The real capacity of storage cards

Canon 5D

A welcome ‘feature’ of the 5D.

I mentioned a while back that I was moving to 2gB cards in the 5D, from the 1gB I started with.

The welcome capacity increase and falling price made sense.


With an empty 2 gB card

The 5D continues to dutifully reports room for 120 RAW pictures on a blank card, yet the other day I again noticed how low that estimate is. Obviously, the size of RAW files will vary depending on the scene, but the difference I noted is anything but insignificant.

Apple’s Disc Utility reports available space of 2.039gB on a formatted 2gB Extreme IV card, so that means Canon is assuming an average file size of almost 17mB per image in computing the above count. Nice that they use so large a size as it means they are erring on the side of safety.

Here’s the scoop:

That’s actually 140 images plus a count of two for the containing folders. And there’s still 350mB of free space left! Do the numbers and you come up with a capacity of 171 pictures, or 42% more than that original estimate. Of course, this will vary with the scene photographed, as more detail and color translates to a larger picture file size.

Now that’s a ‘feature’ I can handle any day.

Follow-up:

Check Comment #1 for a very interesting discussion of some of the underlying reasons for varying file sizes.

The pictures I was writing about in this case were studio photos taken with studio flash with just the subject’s eyes critically sharp (the 85mm Canon f/1.8 lens was used and 18″ x 24″ prints were a breeze to make and superb in every way) and everything else pretty much out of focus, so it seems the low ‘sharp content’ correlates with the small file sizes. The 10mB file above was a blank where I pressed the button before the flash was ready to fire, so that would appear to be the base file size of a 5D image. The Comment suggests that, for identical subjects, two lenses of like focal length will result in the one with higher micro-contrast generating a larger file size – an interesting ‘test’ method for this variable.

One thought on “The real capacity of storage cards”

  1. Thomas, the RAW files are packed (lossless packing like zip) and the file size is determined by the content of the file. File size is smaller if any of the condition(s) below is/are true:

    • picture has a lot of smooth out of focus areas
    • lens is not very sharp
    • lens has poor micro contrast (can still be “sharp” – applies to most of Canon lenses)
    • there is not much noise (bigger ISO > bigger RAW file size)
    • too small aperture (f/11 or smaller) is used and diffraction eats all the small details in photo
    • tripod (and flash) is not used (traditional rule 1/focal length does not work if you want every pixel to be sharp, this is based on ancient sharpness requirements with film and small print size. The circle of confusion is about 0.3 etc. which does not apply if you print larger than A4/Letter size)
    • even very slight focus errors will cause softness causing smaller file size (micro focus adjustment on 1D Mark III is really good to have knowing quality control of Canon lenses).

    I have never got more than 16MB files from Canon lenses when using ISO 100 with 5D but for Example Carl Zeiss Distagon T* 2.8/28 constantly produces files which size is over 19MB, and sometimes file size gets to 22MB. Also difference in large prints (e.g. 75cm x 50cm / 30″ x 20″) you really can see the difference between Carl Zeiss and Leica lenses compared to Canon lenses. Canon lenses are optimized to produce big contrast but they are really poor in micro contrast and small details. Of the lenses that you have only the Canon EF 200mm f/2.8 L has decent micro contrast; you may end up with a file size close to 16MB if you take a photo in which everything is inside DOF, there is a lot of detail and a tripod is used.

    Also it seems that the anti-alias filter has something to do with this: I switched to the Canon 1D Mark III and even though it has 2.5 fewer megapixels yet the file size is quite close to the 5D’s file size. Also due to live view and exact focusing I get more constant results with manual focusing and I no longer need to focus bracket to get excellent results.

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