[filmscanners] RE: 35mm filmscanners

["Austin Franklin" <austin@darkroom.com>]

> The canon also supports the use of a scsi interface (very fast!!).

Just as a note, typically, the interface between the scanner is far faster
than the scanner actually requires.  It is the actual data acquisition time
(time the scanner is scanning the film) that limits the scanners speed, not
the interface speed.

It's pretty easy to figure out...  If the scanner has, say, 4000SPI, and it
scans the long side of the 35mm film, that's 6000 lines.  Each line is
exposed for what ever the exposure time is set for, let's say 16ms.  There
is some small overhead required to move to the next line, say, 2ms...so 18ms
per line.  That is 6000 x 18ms or 108 seconds, or just under two minutes.

The scan, if it's, say, a raw color scan at 16 bits per color, would be 4000
x 6000 x 6 bytes per pixel, or 144,000,000 bytes.  144M divided by 108
seconds is 1,333,333 M bytes/second.  So, as long as the transfer speed of
the scanner interface can keep up with that number, it isn't the interface
speed that is the limiting factor, simply the exposure time.

Obviously, change the resolution, exposure time, stepper motor overhead, and
number of bytes to suit your particular scanner.

Austin

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