ISO versus card capacity

[John Liddle]

I have not noticed this before, but changing the ISO setting on my 5D Mk3 changes the number of shots available on the card (as displayed on the LCD panel). Auto ISO gives a significantly smaller number than say ISO 100 or 200. The number decreases with increasing ISO and at 12800 ISO the capacity displayed is the same as for Auto.

Why should this be?

[paul linton]

Re: ISO versus card capacity

Well blow me down you're right John.
I've just tried some setting.ISO 3200 i have 419 left/ change to ISO 800 i have 453 shots left I never knew that how very strange.

Paul

[jimsphotography]

Re: ISO versus card capacity

I think ( meaning I'm not 100% sure) it has to do with noise and artefacts. Low ISO has less noise etc and therefore more areas that are uniform and can have the data compressed. If the uniform areas are peppered with noise then it's harder to compress the data from the uniform areas of the image.
In much the same way that to images at the same ISO - one containing lots of blank uniform blue sky and one with no sky buts lots of complex detail ends up as a slightly bigger JPEG file.
James

[nickorando]

Re: ISO versus card capacity

Yup. There's just more data in a high ISO file. Same in RAW. I first noticed this with the 10D - notice it less now, as my cards are all a lot bigger.

[John Liddle]

Re: ISO versus card capacity

I should have made it clear that the effect I describe is in relation to the camera being set for RAW only. I can accept the arguments relating to jpg files but I cannot see how these can aplpy to RAW files, which as far as I am aware are not compressed.

[nickorando]

Re: ISO versus card capacity

I should have made it clear that the effect I describe is in relation to the camera being set for RAW only. I can accept the arguments relating to jpg files but I cannot see how these can aplpy to RAW files, which as far as I am aware are not compressed.

Why not? The amount of data in any file has an impact on the size of that file, compressed or not.

[John Liddle]

Re: ISO versus card capacity

Why not? The amount of data in any file has an impact on the size of that file, compressed or not.

Indeed, but as I understand it, a RAW file contains the radiometric output from each pixel i.e. information defining the colour and intensity of light detected by each pixel. This (small) amount of data for each pixel will not change in size irrespective of the nature of the light detected.

Taken from a Canon tutorial:-

"The raw data comes from the millions of pixels that make up the camera sensor. Each pixel is photosensitive and responds to light by generating a small electric current. The value of each current is converted to a digital format. This mass of data forms the bulk of the raw image file. In addition to all the captured data, the raw file also includes information about the camera settings at the time of the exposure."

There is some incredibly detailed information (which I do not pretend to understand) on the Canon CR2 RAW format here:

http://lclevy.free.fr/cr2/

So I still remain puzzled as to why ISO setting should affect file size.

[Woolley]

Re: ISO versus card capacity

But it also creates a jpg for viewing on the screen, might that have an effect?

I think also not all data is the same value, so different colours take up a greater amount of data. Black, as in the absence of colour is less data, and high ISO is less black.

Or I might have made that up! Just have this vague notion I read something to that effect once.

[nickorando]

Re: ISO versus card capacity

Yeah, there's an embedded JPEG for viewing.

[collywobbles]

Re: ISO versus card capacity

I assumed years ago that the higher ISO setting produces a larger file. Just MHO

[John Liddle]

Re: ISO versus card capacity

An update - I have put the question to the Oracle (Canon Customer Support) and after an additional circuit of the mulberry bush because they didn't read the question properly, my query has now been passed on to the "product specialist team" who will, apparently, investigate.

I will update you with the answer when I get it.

[jimsphotography]

Re: ISO versus card capacity

An update - I have put the question to the Oracle (Canon Customer Support) and after an additional circuit of the mulberry bush because they didn't read the question properly, my query has now been passed on to the "product specialist team" who will, apparently, investigate.

I will update you with the answer when I get it.

Excellent - I look forward to that.
Thanks
James

[John Liddle]

Re: ISO versus card capacity

Update No. 2 - I have received a holding response from someone who only works Mondays, Thursdays & Fridays - Roll on Monday!

[El Sid]

Re: ISO versus card capacity

As far as I am aware it's down to the potential noise encountered at higher ISO and is similar to the difference in file size between pictures of subjects with large amounts of plain sky and those with lot's of detail. While RAW files are not compressed in the same sense as JPEG I understand that the data is nonetheless sorted and stored in a manner that minimises the file size. Pictures with low levels of detail such as plain sky can be sorted and arranged in a far more compact manner than those with lots of detail. I first encountered this years ago after taking a lot of close ups on a local beach - the close ups of the sand and all it's grains were considerably larger than those that contained mostly sky.

Noise effectively equates to fine detail and is therefore more difficult to store compactly - hence larger files. Canon presumably have calculated an average approximate file size for the various different ISO and included this in the algorithm that estimates the potential capacity.

TBH I've never really noticed that it varies with ISO but since the potential picture count is very much an approximation regardless of ISO setting I've never really thought to worry about it.

[John Liddle]

Re: ISO versus card capacity

At last - the Oracle has spoken - see below (the email of the 16th was a hastener)

"Dear Mr. Liddle,
Thank you for your email of 16/12/2015.

Please accept my apologies for the delay in emailing you due to our helpdesk Canon EOS 5D III being with the technicians for checking, which is carried out periodically with our helpdesk testing models.

I have given below a summary of the results I obtained when taking RAW images with our helpdesk Canon EOS 5D III. The image number is followed by the ISO setting then by the file size (MB). In each pair or group of three images, the composition was the same while the ISO was changed. All the images were taken in AV mode, and the High ISO Speed Noise Reduction was Standard.

Image #1: 400, 29.8
Image #2: 12800,37.7
Image #3: 800,, 32.5

Image #4: 800,25.9
Image #5: 12800,34

I believe that the reason for the file size difference has to do with the processing required for noise reduction. For more information on Noise Reduction Settings, please see the instruction Manual, p.145-147.
I hope the above information will be helpful."

So there you have it. I would have liked an explanation of the RAW file structure (and confirmation that it houses an embedded jpg version of the image) since that would have reinforced the view that noisy jpg images take up more space, but I don't intend to pursue the matter any further with Canon.