Here's a post I wrote a couple years ago
with pictures to illustrate (some small edits made for context):
Composition is precisely the reason why Constant Height display is important. The rules of composition, regarding the relative sizes of objects in the frame, are the same between 1.85:1 and 2.35:1. The only difference between the two is that 2.35:1 has more picture on the sides.
Here are a pair of images from two similar movies: The Avengers (1.85:1) and Iron Man 2 (2.35:1). Both images are medium shots of the same character. When displayed in Constant Height format, you'll notice that the character is basically the same size, and he's framed similarly in both shots from the top of his head to his waist.
This is the intent of scope composition. You start at the same size you would in 1.85:1, and then add extra width.
However, if you display these same images in Constant Width format, suddenly the 2.35:1 movie is greatly shrunken and diminished in impact. The character looks much smaller, and the scene is less involving.
That's not the case in Constant Height. Iron Man is the same size in both movies in Constant Height. He fills the same amount of your vision. The Avengers is not compromised or diminished in Constant Height. When done properly, 1.85:1 movies are as large as they're ever going to be on your screen. Then scope movies are even wider for added immersiveness, as intended.
Constant Width, unfortunately, will always diminish 2.35:1 movies. They will always be shrunken down so that all objects on the screen are smaller than the same objects in 1.85:1 movies. That is not
how they were composed.
A Constant Width screen starts from the premise that scope movies are less important than and inferior to 1.85:1 movies. If you believe that's true, take a look at the types of movies that are photographed in each aspect ratio. Year after year after year, the directors of big budget, visual spectacle, eye candy movies overwhelmingly choose to shoot them in scope ratio. Consistently, 70% or more of that type of movie are photographed in scope.
Before you stammer, "Bu... bu... but... Why is The Avengers 1.85:1, then?" That was the decision the director of that movie made. The Avengers is one movie in a long franchise in which most of the other entries are 2.35:1. Iron Man, Iron Man 2, Iron Man 3, Captain America, Captain America 2, Thor, Thor 2, The Incredible Hulk, even Avengers 2... All of those movies are 2.35:1.
Scope composition adds more breadth and dimension to the image. It gives the characters more room to move around. 1.85:1 doesn't do that, not even if you project it extra large on an IMAX screen. All that does is zoom up the whole picture and make everything oversized. Proportionally to the way objects in the frame are composed, the characters are still boxed in by limited width.
Constant Image Area projection does not
solve this problem. Constant Height is the only scenario in which Iron Man appears the same size in both of these movies.