20D why do ISO speed changes alter the number of exposures?

I have noticed that when I put a 1G/B card in my 20D and set the ISO speed to 100, the remaining exposures show as 113, as I increase the ISO speed the number of remaining exposures drops down to 100 when the ISO is set to 1600. Now I know the number of exposures is only a rough calculaion, but why does it change with ISO settings and without taking any pictures?

Alan

I think it has to do with the increased sensitivity of the sensor at higher iso's. Increased sensitivity would create increased information (some of it noise?) and therefore larger file sizes. (And the camera is programmed to know that.)

It has to do with compression. A high ISO image has more noise, noise is random, and random data is more difficult to compress than non-random data so it takes more space.


A simplified explanation...

So if you have a pic with a pure blue sky a row of pixels could get compressed as "3072 pixels of color 12,12,252 in a row". In computerese this might stored "12,12,252 x 3072"

But now add noise, so that every n'th pixel is some whacky color, and "n" is random. So now a line of pixels gets compressed as

"12,12,252 x 130", "19,211,43 x 1", "12,12,252 x 642", "231,47,188 x 1", "12,12,252 x 87", "22,119,55 x 1" and so on.

You can see how the noisier, more random line of pixels takes much more room to store the same line of pixels.

So higher ISO images don't compress as well, so they take more room on the card, so the card holds less images.

A data compression algorithm is applied to the image file to reduce the flash space it consumes. The more "detail" an image has, the less savings are possible with compression, and so image files with more detail are larger.

While we wouldn't consider noise to be "detail" from an aesthetic perspective, the camera's software doesn't know any better, and tries to preserve the detail produced by the noise, thus negatively impacting the value of the data compression. The net effect is that for two shots of the same scene, one taken at higher ISO will be noisier and thus less well compressed and thus reqiure a larger file size.

The camera accounts for this when making its "estimates", i.e. converting the available flash space into a guess of how many exposures will fit there.

At least, that's the "conventional wisdom" on this issue.
-harry