Stupid question, but in a movie theatre: How is it that the movie fits on the screen so perfectly? I mean, there’s no white space left unfilled by the image. And, along these lines, with the image being rectangular (with the length being significantly greater than the width), why don’t we notice tremendous distortion?
For clarity for those wondering why I’d expect to see distortion, I can tell you a freind of mine had a wide, flat screen (plasma?) TV where the distortion is hideous. (If Oprah only knew what the distortion does to her figure!) Anyhow, this put the thought in my head so, I’d expect to see just as much distortion at the movies. I WAG the dimensions each frame of film must be proportionally rectangular? (But, are all movie theatres and screens constructed with such precision???) - Jinx
Yes, the projectionists at movie theaters adjust the margins of the image to fit exactly on the screen using a masking device. A tiny bit of the image is lost, but it is not distorted because it is being projected in a way that matches its original placement on the film to begin with. That is, the lens that projects the image reverses the process that occurred in the capturing of the image in the first place. I know about the projection masks because I’m old enough to have seen projectionists fiddle around with those masks before a screening. Presumably, now things are standardized to the extent that that is seldom necessary.
That is because a normal TV picture (4:3 ratio IIRC) was being stretched to fit a widescreen format (16:9 probably) for which it was never intended; this isn’t the case with movie footage, which is specifically filmed in widescreen.
So how do they get movies to fit on a regular TV screen when they are released on the network? - usually one of two methods:
Letterbox format: The top and bottom parts of the screen are masked off with black bars, making a centre section of more appropriate proportions.
Pan and Scan: Much hated by movie afficionados - select a piece of the action that will fit into the right shaped box and throw away the rest - in a movie where there are two characters at opposite sides of the screen, pan & scan will often be implemented in such a way as to show each of them in turn as they speak and this can generate a cinematographic effect quite different to that which the director originally intended.
The images on film that is used by standard movie projectors actually has dimensions similar to the TV. The “Widescreen” effect is created by the projectionist masking off the top and the bottom of the frame. As CC said, they can adjust these masks to fit the screen exactly.
Filmmakers usually count on this, and will leave mikes and such in the shot, as long as they are in the range that’s always masked off. Which is why you have to pan-and-scan when you release it on non-widescreen home video editions.
It is also worth mentioning that the director and cameraman know exactly which bits of the scene they are shooting will be masked off and as a result, the raw footage will often include microphones, cables, camera rails and even staff in these sections - incorrect masking at the time of projection can result in the microphone occasionally dropping into view, or other parts appearing.
IIRC There’s a bit in the (pan&Scan) version of The Fugitive where Harrison Ford is walking into a railway tunnel; you can see the front of the camera trolley at the bottom of the screen.
Posts above explain why the picture doesn’t have “stretchy” distortion, but they still don’t explain the focus problem. The projector’s lens is closest to the center of the screen, and farthest from the edges of the screen. How do they get the entire picture in focus at the same time?
They are projecting a flat image through a lens to another flat image of the same proportion (albeit larger) - there’s no reason why it should be out of focus at the edges if the lens is OK.
Or to put it another way, yes, the edges of the screen are further from the centre of the lens, but so are the edges of the film.
In addition to the masking around the screen, the projector has an aperature plate cut specificly for that projector and theater. The focus of the lens is also perfectly calibrated to the throw of the theater (distance between projection booth and screen). If you have a multiscreen theater and get the lenses or plates mixed up you could end up with a small image with an immense amount of white screen around it, or a too big image that overlaps onto the masking, walls and ceiling.
Also, in a theater, the lens is fairly far away from the screen. This means that there wouldn’t need to be as much magnification to get the projected image to fill the screen; it’s designed to do so.
Actually those two effects don’t cancel out. If you move the subject further away from the lens, the focused image moves closer to the lens, not further away.
But lenses can have slightly different off-axis vs. on-axis focal length. If you design the lens system correctly, you can create a lens system whose off-axis effective focal length is slightly longer than the on-axis focal length. Such a lens takes a flat subject and project it into a flat image. Those are called “flat-field” lenses and used for enlargers and projectors.
On some slide projectors you can select between a curved-field and a flat-field lens. Glass mounted slides are perfectly flat so you want to use a flat field lens. Slides mounted on cardboard frames have a natural curl (curvature) so curved-field lenses work better.
