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I see memory cards and they claim 60x or 100x or 300x, where I assume 'X' means times. Times what?

I'm assuming it has something to do with transfer speeds. Is it only on 'reading', ie, going from the card to the computer. Or is also on 'writing', ie, going from the click of the shutter to the card?

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up vote 12 down vote accepted

It's transfer speed. 100X means 100 times faster than a 1x CD-ROM (whose data rate is 150KB/s).

So a 100x card is 100 * 150 KB/s or 15,000 KB/s or 15MB/s (note that this is data rates, not storage).

These are, of course, theoretical max writes, and real world, average case performance is not that high. You just need to get a CF card that can keep up with your camera's buffer. Once that is achieved, then your burst bottle-neck will be the camera's internal shot buffer.

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Follow-up: be aware that reads are faster than writes, and unscrupulous manufacturers sometimes quote the read speed instead of the write speed when giving this rating. –  Reid Jul 19 '10 at 2:10
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Also, for a useful chart of common "X" ratings to actual speeds, see Wikipedia: en.wikipedia.org/wiki/Secure_digital#.C3.97_rating –  esm Jul 19 '10 at 2:35
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