Curvature on a screen?

Here is what Stewart does:

http://www.stewartfilmscreen.com/eng...e.screens.html

Probably close enough if it works for their $18,000 Cinecurve screens.

But one poster here has suggested the most accurate way is to get the lens first, then project and measure the image height. Find the places where the image height is the same as the center, and that's where the edges should be off the wall. Seems like a reasonable plan.

I think what was being asked was what is an appropriate radius to curve the screen - I couldn't find anything on the Stewart site that mentioned it specifically, but maybe I just missed it. Since the basic idea is to (among other things) make reflected light the same brightness across the screen (angle of reflection = angle of incidence), common sense leads me to believe that the curve should be as if you tied a string to the lens of your projector, and the other end to a pencil, and drew the arc. Not sure if common sense is right in this case. Maybe check SandmanX's thread, I'm sure it must have been discussed before.

Yes, and my link above was exactly to that. Check it again. It give the radiuses Stewart is using, specifically 2 ranges, 2 specific radiuses. Empirical evidence (e.g. Stewards radiuses in teh link above, and others) suggests common sense dosen't apply. Don't forget that a 16x9 PJ projecting on a flat screen produces a rectangular picture. A radius to match the throw as you describe woudl produce an image that was no longer rectangular. The incidental angle improvements are a frige benefit IMO. One of the main reasons for it is to minimize image curving artifacts (e.g pincushion). There is a paper on Stewarts website spouting the benefits from their perspective, but I think they have it pretty watered down, leaving out the technical issues IIRC.

I checked it again, and I still don't see it - sorry, not trying to be argumentative, I am just also interested in the answer to this, and can't find it - I read that page, and the CineCurve Spec Sheet, and the Benefits of Curved Screens, checked the manuals and brochures, and the Product Information pages, but didn't find anything on the radius of curvature. I must be going blind...

The method of determining the correct curvature of the screen is based on considerable lab work at Stewart with a wide variety of lenses, lens combinations, projectors, screen materials and throw distances. Their information would apply only marginally to other screen materials (non-Stewart) due to any effects of the optical coatings. I can tell you from our experience that the radius method outlined above (radius roughly equals the throw distance) is absolutely incorrect. Any screen manufacturer providing curved screens should be able to tell you the correct curvature for their screen(s) and your projector. If they cannot, you might want to 'shy away' from their curved products since they obviously don't have a clue and are marketing curved screens just to 'take a ride'.

If this is a DIY project, you should mount your projector and lens combination and do some experimentation before you build the screen frame.

The poster in question was me. This method works perfectly and determines the depth of screen curve to perfectly compensate for any pincushion distortion introduced by the optical system. Any other method is subject to some degree of error.

It all depends on how much of a perfectionist you are. I wanted +- 1/8" tolerance of the screen edge to the projected image edge over the entire 2.70:1 screen and thats what this method delivered for me.

Vern

Sorry, they changed the page, maybe due to these posts. But I have the old one printed out. It lists 2 radiuses, one for screens up to 9' wide, another for screens above 9'. 40' IIRC on above 9' wide. I'll look at my printout tonight when I get home.

Thanks Vern, I was pretty sure it was, but I didn't want to quote you specifically from my feeble memory . This question gets asked so much it shoudl be a FAQ hugh.

I have a question,
What is the advantage of a curved screen IF your flat 2:35 screen does not have pincushioning or loss of brightness at the sides? Is there any?

There was a whole thread on this right here in 2.35:1

http://www.avsforum.com/avs-vb/showthread.php?t=586590

Should have all of your answers for DIY and beyond...

Thanks for the link, that clarifies things a lot - and now I must learn more about torus screen designs Sounds interesting, I wonder if those would be practical to DIY with a laminate material (if they have enough flex)...off to searching...

Just got home, looked it up. I was right they did change the page. So consider that these curves are no longer on their page. I presume they are right though. I printed it thinking they just might do exactly wht they did, remove it/change it. It said:

I don't know if they would do a custom arc or not. Probably but these would be for the masses wanting a curve and allowing them to do some mass producing. For a 120" screen, I'm betting the 40' radius would be close enough for me. Edit, not really mass producing per se becasue I believe they do not make them until ordered, however curved metal is a expensive beast, so I'll say.... to be able to more quickly make a "standard" curved screen, vs havin an infinite number of curve templates...

