SD card capacity

[StuartUK]

I have a Canon 60D and a couple of Sandisk Extreme 16Gb cards. I normally shoot in RAW mode. With an empty, formatted card, the camera shows 539 shots remaining capacity.

I am well aware of the usual storage seller's rip-off. Whilst, in computing terms, a Gb is 1024 * 1024 * 1024 bytes, when we buy a storage device, we only get 16,000,000,000 bytes.

So, if I check the available space on an empty card as formatted by the camera, I see 15,911,354,386 bytes free. That is already 84Mb less than the 16 * 10^9 capacity. I am not too surprised by this. It presumably represents the space occupied by the directory and file allocation table structures created when the card is formatted.

But, if I check my main photo archive and work out the average size of a .CR2 file (<code>dir *.cr2 /s</code> - will show me the number of RAW files and the total space they occupy), their average size is 24,209,182 bytes (c. 23Mb). Divide this into the free space reported above and I get an expected capacity of 657 files - which is an awful lot more than 539 that the camera reports. So I am only get about 82% of the capacity I would expect from one of these cards.

So what is going on here? I would be interested if users of other cameras and cards see the same sort of differences.

 

[acetone]

The camera give you a guess of how many images the card will store, just go out shooting until the card is full, it'll be a lot more than that first estimate .

Files sizes vary all the time, but I've had more than 700 raw files on a 16gb card even after the camera told be it would only store 500.

Nothing to worry about.

Also you say your using your average file size to work out how many shots, canon will be using the maximum file size to give the number of shots a card can hold.

 

[StuartUK]

I am not worried, just interested!

Yes, I came to the conclusion that it is making its initial estimate using something approaching max file size. Based on my collection of photos, the value it is using to get a starting value of 539 is approx. 2 standard deviations above the mean. That would give a size which would expect to cover around 99% of files.

But where my experience differs from yours is that I don't think my camera recalculates the remaining capacity depending on the free space actually left. It simply decrements the number of remaining shots every time you take one until it hits zero and then displays "Card full".

I must remember to note if this is actually what happens. If it does recalculate based on the remaining space then, sometimes when you take a shot, the number remaining should fail to go down, or even go up!

 

[modchild]

I think the camera tries to work out the minimum amount to fit the card and not the maximum. Thats why some times you can take a shot and the count wont go down. I tried this with a 4Gb SDHC card in my 5D3 set for Jpegs and ended up with over 450 more shots than the card first said.