Curved screen

[Neal Scanlan]

Hello,

Happy holidays to all.

Can I ask a question:

When I am using an anamorphic lens I notice that edges on both sides of the projected image are slightly out of focus. I am assuming this is because I am projecting onto a flat screen and not one that is curved. Is this assumption correct and is a curved screen the only solution to this issue?

[Jim Cassedy]

Assuming it is a standard scope lens or anamorphic attachment, a flat screen is just fine.
Except for the very early days of Cinemasocpe™, when curved screens were somewhat
of a 'fad'', 99% of scope screens had no curvature.

[Steve Guttag]

Neal, that is not an easy answer, particularly with film projection and I think Jim's answer above is misleading. The 1:20 curves at the end of film (last decade or 2) were more on the faddish side with little science behind them, despite a paper on it.

In a perfect world, with a perfectly flat film plane and a perfectly made lens, a flat screen should yield the best results in terms of focus and linearity. But let's move reality into the situation.

1. The film is not in flat plane. Depend on the gate design, it may be curved, in one direction, by design. However, adding into that, the film will swell towards the light (heat) and since shutters have, at least, 2-blades, it pulsates it focal plane. And furthermore, it has been show with high-speed photography, we focus the image when the film is at its most deflected state (on the second opening of the shutter, or 3rd in the case of a 3-wing). So, you are focusing the film when it is in it most dished, non flat state. That will play into the final result and it will vary by lamp size, gate design and heat reaching the film.
2. You have not 1 but 2 lens systems in an anamorphic application (even if they are integrated). Lenses have hard time...they are focusing on a moving object, they are heating up themselves and they need to minimize spherical aberrations as well as chromatic ones. With all of that, it is not uncommon to see some "pincushioning" of the image and it will vary by focal length. A pincushioned image indicates it is preferring a curved field to focus upon. But that is not cut and dried since you are likely not projecting on dead-center...as you curve a screen (and presuming a cylindrical curve to be able to use conventional screens), you will also create some notable geometric distortions as keystone interacts with the curved field that likely will not precisely match what the lens system wants.
3. So, while the prime lens is trying to deal with all of that, you have the anamorphic part. They are normally the worst part of the lens system with the vast majority being simple 4-element lenses. They do not react equivalently to all focal lengths either. Schneider, a top-tier lens manufacturer, who has been noted for making so-so anamorphics even published (and incorporated into their excellent lens calculator) how their compact and full-sized (WA) anamorphics interacted with their various prime lenses. The 2:1 expansion was, at best, an approximation. So, depending on your anamorphic lens, it too will have its own artifacts that it contributes to the image, including stretch, pincushion or barrel (opposite of pincushion), chromatic aberration and astigmatism. After all..it has zero magnification in the vertical plane but a 2:1 magnification in the horizontal (presuming a conventional anamorphic). That is a pretty neat trick. Note too, ANY rotation of lens elements in an anamorphic shows up as an impossible to focus image, even if it is an otherwise good lens design. And to make it an extra challenge, they heat up too...so keeping lens element relationships are extra tricky in anamorphics.

So, could curving your screen improve image focus? Maybe but I bet it will be a VERY slight curve. You can easily prove it to yourself. Get a piece of paper and hold it in front of the soft part of the image and slowly move it closer to the projector (we're talking cm at a time) and see if the focus improves on the paper. If not, curving the screen is not your answer.

I would make sure I'm using good lenses (Schneider or ISCO as your prime lens will be best, I prefer ISCO and you will get a higher resolution with ISCO) and use the best anamorphic you can get your hands on. The compact Schneider was their best offering, in my opinion but every ISCO was better (Ultra, Ultra Star and Blue Star)...that does not mean every example you may find in the wild will be perfect. Be sure to set the distance ring (aka astigmatism ring) precisely by using actual test film, not the markings on the lens. Defocus the ring to infinity...adjust the prime lens to get perfectly horizontal lines. Then adjust the distance ring on the anamorphic until the vertical lines come into crisp focus...go back and forth, if need be to get the best image. If you are using good equipment, you should get a pretty good focus from corner to corner. If not, there may be a lens issue or even a gate/gate alignment issue. That is where getting a sharp image starts.

If you still want to curve the screen, for whatever reason (esthetics included), use the paper test above to see what the depth-of-focus you have and so you know the limits of the system. Remember too, as the edges of the screen get closer to the film plane, the image will get less tall (less throw to those portions of the image) so that may affect how you have to mask the top/bottom of the screen.

fi you really want to go to town and curve a screen for more even light in a large space, that requires more tools to get the best shape and presentation.

[Mike Blakesley]

Our 30-foot screen frame was first installed in 1953 and it has a pretty shallow curve - I've never run a string across and measured how deep it is, but I'd say maybe it's 18 inches at the maximum depth. I've always wondered if it was done that way because that's how they were all done then, or because of focus concerns, or just to make it look better.

[Rick Raskin]

I agree with Steve. In my very old experience, the quality of the scope adapter makes quite a difference. I worked with Kollmorgan anamorphics and the end focus was miserable. Switching to B&L made all the difference. I've heard that in today's world Schneider and ISCO are quite good.

