ISO and file size

[hikari]

I am just wondering about the relation between ISO and file size.....

I have a 350D with a 256MB (blank, formatted) CF card.

Setting the exposure control to 'P', quality JPG fine and ISO 100 the memory card can take 72 images (information on the status display ;))
ISO 200 gives 68 images
ISO 400 gives 64 images
ISO 800 gives 61 images
Finally ISO 1600 gives 58 images

It seems like higher ISO results in less compression of the image.

There must be a good reason for this, but I don't see it yet.

Can someone please fill me in?

[nitsch]

I could be wrong, but I've always thought that this is because higher ISO images are noisier. Noise doesn't compress as well as a smooth area of colour, hence the larger files.

[Incomplete Pete]

As you bump up the ISO the noise levels increase, the more noise in the picture the greater the file size.

[hikari]

Thank you both for responding.

I guess both replies are correct.

Noise is way too random to allow for decent compression, so that must be one factor. I should have known that but I overlooked it. Thanks Nitsch for refreshing my memory!

If the amount of noise alone would increase the file size, regardless of compression, the maximum number of images on the card should vary when the quality is set to RAW and ISO is varied. It is in fact, so the image size is a function of ISO. Thanks Pete for pointing this out.

[robertwgross]

Yes, the size of a RAW file will vary a little with ISO as a result of noise.

The size of the RAW file will vary somewhat based on the content of the image. For example, if you shoot a flash photo of a plain white wall, evenly illuminated, check the file size. Then go out in your backyard and shoot a photo of trees or flowers or grass. Now check the file size if it.

In general, the RAW file of something plain will be slightly smaller, and the RAW file of the complex scene will be slightly larger. Fine detail does not compress so well.

---Bob Gross---