Several Imax theaters also have 35mm projectors, so maybe it was a 35mm print shown in an Imax theater.
Use a moderate image size that will give good screen luminance (ideally 16 footlamberts) without causing heat damage to the 35 mm print. A maximum image size of about 25 x 60 feet is reasonable. Gordon McLeod has had good success showing 70 mm 5-perf prints on the huge IMAX screen at Ontario Place in Toronto. Since the image area of 70 mm is much larger than 35 mm, the energy of a very large lamp can be "spread out" over a larger area of film, giving good light on a huge screen. I recall they played a 70 mm print of "Titanic" to many sell-out crowds a few summers ago. Maybe Gordon can give details.
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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
In the past, the Navy Pier theatre has also shown prints of The Wall, Stop Making Sense, and a few years ago they screened Francis Ford Coppola's personal 70mm director's cut of Apocolypse Now. Now THAT was impressive. The main difference between the director's cut and the regular release version is that Coppola's version has no titles or credits, so you don't see the words "Apocolypse Now" until the middle of the movie, when you see the graffiti on the wall, and the film ends with Martin Sheen leaving the jungle; there is no final attack on the base. I agree with the artistic decisions -- the film is much more powerful that way, and I felt privileged to see such a fine print. There is detail in the 70mm print that I never saw in the 35mm prints.
Jim had a print of Phantom Menace in the booth for a while, but the theatre never got permission to screen it.
There are two main drawbacks to projecting 35mm in an IMAX theatre. First, the screen is slightly curved, so there is a small amount of distortion -- instead of a rectangle, you see a slightly concave rectangle. Also, the theatre was designed to show IMAX shorts, so there are no bathrooms -- you have to go out to the lobby, and there are only side aisles.
Jim was ready for the funny farm after showing nothing but Fantasia 2000 for three months straight, over and over again.
The success of 70 mm screenings like "Titanic" at Toronto's Ontario Place gives me real hope that audiences recognize and appreciate the quality that only 70 mm prints can deliver on large screens.
BTW: Just ONE of those sell-out audiences more than paid for the extra cost of the 70 mm print!
35 mm prints are fine for screens up to about 60 feet wide. My guess would be that your 80-foot wide 35 mm image is certainly less than half of the SMPTE standard 16 footlamberts, and you are probably at the limit of putting 7000+ watts of energy through a postage-stamp sized frame without causing heat damage to the 35 mm print.
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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
PLEASE don't try to fill your full 76-foot screen width with a 35 mm print! It will be dim, and although the picture will be big, it won't look as good as it could with more brightness and less magnification.
For a 61 foot throw, a 50 mm prime with the scope lens will give you a 21.5 x 51 foot image. With a matte screen, a 5000 or 6000 watt lamp should get you close to the SMPTE aim of 16 footlamberts with this reasonably-sized image. A gain screen would likely need less power.
To fill a 76 foot wide matte screen with 35mm scope would need at least 10000 to 12000 watts to get near 16 footlamberts, but the film would be burned to a crisp within a matter of days.
. (Distributors don't like theatres that routinely burn prints). 
Another issue is that really short focal length lenses have very shallow depth of focus. So the "focus flutter" that the very high radiant energy causes will be very obvious, and the image will not be very sharp.
Finally, even IF you could provide a decent 32 x 76 foot image, ALL of your audience will be forced to sit closer than the optimum 2X - 4X image height seating distance from the screen specified in standard SMPTE 196M --- much too close for viewing 35mm images.
There are good reasons IMAX uses 65 mm negative and 70 mm print film for theatres designed like yours! It's fine to show 35 mm prints in IMAX theatres, but for your audience's sake, recognize the limitations of the format, and keep the picture size reasonable.
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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
No wonder you didn't want to tell us the light level you get for 35 mm . Looks like you are using a 45 mm prime for scope?
Watch out for heat damage, especially with B&W or silver-retention prints that absorb additional infrared energy. A 7000 watt lamp at 175 amps is alot of energy to focus on a postage stamp sized frame!
Glad you agree that 70 mm RULES 
on huge screens!
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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
I only have a 4kW lamp installed currently, it's still the original lamp. I am thinking about putting in a 5kW, but I want to wait until my Footlamp meter arrives so I can actually check out the light from the 4kW. It actually looks pretty good with the 4Kw. The screen BTW is silver (has to be for the 15/70 3D).
I should be able to fill the screen (Horizontal only) with 70mm, right? What would the focal length be? What is the equation to figure that out? The 70mm lens that I have actually has a smaller image than the 35mm scope lens. 
For 35 mm, stick with the 50 mm prime lens for scope. The 21.5 x 51 foot picture is reasonably large, and with a high gain "silver" (3D) screen, you should be getting a reasonable light level, even with 4000 watts. The only thing to watch is the uniformity of the illumination. High gain screens are very directional, and unless curved properly, you can have quite a bit of fall-off at the edges, or when viewed from seats far off the centerline of the theatre.
For 70 mm features, IscoOptic does make a very expensive 38 mm Ultrastar HD lens for 70 mm that would give you a 35.5 x 78 foot image. But IMHO it may be wiser to go with a more available longer focal length lens designed to cover 70 mm. For example, a 57.2 mm Isco Ultra MC (designed for 70 mm on a flat screen) would give an image size of 23.5 x 52 feet, and is much less expensive than the 38 mm lens. Check out what Schneider has to offer as well.
For 70 mm prints, the energy is "spread out" over a much larger print area, so heat damage is much less likely than with 35 mm. Just be sure to adjust your lamphouse focus or use a beamspreader to properly illuminate the 70 mm aperture.
Some people may quibble with showing feature films on an IMAX screen, because of the lack of proper screen masking. But as long as you recognize the limitations of the smaller formats, and maintain a good light level and sharp steady image, audiences seem to really enjoy seeing regular films on such a big screen. Good luck!
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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
We make an announcement about the format difference before the show so people know that it won't fill the whole screen. So far no complaints. You really don't notice it once the house light are down and the show is running though.
The reason I want a larger 70mm image is so that it can help me convince the people in charge of the money to invest in a sound system. I don't know why they installed a Simplex 35/70 combo and didn't include the sound system, but I am trying to correct that oversight.