Anyone happen to use Vern's method to meticiously find the perfect radius using a Isco II and a 1.8 x throw (i.e. 54" tall and 14.4' throw)? If so what was it?

Stewart doesn't mass produce. All screens are manufactured to order and all curves are based upon actual requirements, not marketing department "lowest common denominators".

Well, maybe so, and "mass produce" was a bad choice of words, so I edited it above. But, what was above was right from their webpage. Maybe they changed their minds, and didn't want an infinite number of curve templates....

Thanks for the follow-up, GetGray, good info to know as a sanity check.

Don't they use special "ray tracing" equipment to determine the exact curve?

Mark

I think if the throw is long enough, the risk of those distortions/artifiacts with good optics is greatly reduced. But, there is a wow factor to the appearance of a curved screen. I suspect that with a really long throw, the radius of the curve of the screen in not so critical, within reason. True?

Sorry to revive this, but there is an outstanding question that never seemed to get resolved in this thread. Dennis seems to be saying that the CineCurve screens have individual radii, based partly on the projector involved. Elsewhere in this thread there's the statement that Stewart bases it strictly on the screen width. I called them today and an someone said the latter. I suppose it's possible that the gentleman I spoke to was wrong about his own product -- Dennis certainly seems to know what he's talking about. Surely someone who has purchased one of these screens can answer this: Does Stewart ask about your projector or do they just compute the radius from the width? Maybe the answer to this has even changed over time. I need to know because my RS1 is not arriving for who knows how long and I want to know if I need to wait.

That is becuause the exact curve is room dependant. Take any lens and place it in front of a projector in one room, and you get one result. Take the same lens into another room with another projector, and chances are you will see a different result....

Mark

I'm sorry if my question was not clearer... I understand what you are saying, and it is precisely why I am trying to find the answer to this:

Does Stewart take your lens and projector into account when computing the radius of curvature when you order one of their CineCurve screens, or do they just base it on screen width? I'm not asking "what should they do", I'm asking "what DO they do"?

I don't have a Stewart Screen, but my understanding is yes, they customize it to your gear and room as it based on twice the throw, not just the width...

Mark

Anybody out there WITH a CineCurve that can inform this?

You have been asking the screen manufactuer instead of the lens manufacturer.

You radius the screen to the lens specification. Stewart has done their own experimenting and have a range of radiusses typically falling between 30 and 40 feet.

Amore precise formula was obtained from Isco assuming a 1.8 to 2.4 backup lens on the isco 3 2 to 1 TD, with 25 feet TD the radius is 60. I will pass this data to Stewart as they may want to revise their depth of curvatures.

"Our attachment III is designed for flat screens. But can also be used for slightly curved screens.



The attachment causes a small pincushion distortion. This is quite normal for anamorphic attachments.



Additionally the attachment amplifies the distortion of the projection lens.

Because of the 33% wider image when using the attachment the distortion is more visible on the screen.



If you use a cylindrical curved screen then the pincushion distortion will be reduced horizontally by the screen curvature.

The vertical distortion is not affected by the cylindrical screen curvature.



For optimal condition the radius of a 360 degree cylindric screen can be calculated with the formula r(cyl) = distance x 2.4;

For example with a distance of 4 m: r(cyl) = 4 m x 2.4 = 9,6 m.



For calculating the radius of a vertical curvature you have to use the slightly modified formula r(vert) - T = distance x 2.4;

Or calculating the distance: distance = ( r(vert) - T) / 2.4 ; with r(vert) is the radius of the vertical screen curvature and T is the depth of the vertical bending of the screen. Please see the attached drawing: "AnAtt formula vertical screen curvature.pdf".



Seems like the TORUS is dead in the water... It will be interesting to see what the new custom-radiussed DNP .8 all-format Supernova ISF looks like.

Sounds like Stewart does take that into account: link
I'm in the same boat. I want to try and figure out what the right radius would be in my situation but have no idea how to use the calculation posted above by CINERAMAX. Maybe I'm being obtuse but that is not penetrating the gray matter...