Curved screen

[Steve Guttag]

There were some curved field offerings for film. A MagnaCom could be used (within a range) to help with a curved screen. ISCO also had a curve field magnifier (.52X) that did well. However, these were not anamorphic type magnifiers so you are applying the distortion to both planes. You may straighten a horizontal line but you'll curve a vertical one.

I too prefer curved screens and projected onto a Cinerama one for nearly 2-decades. If we had the curved field lenses in digital cinema that we had for film, I'd put them in more. Some of the Schneider large format lenses (short focal length) are designed for a curved screen (but relatively shallow).

My most recent curved screen installation was at the Ventnor Square Theatre in Ventnor City, NJ. It is a 1.4 gain screen with an appropriate curve for best light reflectance. It came out well but there is definitely some bowing of the horizontal lines.

[Brad Miller]

Theatre owner MR Warren many years ago in Oklahoma had a special curved screen lens made for his large format cinemas and now they are owned by Regal.

We used a digital anamorphic on the big houses at the Warren Midwest City, as it was the only way to fill the screen from the short throw. After a good amount of very careful physical fine tuning of the boresight and anamorphic position, we were actually able to get sharp focus in the center and all 4 corners, and it was a pretty deep curve too. I was not expecting to be able to achieve that going in without a special lens.

[Jim Cassedy]

Many projectionists I know hate curved screens.

Good friend Terry was polite enough not to mention my name, but he knows that, in my opinion, curved screens are an optical engineering anathema,
and I despise them with a passion only surpassed by my dislike of child molesters and tax collectors. (and platters!) When done right, with geometrically
corrected prints and specially designed optics, they can be wonderfully impressive. But, the problem is that, except for Cinerama, D-150, Disneyland &
a few World's Fair & special venue set-ups, these image corrective options were not available to 98.4½% of most other theaters. Even though there was
a recommended degree of curvature based on so-many-arc-degrees-per-foot of screen width- - the amount of curvatures among cinemas still varied
greatly, often based only on architectural whim and the desire of some theater owners and chains to 'out-curve' the competition during the days when
such nonsense was in vogue. The only thing that varied more than the curvature, was the friggin' focus, which was a nightmare to maintain and which
I was never 100% satisfied with. (or at least it was always a real P-I-T-A to deal with at the several theaters with curved screens that I pulled shifts at
early in my career) The very idea of a curved screen IMO violates several basic principles of optics, which can best be described as the projection
equivalent of trying to fit the proverbial square peg into a round hole. Even noted projection guru and author Robert Mitchell (I'll bet some of you own
one or more copies of his "Manuals Of Practical Projection" ) once wrote that : " A flat floor, on a (curved) Cinemascope screen looks like the interior
of a bathtub. Flat walls become arches. Rivers flow uphill, and skyscrapers emulate the leaning tower of Pisa"

Personally, my vision of hell includes a room where I'd be forced to spend an eternity trying to focus a scope film with fuzzy edges on a curved
screen (projected from platters! ! ! ) On the other hand, my vision of heaven would include a fully operational 3-strip Cinerama theater.
- - but I'll probably wind up in a digital purgatory, trying to figure out the aspect ratios & frame rates of an eternal assortment of Avatar sequels .

"Exhibit - A "
CurvedScrn.jpg
(Image © 1953 International Projectionist. R.A Mitchell quote: ibid and reproduced
from FILM HISTORY / Vol 16 / John Belton)

[Sam Chavez]

Nicely illustrated Bother Jim.

[Steve Guttag]

Well, no...poorly illustrated and factually incorrect, at least the top portion. The top portion presumes that the projector and viewer are on center line (vertical and horizontal)...how does that work AND not have the person's head in the picture?

The top portion of "Exhibit-A" is an "orthographic" projection. However, when the projector is off center, there WILL be keystone and even if you had enough lens shift to attempt to correct for that, there will be stretch. And then there is the observer's perspective. They do NOT see a rectangular screen. And here is an example with a photo I took while calibrating a projector on a flat silver screen for Avatar 2:

Screen Shot 2023-01-07 at 9.18.54 AM.png

The top of the screen appears level...all of the other sides are anything but parallel or perpendicular to anything. And that is with white light! I was clearly a little off center to the left and below the centerline of the screen but in the middle of the seating area (the spectroradiometer had centerline). The spot being off center and low, backs up the perspective of the screen. The projector is horizontally centered by vertically above centerline (as most are if you don't want people getting into the picture when they stand up).

I absolutely "get" and respect those that prefer the distortions associated with a flat screen and if you are at home and can situate your chair and equipment such that you minimize the distortions...more power to you. But, to think, in a commercial setting, that more than 1 person is getting the perfectly paralleled image is fantasy. Your brain is going to be pretty good at ignoring the geometric issues unless it is extreme...like a steep projection angle where all buildings (or other vertical lines) lean in...which Exhibit-A misses completely. Here is a picture of a test pattern from a projector at a steep angle (top of balcony) to a flat (roll-down) screen:

Screen Shot 2023-01-07 at 11.23.20 AM.png

Curve or no, sides are going to lean in with keystone. Note too, the red horizontal lines pincushion. Lenses are not perfect and lens shift makes things less perfect! Note the center "circle" is now an oval (stretch) and again, my perspective when taking the picture does not convey are rectangular screen either.

