High ISO = Less Card Space?

I noticed this the other day on my 20D.


Shooting RAW at ISO 100 on a blank 4GB Sandisk cf card, I have 447 images left.


Up the ISO to say, 800, and it drops to 416.


That's a big jump. The number of images the card can hold decreases steadily as the ISO is hiked up. Is that normal? Does increasing ISO increase file size drastically enough that it would go from 447 to 416?


I'm more curious than worried. I figure I don't shoot much if at all, over ISO 800, and 416 images is still pretty many.


Just curious.


Thanks,


Rodger

That's normal. Higher iso means more noise. Nose doesn't compress, so higher iso pictures tend to take more space.

Ahh I see. Thanks!

yes, this is true. i notice a larger file size using higher ISO

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Originally Posted by Jon Ruyle

That's normal. Higher iso means more noise. Nose doesn't compress, so higher iso pictures tend to take more space.

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Yes, but my XTi also increases the RAW size as I crank up the ISO while RAWs are uncompressed images AFAIK...

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Originally Posted by STL

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Originally Posted by Jon Ruyle

That's normal. Higher iso means more noise. Nose doesn't compress, so higher iso pictures tend to take more space.

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Yes, but my XTi also increases the RAW size as I crank up the ISO while RAWs are uncompressed images AFAIK...
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Canon's CR2 file format uses a lossless compression algorithm. All of the 1s and 0s are retrievable from the .CR2 files, but areas with consistent patterns are compressed. (JPEG is a lossy compression algorithm, so if you have big patches of blue sky for example, pixels that are ALMOST the same value may get compressed to have EXACTLY the same value. That's why overly compressed images have bands of different blues through the sky.) CR2 also has a JPEG thumbnail embedded inside the file, which is therefore going to vary in size.


I think .NEF and I know TIFF are uncompressed (though NEF might have other embedded elements causing a change in size).


The camera also uses an expected/anticipated file size when calculating the number of pictures left. As you fill the card, you may see a picture be taken but not decrement the card space counter. I was playing with my Pocket Wizards the other night, with the 1D3 banging away at 10fps and watching the flashes trigger. With a 1GB card and L JPEG, I managed to drain the card space counter, causing the camera to pause, but as the files were created and written, the camera realized it had more space, so it took some more pictures on two separate bursts (and the images were moved into different folders). And yes, the PW Flex units were keeping up at 10fps...

Seems to be normal for all the Canon bodies. It's quite noticeable on the 5D MKII using 2G cards. When formatted, the camera will show the card with about 75 available images. If you make a big change to ISO settings, the number of available shots begins to drop away.

What's really funny is that it's unnecessary. For example:

• ISO 1600 - f/4 - 1/500 - 25 MB
  
• ISO 25,600 - f/4 - 1/500 - 45 MB

Both shots have the exact same exposure. The noise is the exact same too (after you correct the brightness in both images to be the same), which is no surprise given that ISO 25,600 is just a digital push of ISO 3200. It has 4 stops of clipped highlights, so it actually has less image data, yet it's almost twice as large. If Canon offered Metadata ISO, the filesize could remain the same, and the highlights would not be deleted needlessly.

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Originally Posted by Daniel Browning

What's really funny is that it's unnecessary. For example:

• ISO 1600 - f/4 - 1/500 - 25 MB
  
• ISO 25,600 - f/4 - 1/500 - 45 MB

Both shots have the exact same exposure. The noise is the exact same too (after you correct the brightness in both images to be the same), which is no surprise given that ISO 25,600 is just a digital push of ISO 3200. It has 4 stops of clipped highlights, so it actually has less image data, yet it's almost twice as large. If Canon offered Metadata ISO, the filesize could remain the same, and the highlights would not be deleted needlessly.
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They have the same exposure data, which suggests to me that the second shot had less light (maybe 16x less light). With less light, the sensor would be closer to its noise floor, as would the electronics further down the chain. More noise would then lead to a larger file.

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Originally Posted by peety3

They have the same exposure data, which suggests to me that the second shot had less light (maybe 16x less light).

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They have the exact same "exposure", which means the exact same amount of light. (The "data" value is not part of exposure; it is part of "brightness"). The ISO 25,600 shot has a four-stop (16X) increase in gain, so the data values are 16X higher than the ISO 1600 shot, even though the amount of light is the same. The SNR is the same too, but one must equalize the brightness for a visual comparison.