Thin 70mm film stocks and focus uniformity issues

[Steve Guttag]

How many of you that are running the NEW 70mm prints (e.g. Disclosure Day) are noticing that the rather thin film stock they are using for 70mm releases are playing havoc on your focus uniformity?

If you thread up a vintage print...no problem. If you thread up a vintage 70PA loop...no problem.

Thread up a "modern" 70PA loop (something from BL&S, for example...made in the last 5-years)...maybe a problem

Thread up something made in 2026, problem.

I first encountered it on a pair of Kinoton FP75E projectors. it was near impossible to get the image focused in the middle as well as the top and bottom of the picture. I realigned the crap out of the lamp, reflector, gate...you name it. I used most every trick in the book...alignment projector, laser, actual mechanical gauges to verify that the gate, lens and reflector are all in line/perpendicular...etc.

But, when credits rolled, in particular, horrible results. These machines had been in for the better part of 2-decades without complaints.

The fix? loosening up on the lateral guides (ceramic discs in the case of Kinoton) so they weren't pressing as hard and bam...focused and steady.

Okay...the Kinoton is more esoteric and not bound to be in too many venues. But the Century JJ? It is probably one of the most popular machines in the world and definitely in the USA.

I was on a call recently for other matters and ran target...no problem.

(note, the photo was a quickie from my phone through the port...etc.):

image.png

Setting aside centering and my phone being out of level. There is nothing about the focus there that jumps out and says that there would be a problem.

But, on the credits, this:

image.png

WTF?

The thin film like to slap a lot too.

I've run a few JJs in my time and a particular pair for nearly 2-decades...on a Cineerama Deep Curved screen...I've never had focus issues like this on release prints. It has to be buckle in the gate (thin film combined with the heat from the lamp on darker film to absorb it).

Are any others out there experiencing this?

[Julian Antos]

At Music Box, we’ve had focus uniformity issues on 70mm for years. No matter how well everything is aligned, the center top and bottom (up to a little past the 10% line) are very soft in comparison to the center and edges. It’s not horrible, but an annoyance since we want everything to be perfect.

We recently had some more severe focus issues and while troubleshooting noticed that older polyester prints were fine, but newer ones were “breathing”. New test films were fine, both as a loop and running a length of 100’.

I did see Disclosure Day last night at an AMC running 70mm and the focus was uniform with no breathing, and sharp top to bottom.

[Marcel Birgelen]

I've seen similar issues at a venue also running two F75Es while watching The Brutalist in 70mm. It wasn't always obvious but it was indeed like parts of the image weren't really in-focus. I've never seen anything else like that with other 70mm shows.

What's the theory here? Maybe the thin vinyl backing simply isn't sufficiently stable and starts to warp in front of the gate?

[Jim Cassedy]

We've been running three or four 70mm Disclosure Day shows since June
11th.( We woll be hitting our 30th showing tonight) We haven't had any focus
issues here. The print is on Kodak 2383 stock. Are they making that thinner
now? I haven't handled the 'body' of the print, only the extra long red/green
protection leaders. The rest of the print runs and sounds normal when playing
here. In fact, just the other night, as I was waiting for the credits to end, I
remember being very happy with the sharpness of the white-on-black text,
and with the fact that they were great for checking travel ghost. We've got
JJ's here, and lenses are Isco UItra-Star's, with an appx 80ft throw on a
45ft wide screen. I did have a tricky time keeping focus on HATEFUL 8 at
another venue until the late Sam Chavez came and adjusted the guide
rollers and rails in the film trap, which were apparently 'cupping' the film
very slightly.

[Victor Liorentas]

I have been running almost everything since and including Oppenheimer to Disclosure Day at the Varsity Toronto on the Vic 8 with great focus on everything. Hyland Cinema London I've run Kill Bill , Marty Supreme, One Battle After Another and Project Hail Mary with great focus also on a Vic 8. I use 130mm ISCO at the Hyland and similar focal length at the Varsity.

[Steve Guttag]

interesting. The Vic 8 is a notably different gate/trap design (rigid trap with the spring loading on the gate/runner side with pads. Depending on vintage, I seem to recall Cinemeccanica not using a lateral guide roller or it not being very strong.

I can't speak to the gate alignment on this particular JJ...I was there for an issue on the other side and when the target looked fine, I moved on (this is not a machine i normally service). I did verify that the studio guides were spread as far apart as possible. Gate depth and lateral alignment could have been a contributing issue. The film stock is definitely thinner than it used to be, however.

[Marcel Birgelen]

The film stock is definitely thinner than it used to be, however.

So, what was, according to you, the first film that you noticed that was being released on this "new", thinner 70mm stock?

Maybe the IMAX idea of sucking the frame to the back-plate for stability and focus really is a thing.

[Steve Guttag]

I don't know which was the first. Oppenheimer did give some issues though. Brad Miller might have better data on the evolution of 70mm print film thickness.

[Ryan Gallagher]

We had some focus issues with our JJs on and off in 70mm until we relaxed the studio guides out to their most outboard position (or removed them entirely). I could easily see lateral rollers setting or spring strength/lubrication contributing too. No doubt our setup needs some gate and trap alignment with all those bits. I don't have any purpose built tools for such detailed alignment work, and it will be a year before I (or ideally a certified tech) get to touch them (or an upgraded pair) after our renovation.

