Any reason to shoot film nowadays?

Started Apr 12, 2013 | Discussions thread
Iain McClatchie
New MemberPosts: 3
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Re: Only large format...
In reply to Joseph S Wisniewski, Apr 15, 2013

Suppose you wanted to build another KH-9 Hexagon.  That's right, the spy satellite.  From 1972 to 1986 the U.S. used 19 of these things to shoot the entire U.S.S.R. at 2 foot resolution, in stereo, every few months.

Film is a very good fit for this admittedly extreme problem.  The worst issue is getting all that imagery down.  A set of 7000 negatives, each 155 x 3190 mm (how's THAT for large format?), together covering 20 million km^2 is the equivalent of at least 30 terabytes.  It's not really practical to send that much data over a radio link in a couple of months.  So they wound 80,000 feet of 6.6 inch film onto a reel and physically dropped it to Earth in a re-entry vehicle.  They would then snag the capsule in midair with the occasional Russian submarine directly below hoping for a miss.  This worked.

Now consider trying to do it with digital technology.  Only recently it's become possible to write the data to disk, but those mongo frames (36 gigapixel equivalent) were shot every 1.7 seconds, so the write bandwidth for a digital system would have to be at least 2700 MB/s.  That will require at least 30 hard drives.  Or you could do it with about $50k worth of SSDs, which might be more compact and wouldn't require air pressure.

If you built it with a digital sensor, maybe a TDI CCD chip, the pixel rate is an incredible 40 gigapixels/second.  The fastest TDI sold today reads 12 kilopixel lines out at 90 KHz.  A TDI sensor for a KH-9 replacement would have a line rate of around 1 MHz.  Each line would be 42 kilopixels.  You'd probably need something like 1000 A/D converters.  A JPEG compressor for that stream is buildable but would suck a kilowatt.

So it can be done with digital today, but it's only in the last few years that a digital solution has been practical.  The film advantage for large format has endured for a really long time.

-Iain

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