[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: CBS high def test

Howard Lukk wrote:

"What is the resolution of 35mm academy and Super 35mm film after it has
been put into a normal camera, taken down to your local lab and processed?
Is it 2k x2k? Is it 4k x4k? What is resolution independence?
I am sure a lot of it has to do with the film stock. I have always been
curious about where these numbers come from."

Film manufacturers publish charts called modulation transfer curves, which
plot the response of the film against the spatial frequency. This in turn
is defined in cycles per millimetre: imagine a test pattern of alternate
and evenly spaced black and white bars. A single cycle would be from the
leading edge of one black bar to the leading edge of the next. Spatial
frequency is how many complete cycles - sometimes called line-pairs - the
chemistry of the film can resolve per millimetre. Note for purists: the
true definition of a modulation transfer curve uses a sinusoidal signal
rather than a square wave.

The modulation transfer curve for a popular film stock, like Kodak
5293/7293, gives 100% resolution up to 20 cycles per millimetre, very good
response to 50 cycles per millimetre, and around 40% response at 100 cycles
per millimetre. If you take 80 cycles per millimetre as an acceptable
resolution and multiply it by 24mm, the width of the film frame, you get
1,920 cycles. To stand some chance of resolving that as distinct cycles of
black and white rather than just shades of grey, Mr. Nyquist's theory says
that you need at least twice as many digital samples, which is why people
talk about 4k sampling.

While I am in the process of boring you all, can I just say two other
seemingly obvious but often forgotten points. First, people - indeed Howard
did - talk about "4k x 4K scanning". This is a convenient reminder that
digital film systems generally use square pixels, unlike video, but it is
not strictly speaking true. A Super 35mm film frame is roughly speaking 4:3
in aspect ratio, so it actually is 4k x 3k.

Second, the limiting factor is the chemistry of the film, the amount of
silver bromide crystals that nice Mr. Eastman (or that nice Mr. Fuji) can
stuff into the gelatin. You cannot stuff more crystals into 16mm film than
35mm - if you could, they would make super-high resolution 35mm with the
same technology. So, if 4k scanning is 80 cycles per millimetre multiplied
by 24mm for a 35mm frame, for Super 35mm it is 80 multiplied by 11.76mm or
940 cycles, suggesting 2k scanning is plenty. Do not let anyone try to sell
you a 4k 16mm scanner!

Sorry. I know this is long and boring. And I bet by the time I post it
someone from Kodak will have said the same thing with far more authority.