Thin 70mm film stocks and focus uniformity issues

[Steve Guttag]

One of the complaints about a 3-wing shutter in higher lamp applications was that it pumped the image 3-times and didn't allow as much relaxation between openings such that it was deemed to adversely affect focus.

i think you'd be surprised at how little film needs to deflect to induce a focus issue. If it was strictly light related, I would expect the focus to be most affected in the center versus other parts (the hottest part of the light beam). But, when you have things like lateral guides trying to pinch the film and then add in the gate/tension bands, to a degree, you are already trying to introduce a little bit of curl and then add heat into the mix. The more even the pressure, I would presume the more even the deformity. On the FP75E where I had the issue, the focus deformity was more even (you could have center..or top and bottom but not both. Relaxing the lateral guides addressed the problem with no negative side effects (lateral motion was still rock solid).

[Jerry G. Axelsson]

Steve, would you say that a Kinoton FP-30D for example would also have a tendency to have focus issues with larger lamps as well if it is equipped with a three-blade shutter?
The second question, what do you classify as "larger lamps". 4KW or more?

[Steve Guttag]

I'd think 35mm would have FAR less issues than 70mm. The supports for 70mm are quite far away and you have this large area where the light is contacting. For 35mm, comparatively, you have half the width to support. So, I would not expect an FP-30D to have an issue or as much of an issue. The thinner film stocks could still cause grief but less so. As for larger lamps, yes, 4K and up (and, perhaps, down to 3K). The exception would be the Kinoton-E projectors because of how they handle 3-wing is more of a duty cycle thing...they behave differently than pure mechanicals. Still, you will have three deflection points on 3-wing and the most deflected point is where people focus on.

I would also expect 3-wing shutters with larger lamps to be more of an effect on straight gates where the film's center is allow to pump more.

[Tj Hopland]

Does anyone know how many batches of the BLS test film there has been and where they think they rate in this thickness game? Reason I'm asking is I was just working on a setup and used some of that film for the first time and I thought it seemed a bit on the loud side when running. I didn't source it so I have no idea how old it is or how its been stored. Stability and focus seemed fine so i kinda just figured I had forgot what 70 sounded like, too much time around the fan filled digital boxes.

When it came time to do some training rather than extra wear and tear on those new loops or a print I grabbed some 15/70 leaders we used to use back in the platter days and let everyone get a feel for threading with that and that stuff runs nice and quiet like I expected. So this tread and that experience got me wondering about that test film. Anyone else think it runs on the loud side or know there is or was a loud batch or that the stuff gets like that under certain storage conditions?

[Ryan Gallagher]

Most of the test film (especially PA35/RP40, RP90) is thicker base than release prints, both poly and acetate.

i always have to final focus with a loop of something else, or you’ll often catch focus issues once the film starts.

[Tj Hopland]

I wasn't questioning the focus, it was the how loud this loop was running through the projector that got me curious if perhaps the noise is also related to the thickness. Had to go back and look at Steve's photo to remember what they called it, looks like BLS 1591 and I guess I'm just assuming BLS is Boston Light & Sound. I'm assuming this was something BLS had made back for the H8 run because even then the SMPTE stuff was hard to come by.

I have no idea if they just made one batch back in 2015 or if there have been subsequent batches possibly from different labs or on different stocks. Performance was fine so its doing its job. I was just curious if any of the prints were going to possibly made from the same stock and also be on the loud side running though the machine.