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This topic comprises 2 pages: 1 2
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Topic: Absolute screen size and chord depth
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 07-04-2006 09:27 AM
It was Schneider, Harkness, USL (they at least supplied the light meters) and Famous Players....the experiment was more than flawed and the "conclusions" were almost embarassing. Their "optimum" curve was what formed the 1/20 rule. If you use the Schneider lens program and have it calculate the curve, it will give you the 1/20.
The flaw in the plan was that they kept their projection point fixed so as to not take into account that where the light starts doesn't have an affect on where it ends up.
To calculate a curved screen for optimum light, one needs to take into acount where the projector is, what the dispersion of the screen is and where the seating audience is. At that point, you can think of the screen as a big light reflector (which it is). Thus, a long skinny theatre can typically have a more radically curved screen than a short fat one (presumming the projector is always at the rear most part of the theatre).
Some lenses do better with curved screens than others...Schneider does not do as well as ISCO as the radius becomes notably shorter than the throw. I've also noted that Schneider is hypersensitive to being absolutley square to the projection plane. ISCO clearly has designed their current lenses for curved film planes.
If your screen gain is 30% or less, then you really can't curve it very much (and why would you want to since curving the screen will introduce geometric artifacts).
Personally, I love curved screens...deep curve ones too. But you can't approach this stuff with mere rules of thumb...you have to really know what you are doing to make them work properly.
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Brian Guckian
Jedi Master Film Handler
Posts: 594
From: Dublin, Ireland
Registered: Apr 2003
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posted 07-08-2006 01:43 PM
quote: Steve Guttag
All lens manufacturers and every engineered projection process has come to the same conclusion that the screen has to be curved for optimum presentation...how much to curve it though varies greatly.
I'd like to pick up on that - is there a formal mathematical relationship (of which the mathematical formuale quoted earlier could be a subset) between say the throw, screen width on Scope, screen gain, and other factors which could be derived, and indeed, agreed?
If the 1/20 rule is flawed (albeit from a light distribution point of view) then is there a better mathematical model out there?
I'm thinking about a situation where you have enough light to use matt white, but you want to curve the screen "for optimum presentation" - and indeed curved screens being more aestheticaly pleasing. What also if you're happy with light uniformity but want to curve the screen for best focus?
It just seems curious that there is an accepted formula for deriving gain screen curvature, but there's no agreed standards for other forms of curvature, with different methods being presented in this thread.
Is it due to the specificity of lens types and designs?
Another thought that occurs is that nowadays, if you install D-cinema alongside 35mm on a curved screen (gain or no) are the lenses on those projectors designed for flat or curved screens?
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 07-09-2006 10:41 AM
Brian,
In my best double-speak...yes and no. There is a formula for a moderately gained screen (Pearlescent) that John P. mentioned and is documented by the SMPTE. However, it is an approximation based on actual measurments in an attempt to have a rule of thumb. However, even that document tells you to do "ray tracing" to achieve a properly curved screen (for best light).
I developed a calculus based program in college to do it but you do have to know the parameters of the room and the screen dispersion. ORC also had such a program which tended to come up with curves similar to mine (close enough that I don't think the frame manufacturers could make the difference between the two). Both programs are based on the concept of sending a ray of light towards the edge of the screen and seening where the cone of light reflects back. Then increasing the curve until the maximum amount of the seating area is covered.
The 1/20 rule is flawed in that it is based on nothing...just a number thrown out there...you can't come up with it from the data that was taken (hence my embarrasing comment above). It has not scientific anything behind it. What it DOES allow you to do is proclaim "Curved Screens" without going to gain screens and you can't get into too much trouble with it. Just about any lens will focus reasonably well on it (since the 1/20 rule often gets you something near a throw=radius curve).
As for curving for "optimum presenation"...that can't be done...I like curved screens...some HATE them...so what is optimum? If you have an extreme projection angle, curved screens (all of them) will accentuate that with the part of the image furthest from the projector being the most "bent."
As for getting curved screen advocates to agree on a common set of rules/formulas for "optimum" curve...forget that too. What is important to you? Light? Focus? Using the screen to put you IN the picture (deep curves like Cinerama)?
