TheaterBlog - The Screen Part I: Choosing an Aspect Ratio

When I started planning the theater what I didn't want to have was a room with a big TV in it; I could have that anywhere, but for this room I wanted the theater experience.  A big part of that meant a large screen, but size was only part of the equation: aspect ratio mattered.

Movies are filmed in a variety of aspect ratios, while current HDTV resolutions measure 16:9 (1.78:1), the same can't be said about movies seen in the theater.  Let's have a look at some recent releases and their corresponding aspect ratios:

Rambo - 2.35:1

27 Dresses - 2.35:1

Cloverfield - 1.85:1

Untraceable - 2.35:1

Bucket List - 1.85:1

Juno - 1.85:1

National Treasure: Book of Secrets - 2.35:1

There Will Be Blood - 2.35:1

Mad Money - 2.35:1

That's right, even though we just spent the past several years making the move from 4:3 displays to 16:9/16:10 displays, there's content available in an even wider aspect ratio: 2.35:1.

I don't expect to see widespread movement towards 2.35:1 displays anytime soon since only a subset of movies are filmed in this wide, also referred to as Cinemascope, aspect ratio.  That's why, even on a 16:9 display, you get black bars at the top and bottom; you're watching content that's filmed in an even wider aspect ratio.  

Keep an eye on the screen the next time you're at the movies; chances are the previews are in a different aspect ratio than the feature.  Movie theaters handle various aspect ratios by opening/closing curtains to mask the black bars.  There's nothing wrong with the black bars themselves, but projecting black is tough to do.  If you move a light absorbing black curtain in the way, you get much better perceived contrast ratio since a black mask is always darker than any black a projector could produce.  

As soon as I heard that it was possible to build a home theater with a 2.35:1 screen, I knew it was what I wanted.  All logic went out the window and emotion took over: I wanted a superwide screen.  

There are a couple of problems you run into with a 2.35:1 screen, the first being that there are no consumer projectors available that will project a 2.35:1 image.  The name of the game is 1080p and that's 1920 x 1080, a 16:9 ratio.  So how do you get a 2.35:1 image from a 16:9 projector?  There are a few ways:

Manually Zoom the 2.35:1 image so it vertically takes up the entire screen.  You maintain 1:1 pixel mapping with this method, but you aren't using the full resolution of the projector to display the screen.  The projector is still projecting the black bars at the top and bottom of the screen, they are just zoomed off the screen so you can't see them.

Use a video processor to scale the 16:9 image vertically, then use a horizontal expansion lens in front of the projector to stretch the scaled image horizontally.  This is referred to as a Constant Image Height (CIH) setup, since both 16:9 and 2.35:1 content are viewed at the same height.  Only the width changes and masking systems are sometimes used (like in theaters) to improve the experience.  

Alternatively you can use a vertical compression lens which works similarly to #2, except instead of stretching the image horizontally you compress the image vertically.  This is known as a Constant Image Width (CIW) setup as 16:9 and 2.35:1 content will be the same width, but the screen height will vary.  

The zoom method seemed a bit cumbersome to me, so I started looking at anamorphic lens systems.  The CIH/CIW methods made the most sense to me as you are using the full resolution of your projector on visible content, not for projecting black bars.  Many 1080p projectors are now coming with built in vertical scalers for CIH setups, not to mention that a HTPC could do the same task without a problem.

I liked the idea of having a fixed height screen with variable width, I figured I'd include a masking system to complete the effect.  While 2.35:1 content would take up the full 11 feet of screen, 16:9 content would only be about 100" wide.  I'd need automated masks on the sides of the screen to mask off the black bars when viewing 16:9 content, but how hard could that be?  

It turns out that 2.35:1 screens with automated masking systems are very expensive.  To make matters worse, I wanted to complete the cinematic experience with a slightly curved screen.  At the end of the day I was looking at $10K - $20K just for the screen and masking system if I wanted to buy one ready made.  I wasn't about to spend that sort of money on something with no modern day electronics in it, so DIY it is.