General optics/lens question - help!

[Robert Edward McClure]

A good friend and I were recently discussing the effects of front tilt upon 'film plain.'

In connection, my friend said he didn't understand why, for example, the image projected onto a motion picture screen by an m.p. projector (and lens) was able to be in focus across the projected image plain. After all, he cited, the screen is 'tilted' relative to the film plain. IOW, the projection lens is (usually) higher than the screen - and so must be tiled downward to correctly project its image to the screen.

I was not able to explain 'optically,' I realized, how or why the projected image could remain sharp across the plain of the screen. Related to 'depth of field,' or 'depth of focus' considerations?

What is the simple 'optical' explanation?

Thanks!

Robert

[Mark Woods]

The lens is either tilted or shifted to become parallel with the screen. Some are not.

[Skorzen]

I have done a lot of work hanging projectors in classrooms and I would say that the image is not in focus across the screen. It may have what some people percieve as "acceptable sharpness" but the fact of the matter is that it will not be equally across the screen. This is why when we were installing projectors we were very careful about putting it in the right spot. If you have to use keystone correction than you don't have the projector located properly in relation to the screen. Some projectors do have the option for shifts, and I would not be surprised if some can tilt as well, although I have not worked with any that tilt.

[Gene McCluney]

The "focus" of a film projector is a compromise. Some compensation is of course due to the depth-of-field of the lens and the great distance to the screen, but in general there is a bit of softness..you just try to strike a happy medium. Ultra-high quality installations, particularly back in the days of 70mm and curved screens actually used lenses corrected for the curvature of the screen, and the projectors WERE installed to project straight out to the screen, not up or down.

Back in the old theatres I used to work in (which were installed in the 1940's and 1950's) the projectors were installed where it was convenient, regardless of the amount of distortion it gave to the image. One theatre had the projectors at the back of the second balcony, projecting way down to the big screen. The aperture plates were trapezoidal in shape to make the image on the screen rectangular. Focus was approximate.