Archived posting to the Leica Users Group, 2000/10/30

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Subject: [Leica] Film megabytes and some curious contradictions
From: "Erwin Puts" <imxputs@knoware.nl>
Date: Mon, 30 Oct 2000 21:57:36 +0100

I a am surprised. A few years ago I gave some figures on this list and noted
that Kodachrome 25 had a byte count of 135 MB which is an amount digital
capure would not reach in eons. At that time I got ridiculed and some even
would me have ostriced from this list as being The Idiot of Tradition. Even
recently my ideas and facts about the level of image quality that can
produced by film and Leica lenses have been questioned to the extreme. Now I
present details from "conventional wisdom" (not my ideas, but industry
figures) and again I am approached as the person who should do his numbers
again.

What is the amount of bytes for film. Some say you should count the grains,
but that is the wrong apporoach, as the individual grains are very small,
often in the region of less than one micrometer in diameter. In practice,
the smallest image point needs a number of grains to become visible, so this
does not count. The best equivalent is the resolution, that is a square grid
superimposed on the film area, as this is the exact replica of a sensor
grid. Now current film (BW or slide or color neg) has resolution figures
around 100 lp/mm and Techpan has 200 to 300  lp/mm. Follow this: the
negative area has 24 x 36mm. Every mm holds 200 different lines. that is 24
x 200 x 36 x 200 bytes = 4800 x 7200 = 34.560.000 bytes. We need three
colours, that is 34.560.000 x 3 = 103.680.000 thus more than 100 MB for a
colournegative or slide.
This figure is close to my original and ridiculed calculation.
Now for some  steps. It is very difficult and often considered unnecessary
(pace Mike Johnston) to get on film more than 40 line pairs or 80 lines. And
if we be even more general, 20 lp/mm are the best. most people would dream
about. Assume now 20 lp/mm or 40 lines per mm (the best you can get in hand
held picture taking, generally speaking).
Again: 24 x 40 x 36 x 40 = 960 x 1440 = 1.382.400. Three colours would be 3
times this number, which is 4Mb. And with 40 lp/mm we et 5.529.600 bytes
times 3 = 16.588.800 bytes or 17 Mb, quite close to the 20 MB I quoted as
the conventional wisdom.
So any digital capture of 20Mb would be close to the resoluton of 40 lp/mm
that some on this list would consider as the most one would want in 35mm
photography and to go beyond this number would qualify as being a freak
obsessed with lines and test targets and all that. Reread  the mails by Mr
Johnston and Mr. Grant and Mr Goodman at all.
So I accept this proposition(as does the industry as a whole) and I start
from these figures (20Mb) and now I am questioned by some others, who say
that my figures are wrong and that I should start with a much higher number
of Megabytes to do justice to current thinking.
Now I am totally at a loss.
Some would hold that 40 lines is the best you need in typical Leica
photography, which amounts to a 4MB picture, easily reached by todays
digital cameras of 3.3 to 4 Mb.
Look at the facts: I noted earlier  that you should aim for the 135MB
possible with Kodachrome or even better hi-res BW film. I am shot dead for
this proposal, as it exemplifies The Idiots Approach To Ridiculous HiRes
Photography.
Then I note that maybe 20Mb would be a sensible compromise and I am again
shot dead because film can handle 100Mb or more and now it exemplifies The
Idiots Approach to Ridiculous Claims by Digital Photography.
I am confused!
Can anyone explain?

Erwin

Replies: Reply from Mark Rabiner <mark@rabiner.cncoffice.com> (Re: [Leica] Film megabytes and some curious contradictions)