Image Processing Reference
IMAX 70 mm format
69.6 mm x 48.5 mm
Figure 4-6 IMAX film dimensions.
IMAX programs tend to be quite short. Due to the nature of the projection equipment
and the way the film is spooled, older installations were limited to a maximum time of 120
minutes. Upgrades to the projector extend this to 150 minutes. IMAX movies are also pro-
jected at 48 frames per second. This will reduce the available run time to 75 minutes.
Some IMAX programs are shot in 3D, giving a separate image to the left and right
eye. Compression work to display this content on normal TV sets begins with a 50%
reduction right off the bat, simply by choosing one or the other image. The 48 fps frame
rate is used in IMAX presentations to produce a pair of images for 3D, interleaved, with
left and right on alternate frames, effectively presenting a 24 fps frame rate.
One format of note is VistaVision. This is shot on 70mm film stock running horizontally,
which is the same way IMAX film travels through the projector gate. In this case the for-
mat is an ultra-wide aspect ratio. In fact, VistaVision uses one of the largest possible imag-
ing areas of any film format (the Todd-AO format is a similar size). While it was used by
production companies in the 1960s, all the available cameras were later acquired by
Lucasfilm during the production of the early Star Wars movies because they found it to be
ideal for use in special effects work. They adapted and modified the format throughout
their work on these films. Figure 4-7 shows an example of the VistaVision film format.
Working digitally makes most of these formats obsolete, so optical film is usually just
35 or 70mm for projection.
The imaging area on 70mm film stock is approximately 48.5mm by 22.1mm. If this
were scanned at the maximum 115 pixels per mm, resolution would yield a picture that is
The corresponding imaging area on 35mm film is of course much smaller at 21.0