Our most contemporary prints we've run in 70mm lately were Close Encounters, Searchers, and NxNW, but I can't speak to if any of them were on the "thinner" stock unless I go back to our reports and see if I wrote down the full edge codes.

It would be interesting to find a scene sequence within such a print where it goes from a very bright emissive scene to an extremely dark scene. Or better have some test footage printed that has focus patterns on white and inverted on black. I'd be curious to know if we are just struggling to control film movement due to dark images and heat in some setups.

[Stefan Scholz]

If we remember the bimetallic effect on projected film - emulsion and base have different expansion rates - even with the smaller 35 mm gate, each image is about 2/3 of the projected time out of the depth of field using the standard f 1.8 lenses. The problem is even more dominant with 70 mm film, the thinner the easier to warp in parts of the image. It is not infrared, it is absorbed energy, with B&W title sequences, that's the worst condition. Thicker acetate film eventually had the better mechanical warping resistance, than today's super flexible polyester stock.
It often helped to use stepped down 8 perf lenses, which offer a better depth of field. I thought the were standard in the venues that still did 70mm, but no, from legacy lenses to 35 mm lenses, 35 mm with magnifying converter etc a lot was used, not good enough for the purpose.

[Steve Guttag]

Whatever tangent you are going off on Stefen...this isn't that. If it was, you'd have a more uniform focus issue, but it isn't. If relaxing the lateral tension "fixes" it on a pair of projectors, it isn't a lens issue. If you are getting the maximum resolution of the test film with your lenes, swapping lenses isn't going to fix it. I don't know what your fascination is with 8P lenses is but other than more ability to lens shift, they aren't going to buy you much. The 70mm rated lenses (typically 105mm - 150mm, for typical lengths, and there are shorter EFs, 100mm and below but where always in much less supply), will handle 5/70 just fine. I tend to prefer ISCO over Schneider but not to the exclusion of Schneider. Schneider also has the benefit that if you do want to stop the lens down, their 100-150mm lenses have a convenient place to put an iris and they offered them for multiple sizes (but not all) that could be field installed in seconds.

Wanting 8p 70mm lenses is wishing for unicorns. It isn't that they don't exist...it is that they were never made in huge numbers so they are harder to find and the EFs are very limited. If you have access to them, then great. But, for this particular problem, they are not the solution.

[Marcel Birgelen]

If it would be heat-related, then wouldn't you expect some permanent damage in the form of warped film? I doubt that polyester film, that gets warped out-of-form will perfectly spring back to its previous form once the heat is removed. The Vic8 and FP75E do have water-cooled film gates, I wouldn't expect heat to be an issue on those machines.

While most likely not a lens issue, a lens with a broader depth-of-field could alleviate the issue, although I'd consider it a hack rather than a solution, you're still giving up a considerable amount of resolution, one of the hallmarks of this entire 70mm endeavor.

[Steve Guttag]

Marcel,

Film will always swell towards the heat. If you were to take a high-speed movie of the film in the gate such that you can see the shutter slowly spinning, you'd find that each time the shutter opens, the film swells back towards the heat...then relaxes when the shutter closes. Another fun-fact, when we focus the film, it is when the film is in its most deflected position.

The heat is causing the film to want to deform and the gate/skate/trap is not adequately controlling it to keep it uniform. And, in the case of lateral guidance that pinches the film, can induce deformity.

Water cooling helps with metal parts expanding and keeping them from transferring heat to the film's edges...it does nothing for the image area, which isn't contacted by anything that receives water cooling. Century's "Cine-Focus" which applied positive air pressure on the lens-side of the film was an attempt (and semi-successful) at minimizing the pumping of the film as the shutter opened/closed. It also helped push heat away from the image area.

in the case of Kinoton, water cooling can also help with the heat that reaches the delin parts. For metal gate machines, It can help with expansion of the metal under the heat as well as not causing people to burn their fingers handing the parts...which again, will contact the film's edges too.

[Ryan Gallagher]

Maybe a crude experiment if these are turret model JJs where access is possible, would be to run some compressed or canned air at the film in the gate during a credits sequence on a machine exhibiting the problem. If focus issues goes away possibly entirely heat related film movement? That blur pattern photographed however gives me the sensation it is not that simple. Multiple variables perhaps.

I read it as out in the upper left and bottom right, with a diagonal pattern that is closer to focus.

if I was diving really deep on the problem and alignments were confirmed good, I’d be tempted to roll some problem film with the lateral guide rollers removed. If that was the source of pinching perhaps a weaker spring could be swapped in on the side that moves?

[Marcel Birgelen]

Steve, thanks for the writeup... I'm aware of what you're writing... it's basic physics, you hit something with a high-energy photon beam, the least what you'd expect is that it expands. Even what Stefan said has merits: the black part of the film will absorb more energy than the transparent part. Still, I'd never expect this expansion to be causing warping as dramatic as in your movie credits picture, at that level I'd expect some permanent damage.

I've "googled" for some slow-motion footage of a 35mm or 70mm projector. It would be interesting to see if you can see how the opening shutter affects the surface of the film. I guess that's not that easy to film this, since the extreme brightness of the surfaces involved. Maybe we should drop the Slo Mo Guys a message, would be an interesting episode.