Film-Tech Forum ARCHIVE: Opinion on short throw lenses for 35mm projection sought.

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Posted by Jerry Axelsson (Member # 3175) on 11-17-2019, 03:28 AM:

Hello analogue enthusiasts.

I wanted to hear your views on short throw lenses for 35mm projection.
I have heard that several projectionists and technicians seems to have a consensus that a 1.85 lens should not have a shorter focal length than 45mm.

In your views, what type of image quality degradation occurs if you go lower than that focal length.
Some multiplexes had smaller cinemas with short throws going down to 30mm focal length for FLAT.

If you for various reasons should go below 45mm focal length, which type of lens will give you the best overall result to deal with that?

Grateful for your thoughts.

Posted by Paavo Mardi (Member # 4962) on 11-17-2019, 07:36 AM:

With short focal lenses picture distortion will occur. Projector must be perfectly centered, deep downward projection is bad. As you are from Sweden you will probably show subtitled movies. With off-center projection subtitles will be inclined. Deep downward projection will make subtitles curved.

Posted by Helmut Maripuu (Member # 2281) on 11-17-2019, 07:39 AM:

Sandrew's cinemas in Västerås had 45 mm lenses + zoom.
-The diameter of the 1:85 lens was 20mm.
-The diameter of the Cs 1:30 70 mm lens was 34 mm.
-The diameter of 35mm 1:37, 16mm projector is 12mm.
-The diameter of 25 mm, 1:87 is 10 mm.
All ISCO.
I think the heat will damage the lens. Guess you can use 45mm zoom and Isko zoom. Don't know if Sf bio still has lots of Sandrew stuff in stock.
Ps. Do you know where to get 35 mm; s projectors. Would like to have a dozen that I could save for the future.

Posted by Mark Gulbrandsen (Member # 72) on 11-17-2019, 09:14 AM:

I had many theaters with short focal length, even scope lenses with 40 and 45mm backups on them. Many flat lenses 30mm up to 45mm. Very careful projector to screen alignment is necessary to prevent excessive distortion. Keep in mind the lenses them selves do not introduce very much if any barrel or pincushion distortion. And they can provide pretty even light. They are corrected for all that. I am speaking of later Schneider and Isco lenses... Isco being the better choice if you can find them.

Mark

Posted by Steve Guttag (Member # 268) on 11-17-2019, 11:39 AM:

40 and 45mm Scope lenses eh? Yes, possible, but VERY rare...ISCO could go down to 42mm on 35mm Scope.

So to the question of short EF on 35mm.

Most sub 50mm lenses were a bit dicey in their quality of image, the ISCO Ultra-Star/Plus being notable exceptions. Schneider clung to their standard barrel size which put a LOT of heat on a small area and they weren't very good at stability to begin with so it is easy to crack a Schneider from the heat. ISCO...on the Ultra-Star Plus series (red lenses) made the inlet pupil larger on those smaller EFs and handled the heat MUCH better...they built in the magnifier and thus moved the focal point forward (which also helps to get the image out of the projector before vignetting). The ISCO Ultra-Star Plus can go down to 25mm though I think the shortest I used was a 42.5mm.

For short EFs (below 40mm) I tended towards using a longer prime lens and a special ISCO magnifier that had a .52X magnification. The magnifier was available for either Flat or Curved screen and would allow one to go down to 31.3mm on 35mm film using a 60mm ISCO Ultra prime lens. The magnifiers started life as part of the T-Kiptikon series but ISCO eventually made them available as screw on attachments for 70.6mm barrel lenses. They were good for 70mm film too.

What I liked about using that magnifier, aside from it being very good, is it allowed larger prime lenses that could deal with the heat better and typically were better lenses to start with. And, if you did curved screens, you had options. That said, if your screen is curved, then the much maligned Magna-Com 65 can be a great solution as they interact with curved screens well as they tend to pincushion the image some.

Posted by Marcel Birgelen (Member # 6801) on 11-17-2019, 01:48 PM:

When I worked at a small quad-plex back in the 1990's, we had one small, oddly shaped room with a short throw prime lens. I think its focal length was 40mm. The lens was a Schneider and it had cracked more than once. Besides that, it suffered from vignetting and visible optical aberration at the edges. Eventually, the room was closed as a new, larger room was opened and the projector repurposed. Those lenses were suckers, so try to avoid them.

I don't have any experience with ISCOs at that focal length.

Posted by Jerry Axelsson (Member # 3175) on 11-18-2019, 12:52 AM:

Thank you all for your input answers.

A person with longer service experience in the business mentioned his rule of thumb to me: The screen width should not really exceed more than the 2/3 of the throw in a auditorium.
This rule helps you stay clear of the shortest focal lengths in lenses as well.

Posted by Harold Hallikainen (Member # 5405) on 11-18-2019, 12:31 PM:

This reminds me of something I ran across recently in work on the SMPTE Recommended Practice for measuring screen luminance. It seems that with a lambertian surface, the luminance will be the same at any viewing angle (so every seat in the house is good), but the luminance varies with the screen position depending on the projector throw. The closer the projector is to the screen, the more the sides "fall off" from the center luminance. That's m current understanding, anyway. But, since this fall off is the same for every seat in the house, it COULD be compensated for in the projector software or could (as was done in the 1950s) be compensated for by varying the density of screen perforations (more perforations in the center).

Harold

Posted by Steve Guttag (Member # 268) on 11-18-2019, 12:41 PM:

1) Matte-white screens (Lambertian) are on the decline, unfortunately.

2) Most Matte-white screens for cinema, aren't (they aren't gain of 1.0 nor do they have a near 180-degree scatter).

So trying to compensate for it is an attempt in futility.

Posted by Harold Hallikainen (Member # 5405) on 11-18-2019, 04:51 PM:

I agree. It's just that I had not considered projector throw in luminance uniformity before. If uniformity were not messed up due to screen gain (where luminance at a particular place on the screen depends on where you are sitting), it would, I think, be possible to compensate for drop off when moving from the center of the screen. Another interesting area is emissive screens. The LEDs are not isotropic sources, so the "hot spot" follows you around the auditorium. If you are on the left side of the room, the left side of the screen is brighter than the center. I don't know how much of this effect there is, though.

Harold

Posted by Steve Guttag (Member # 268) on 11-18-2019, 08:20 PM:

I've done a bit of study on gain and curved screens to focus the light. The real pro was Glenn Berggeren. A flaw that the likes of Schneider made when they did their study was to not realize that projector location, seating location, in conjunction with the screen's reflective properties all work off of each other.

I wrote a small program in Mathematica several decades ago that would allow one to select the optimum curve to attain even light from side-to-side (cylindrical curve).

Since I also worked weekends at a Cinerama theatre, I also got quite a bit of exposure to the "joys" of short EF lenses and how they react on deep curve screens.

Posted by Mark Gulbrandsen (Member # 72) on 11-19-2019, 10:00 AM:

quote: Steve Guttag
40 and 45mm Scope lenses eh? Yes, possible, but VERY rare...ISCO could go down to 42mm on 35mm Scope.

Yep! My most hated screens and very expensive lenses. But the space was what it was and presenting the entire scope image was worth the cost. Both places were balcony's made into a third screen.

Mark