[René van Vught]

Hi Steve,

For many years I have been following this forum, analog 35mm film projection has gripped me over 50 years.
I must compliment you on your always very knowledgeable and comprehensive explanation of this fascinating world.
You keep the analog presentation alive. Chapeau.

[Neal Scanlan]

Hello All,

Thank you very much for your brilliant help and I think Rene's eloquent post says it all.

Concerning lenses: Is there an advantage to using an integrated anamorphic lens over an anamorphic adapter?

[Steve Guttag]

No, not unless it is a short focal length. The integrated anamorphic came about to close-couple the prime lens to the anamorphic lens for the shorter EFs in order to get the image out of the lens. Except for 3 focal lengths of the ISCO Blue Star line, all compact anamorphics I've seen are just 4-element lenses. I'd say that ISCO's integrated anamorphics were really just their 16mm anamorphic affixed to the prime lens.

With ISCO, they issued integrated anamorphics as a single line item and are not designed to be disassembled. Conversely, Schneider offered their compact anamorphics like their full size as something one could combine with any of their prime lenses. They offered close-couple adapters where one could unscrew the lens-portion of their prime lens from the larger 70.6mm barrels to get the elements very close to the anamorphic.

I, often, used the Schneider compact anamorphic for 16mm anamorphics as it opened up the entire 35mm film lens line to 16mm...which often had limited choices.

I'm sure some used the integrated anamorphics when installing lens turrets as a means for ease of use as well as to avoid vignetting the flat image. If one has a short EF lens and the image is expanding rapidly, a full size anamorphic could occlude part of the flat image's picture, in a lens turret. Integrated (compact) anamorphics keep the OD at 70.6mm, which would minimize that chance. And, if one used the lens as a "crank" for the turret, they aren't going to unspin the anamorphic attachment from the prime lens...adding to yet more alignment issues.

[Randy Stankey]

I can't say anything more than other's have already said (especially Steve) but, from my perspective, I'd suggest checking to be sure your lens set is okay, properly installed with astigmatism adjusted correctly. ("Plumb, level and square," so to speak.) If that doesn't work, I'd move to the film trap/gate to check that everything is square in that area. After that, I'd check the shutter and lamp alignment to be sure that there isn't too much heat getting to the film, causing hot-spotting and buckling.

To highlight something Steve mentioned, when I was learning to run the Norelco AA's at the local Warner, the guy who taught me said to focus the picture by looking for a "donut" of sharp focus around the center of the picture. It has to do with the way film buckles as it heats up in the aperture. The human eye seems to "remember" the last thing it saw before the shutter closes for the last time, before the next frame is advanced into place. Contrary to intuition, the film NEVER stays truly stationary in the gate, even if you think it "should" be. Inertial forces and heat expansion keep the film moving, subtly, during virtually every phase of the projection cycle. What the projectionist should try to do is to find that "last" image that the viewer's eye will "remember" best and focus on that.

Some people can adjust focus intuitively. Others need to be told. I happen to be one that learns best by doing but I often need to be told what to look for before I truly grock the concept.

[Frank Cox]

I always looked for a straight edge in the foreground to focus on. Ideally someone's necktie or the crease in his pants, believe it or not. But any good foreground straight edge would do.

[Randy Stankey]

I do the center/donut thing but I also look at the picture, afterward, to see if it needs any tweaking. Like Frank says, look for other objects or areas around the screen to focus on and try to find a “happy medium” between the more objective “donut method,” and the more subjective look of the overall picture.

As people say, it’s part art and part science.

[Steve Guttag]

Believe it or not, how the film expands to the heat is even more uneven due to the optical soundtrack. The film is held in the gate by the edges...symmetrically. However, due to the soundtrack offset, the light through the film (and heat) is offset for the soundtrack...so, the film does not swell evenly towards the light/heat.

[John Eickhof]

I remember decades ago reading a booklet published by Fox regarding the introduction of Cinemascope, There is a section ( 4.0) regarding screens, the theatres I worked in when I was young all had curved screens, not a deep curve but like the curve in the screen at the Roxy in Forsyth MT. Section 4.0 is worth the read, it compares image quality on a flat screen compared to a curved screen and recommend the curve be a radius equal to the throw from projector to screen, it cited focus issues and light issues! Fox advised a slightly curved screen would exhibit better than a flat screen. Here's a link: CinemaScope: Information For The Theater (widescreenmuseum.com) I have always noticed a gently curved screen always looked better to me especially with older lenses like the B&Ls and Japanese types. I also remember if you ran the Fox Cinemascope test film ( pre RP-40 ) the frame lines at the to and bottom were slightly curved tey straight in the center of the image area.

[Steve Guttag]

That article seems to be trying to use the "Miracle Mirror" screen and denotes a relatively narrow reflectivity. As screen gain goes above 1.3, curving the screen is pretty much required to achieve any degree of uniformity.

[Terry Monohan]

I love curved screens. Ever since my first Cinerama /Todd AO movie the larger and deeper the curve makes a movie stand out for me.

Many projectionists I know hate curved screens. If you have the correct lens to take out some of the keystone effects on the bottom side corners and you don't have much of a deep projection slope angle the curved image will look great.

Too bad so many USA theatre circuits just put in a small flat screen with no masking or curtains these days.

Many new Europe cinemas have deluxe huge curved screens with the proper corrective lens.

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.