Even though there was
a recommended degree of curvature based on so-many-arc-degrees-per-foot of screen width- - the amount of curvatures among cinemas still varied
greatly, often based only on architectural whim and the desire of some theater owners and chains to 'out-curve' the competition during the days when
such nonsense was in vogue.

There is some truth there and some misconception.

The reasons for curving a screen vary. Some are for esthetic reasons, which are as valid as they are for flat screens. I, personally, prefer the look of curved screens. I just don't like the geometric distortions to the image that they bring...such the dilemma! I hope when emissive screens are more than experiments/beta and have sound coming through/from the screen that we can have a curved, distortionless screen.

In earlier days, some of the reason for curving a screen had to do with the limitations of the lenses of the day. Also, as mentioned in another thread (I think) film, when exposed to heat/light is not flat in the projection gate so a curved screen can help with that.

I don't know how many architects design curved screens on a whim. I haven't encountered that. Yeah, on a set of blueprints/drawings, the screen is often depicted as a curve but without any radius or anything. There was definitely a degree of curve screen trend out there in the 90s/2000s.

Schneider Optics (mostly at the behest of Harry Mathias) did a study and determined that a 1:20 ratio of width to chord-depth was optimal. The problem is, the study didn't really come to that conclusion. There was no real science in it. Yeah, they measured a lot of things but seemed to be looking for a conclusion and when that didn't work, applied the desired conclusion anyway. One thing about the 1:20 screen...it doesn't do much harm! I see them, mostly. They don't help with much...maybe some focus, in particular situations...they spread the hotspot, slightly, in silver (or high-gain) screens but you do get the geometric distortions (just less so) that curve screens bring. To come up with a curved screen, you need to know:

• Where is the projector in relation to the screen.
• Where the audience is in relation to the screen.
• What the gain of the screen is (and its distribution)
• What the depth of focus is of the lens in use.
• How does the lens view what it is projecting onto (does it pincushion the image, leave it rectangular).

A simple 1:20 rule ignores all of that and just comes up with a fast formula to curve the screen...any screen. If you think about it, processes like Cinerama, D-150, Circlevision...have all of those items baked into their processes.

A legitimate reason to curve a screen is for best light reflectance. I've used this a bit, over the years. Less so in the DCinema age because lens wise, we don't have the tools as much as we did for film.

Screen Shot 2023-01-07 at 9.24.16 AM.png

This is certainly more even light than the flat silver screen above. Yes, it has curved top/bottom, from the viewer's perspective but, as one can tell, the sides look plumb but that has to do with where the photo was taken relative to screen centerline. Both are silver screens.

Even from the side, I'd take the curved screen (particularly with higher gains) over flat:


Screen Shot 2023-01-07 at 9.27.27 AM.png

The light remains much more even than a flat screen.

Now, getting light even does require a bit of math to figure out the best curve that works with the parameters listed above. On this one, I went a little too far (or the screen manufacturer's data was a little off). If it were a little more shallow, the hotspot wouldn't be on the far side and the uniformity from straight on would have been a little better.

Screen Shot 2023-01-07 at 9.31.13 AM.png


Screen Shot 2023-01-07 at 9.31.38 AM.png

Images can look sharp on a curved screen too.

Perhaps with "warping" we'll be able to electronically fix the various distortions associated with curves and keystones though I'd like some curved field lenses to do much of the work optically. Electronic warping always takes a resolution hit so the projectors will need to be in the 8K-16K range to really do electronic warping and still have a high resolution image.

I do say, if you are going to curve a screen...try to get something out of it...like a more even/bright picture that uses less lamp/laser power.

[Sam Chavez]

Steve is no fun at all. Takes everything way too literally. I was commenting on the art of Jim's work and the nostalgia of these ancient illustrations.

I must commend you on your body of work in getting optimal results for your clients. No one else that I know goes to this much effort. We all yearn for simple guidelines in an area that doesn't want to yield them.

In the end, it's just pictures on a wall immersing us in a story. If the story is compelling enough, we'll watch it in a tent.

[Steve Guttag]

That's not true! Here is the aforementioned Ventnor Square theatre 1...curved front end, curved screen, has a curtain and Dolby Atmos...lotsa fun!

Screen Shot 2023-01-07 at 3.06.42 PM.png

[Marcel Birgelen]

In the end, it's just pictures an a wall immersing us in a story. If the story is compelling enough, we'll watch it in a tent.

Welcome to the Dark Side, Sam. We have cookies, tents with warp-free curved screens AND Dolby Atmos.

Still, Steve has a point... Unless someone develops a light-field display that essentially projects an individual picture for everybody in the room, there will always be geometric distortions in a 2D picture projected on a plane in 3D space, no matter if that plane is a perfect rectangle, a cilinder, a paraboloid or some randomly shaped blob. Fortunately, most of those distortions aren't really obvious to the average moviegoer, unless we start projecting something very big on screen and fringe situations start to happen. Furthermore, our brain seems to be able to cope better with trapezoid distortions rather than curves in objects that should otherwise be straight...

[Neal Scanlan]

As always the discussion is so interesting and informative.

Thank you all