However, most of the curves fall into one of the ones I listed above...Focus, Light, effect.
As for DLP...their lenses are made with the thought of a flat screen, primarily. I do note a mild pincushion when the anamorphics are used but that probably has more to do with the anamorphic lens design than the prime lens. There ARE warping systems to pre-distort the image as it leaves the projector so it looks proper on the screen (not to be confused with the "masking" function found on most DCinema machines that merely blocks off the image to fit the curved screen like an aperture plate).
There are lenses for 35 and 70mm film designed specifically for deep curved screens and they do quite well. In fact, a Magna-Com 65 will do an excellent job of mating up with a Cinerama screen for 1.85 images..good focus and low distortion. Note these lenses are not like anamorphics, they distort the image in all directions so while the horizon lines will fit well, the vertical lines will be pincushioned. With wide aspect ratios, it is the horizontal that is most perceived to be distorted.
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Brian Guckian
Jedi Master Film Handler
Posts: 594
From: Dublin, Ireland
Registered: Apr 2003
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posted 04-07-2007 08:24 AM
I just wanted to bump this thread up from last time to ask if elliptical or even parabolic* screen curvatures have ever been tried in the field.
If you're looking at deep curve screens for effect (but not as deep as Cinerama), one of the things I've noticed is that ratios like 1.33, 1.37, 1.66 and 1.85 are subject to the same angle of curvature as wider 2.39 or 2.21 ratios, because the curves are cylindrical.
Thus the narrower ratios are subject to a greater degree of curvature proportionate to their width (I'm not expressing this well, but it is an aesthetic issue, as well as a design issue in terms of depth of focus for each ratio).
In theory an elliptical curve (for example) would present a flatter field towards the screen centre for narrower ratios, whilst still giving the immersive, or psuedo-immersive effect for the wide 2.39 or 2.21 ratios, or even if you wanted to show 2.76 Ultra Panavision.
Would a non-cylindrical curve of decreasing radius towards the edges of the screen present even more design issues than a standard cylindrical curve?
Just curious.
(* Curves derived from the parabola; the parabola itself being roughly u-shaped and probably unuseable)
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Frank Angel
Film God
Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999
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posted 04-08-2007 07:29 PM
In two of my theatres, the screen must be flat because of the limitations of the fly system -- there is not enough room for a curved screen unless you take up three pipes, something the lighting guy and our rigger would kill if I even suggested it.
BUT, for me, the curve is so much of an aesthetic plus, that I refuse to abandon it. What I did years ago when this theatre was first outfitted for film projection, I decided to try using a perspective curve to give the illusion that the flat screen is actually curved. It's very simple, you just slightly curve the top and bottom mask so that the masks are dipping in toward the center of the screen on both top and bottom. On a 40ft width screen, about 4 inches into the center of the picture is all that is needed to create the "curve." This small cheat makes the screen look as genuinely curved as any real curved screen -- that mild curve of the old CinemaScope screens. And it gives the same dramatic and immersive feeling that a real curved screen does. Yes, it's an illusion, but it totally works. Ask anyone who has seen a movie projected on it.
Nice thing is, although you do loose 8 inches at the center of the picture (insignificant on a 17ft + high screen), you don't introduce any of the geometric distortion that some complain is created on real curved screen, even on the mild CinemaScope curve. On a perspective curved screen, lines remain as straight as they normally are.
In the beginning when in decided that I wanted a perspective curve on the first screen I put in our big theatre, I was a mere babe and was very nervous about doing something unconventional with the install. When I told the contractor crew that this is what I wanted, the whole bunch of them balked and poo-pooed it --told me it was never done. I held my own and insisted; the result proved to be a very positive and it has served us very well for decades.
Over the years many directors have screened their films in our facility and we've never yet gotten a negative comment about the loss of those 4 inches on top and bottom, even from the guys who made the films.
Coincidentally, as I write this, we are putting in another new screen in this same theatre and just a few days ago I had the shop cut the masonite for the bottom and top masks with this same curve. It will grace this screen again for the next couple of decades! ![[thumbsup]](graemlins/thumbsup.gif)
Once the screen and masking are up, I will post some pics of it -- perspective curve and all.
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