't ALSO help keep it from buckling or waving as runs but I thought this was the main reason.

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 10-24-2002 03:28 PM

Perhaps Dwight Lindsey of Schneider, or someone from Isco can jump in here.

AFAIK, a lens can be designed assuming EITHER a perfectly flat film plane, or some film curvature. I recall that one of the reasons modern lenses are so much better than those of a few decades ago is the recognition that the film is NOT perfectly flat at the moment of projection.

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Tim Reed
Better Projection Pays

Posts: 5246
From: Northampton, PA
Registered: Sep 1999

posted 10-24-2002 04:11 PM

Lens speed is a factor, too. Focus is more critical with faster lenses.

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Better Projection Pays!

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Steve Guttag
We forgot the crackers Gromit!!!

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

posted 10-24-2002 05:20 PM

I'll go round and round on the "flat-field" issue...

First off...as the film is heated in the aperture...it swells towards the light source so even with a straight gate, the film is not flat. Furthermore, as the film is pumped by the shutter (it's on and its off...) the film is in in constant motion fore and aft. The relaxation of the film between the shutter and flicker cycle is not enough for the film to completely relax. With 3-blade shutters this can be even worse since the film is hit three times and the film is actually moving to different extremes each time.

It is easy to see that with higher wattages, the effects become more noticable. Many screening room projectors are still straight gate to this day. Since they are in the 500-1000 watt range, the heat really isn't an issue. Order a Simplex PR-1020 and see what you get. Studio gates also have overlapping pad areas.

Curving the gate stretches the film and controls it a bit better. In fact, VistaVision really has it best since it curves the film on the long direction.

Steve

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"Old projectionists never die, they just changeover!"

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 10-24-2002 05:22 PM

But I believe BTS SUpra's had terrible problems with there curved gate not holding focus well according to an article I read once in the BKSTS

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Steve Kraus
Film God

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

posted 10-25-2002 11:33 AM

If lenses are now designed with for a curved focal plane then one has to assume that focus with a straight gate is going to be compromised. As Steve points out, even with a straight gate there is going to be some curving from heat but surely that is not as much as with a deliberately curving film path. Let me anticipate those who are going to point out the wonderful depth of field of many lenses by reminding them that depth of field refers to how quickly you go out of focus as you move away from the plane of focus (points become circles of confusion) and does not itself change the fact that there is still a definite plane of perfect focus and if that plane is curved a lot more than the film does on a straight gate then it has to compromise focus across the frame.

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Pat Moore
Master Film Handler

Posts: 363

Registered: Mar 2000

posted 10-25-2002 12:25 PM

It's not that lenses are made to look at a curved plane as much as they will accept an image to be within a certain area, +/- the perfect image plane, and still be able to focus the image on screen. That's why the same lens works on a flat or curved film trap, as long as the film is within its "depth of field". The lenses of today are designed to accommodate that "flex" of film in and out of the desired focal plane. Typically, "slower" lenses (higher f-numbers) would have larger depths of field while passing a bit less light.

This is what makes motion picture film projection lenses unique, their ability to accept the fact that the film image is a rather fast-moving target and that is does flex in and out of a focal plane. All these lenses look great if they are looking at a glass slide. The difference is when they are projecting the moving picture image and how they handle that.

Pat

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Richard Fowler
Film God

Posts: 2392
From: Ft. Lauderdale, FL, USA
Registered: Jun 2001

posted 10-25-2002 02:12 PM

The Supra projector which was the first to have a curved gate in a modern sense did have to work against lenses of the day and hot reflectors. The trap was fairly large which may have contributed to the flutter problems....you have to start somewhere

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Christopher Seo
Jedi Master Film Handler

Posts: 530
From: Los Angeles, CA
Registered: Jun 99

posted 10-25-2002 03:16 PM

I'm curious as to whether a higher gate tension is needed to maintain a steady image in a curved gate. It would seem that the film tension is relatively high at the aperture, where the gate pushes in the farthest on the trap bands, and less at the top and bottom where the gate rails curve away from the trap. On the other hand, a straight gate should probably impart uniform pressure to the film throughout the gate and would thus seem to require less overall tension.

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Pete Naples
Phenomenal Film Handler

Posts: 1565
From: Dunfermline, Scotland
Registered: Feb 2001

posted 10-25-2002 04:08 PM

BTH SUPA did indeed have dreaful problems, well I think it was early versions that did. If I recall right the aperture was in front of the film, so more of the film was hit with heat from the lamp. There were mod kits for early ones, and later ones had the design altered somehow. I can't comment in more detail as I only ever saw one pair in the flesh, they were earlies, and to be quite honest, once was more than enough.
Likely as not, Bernard can tell us more of the SUPA, I'm really not old enough!

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