Film - A Practical Guide to Video and Audio Compression - page 51

1536 for pro-
duction work. Anamorphic compression (squeeze prints) allows a wider aspect ratio to be
worked on. The final raster is computed digitally or squeezed optically during printing.
The normal aspect ratio fits reasonably well on a TV monitor and many TV programs
are shot on 35mm or 16mm normal aspect-ratio film stock.
Wide screen is accomplished either by shooting on wider film stock and maintaining
roughly the same height, or by applying an optical distortion to squeeze a wider image in
the horizontal axis into normal aspect-ratio film stock. This is called anamorphic distor-
tion, and the same effect happens electronically when normal aspect-ratio video is
stretched from 4:3 to fit a 16:9 wide-screen monitor.

The physical imaging area of 35mm is conveniently mapped to 2048 ×

4.6.2 Anamorphic Printing

Anamorphic lenses were available in the 1960s as a retrofit accessory so that you could
attach them to a 16mm movie camera. The same anamorphic lens is designed be attached
to the projector. This leads to some distortion if the axis of the lens is not precisely per-
pendicular, and the image will appear skewed if it is not fitted correctly. The effect is
shown in Figure 4-5. Professional anamorphic lenses are integral to the camera optics and
are factory aligned.

4.6.3 IMAX

The largest cinema format in current use is the IMAX theatre presentation. This is pro-
jected onto a screen that is 215 meters wide by 156 meters high. The audience sits abnor-
mally close to this screen, therefore artifacts in the rendering process will be more visible
than usual. For this reason the film must be rendered at quite a high resolution.
The film is presented on 70mm stock with the picture oriented sideways, and it
passes horizontally through the projector. The frames occupy an area that is roughly three
times the size of a normal 70mm frame, with a corresponding increase in resolution.
Figure 4-6 illustrates a single frame of IMAX film. Note the placement of the small
circular holes. This ensures that the operator loads the film the correct side up.

Correctly adjusted Off axis by 15 degrees

Figure 4-5 Incorrect anamorphic stretch.