[Frameworks] Stock and exposure for shooting image onto optical track

[Scott Dorsey]

The soundhead is reading only intensity, that's all it does.  The faster
there is a change in intensity, the faster the output of the soundhead
changes.  It's just a photocell sensitive to only one color of light, 
looking through a narrow slit.

In the case of a variable-area track, this means that both distortion and 
high frequency response are very much affected by having a crisp image with
very sharp outlines.

That's why the sound recording stocks are designed to have as fine grain 
as possible, no edge effects, and very very high contrast.

The older sound recording stocks were lower contrast than the current
EXR stock... and also many of them were on acetate base, which tended to
expand and contract with humidity.  The newer sound recording stock gives
a much more precise and sharp outline.

If the sound recorder were absolutely perfect and there was no light leakage
inside it, then it really wouldn't matter how contrasty the sound recording
stock is.  But, it's not a perfect world.
--scott