Image Processing Reference
Nyquist and Grain Size
The concepts developed by Harry Nyquist regarding sample rates needing to be twice the
resolvable detail in order to preserve information apply to film as much as they do to
sound and video. In this case the sampling needs to be twice the grain size or better. To
arrive at a suitable scanning resolution for your footage, you need to know the film stock
characteristics and how the processing will affect the grain. Then, given the picture size,
you can work out how big the grain particles are. Double that and you have the target res-
olution that you need to scan with. Figure 4-3 shows the relationship between the grain
size and the scanning grid.
The vertical pixel dimension is governed by the aspect ratio of the frame and is set
up to deliver square pixels.
These formats have been around for a very long time. Most footage exists on 35mm,
although there are variants such as Todd AO, 70mm, Vista Vision, Cinerama,
CinemaScope, TechniScope, and others.
Hollywood movies shown in cinemas are usually presented on 35mm film projected
with an 1.85:1 aspect ratio or stretched to 2.39:1 with an anamorphic lens (CinemaScope).
Most cinemas will use those two formats, but a few will also support 1.37:1 Academy
Aperture 35mm format as well.
Note that for some formats, the negative master films have sprocket holes that are a
different shape or size. You must be careful to ensure that they are not conveyed through
a film transport designed for a different sprocket format. Mismatching the transport and
film formats will permanently damage the film the first time it is used. If this is the mas-
ter negative, the damage may not be repairable.
Figure 4-3 Film grain and nyquist scanning limits.