ISO & file size

[chphoto]

If my 400D is anything to go by, the ISO has an effect on the image's file size. I put in an empty 2GB card and had space for 200 10-megapixel RAWs when set to ISO 100. However, when changed to 1600, it changes to 179. ISO 200 gives space for 197, ISO 400 gives space for 193, ISO 800 gives space for 188.

Can anyone explain the science behind this? :bonk:

 

[petersmart]

I have the same with JPEGs on my 450D.

I have a 16Gb card and there is a difference of 1000 pics between ISO 100 and ISO 1600.

Perhaps the extra noise is the cause since the files get bigger as the ISO (and hence the noise) increases.

.

 

[Thmaga]

My guess is because higher ISO's are more grainy, the grain takes up more disk space, as opposed to a flat area of colour.
Just like a photo with more detail is a larger size compared to one of say a blue sky, the grain is like lots of extra detail to be stored.

 

[Musicman]

Yes, exactly. Canon RAW files are compressed using a lossless algorithm.

The more randomness there is in any kind of data, the harder it is to compress. Photographic picture noise is random data.

 

[loplyg]

Noise is the arch enemy of compression.

On high ISO even a plain, overblown white background might actually be a zillion slightly different dots, which have to be stored and kept, rather than rounded up to a 'big white area'.