The problem is that, as Secure Digital (SD) cards have evolved, they have picked up:
- a series of different capacity ratings - the original SD (max 1Gb), followed by SDHC (High Capacity - max 32Gb) and now SDXC (eXtended Capacity - max 1Tb),
- three different physical sizes - the original "standard" size (32x24mm), mini- (21.5x20mm) and micro- (15x11mm),
- and a series of different write speeds. They were originally described as "xn" indicating how much faster they were than a CD drive e.g. "x24". This was replaced by "class" ratings (class 2, class 4, class 6 and class 10) which indicate their write speeds in Mega-bytes/second (so a "class 10" card can writes 10Mb/s) and now we have a new UHS (Ultra High Speed) rating system. UHS-1 writes up to 50Mb/s.
For my Canon EOS 60D, the specification on Canon's web site states (under STORAGE):
- Type: SD card, SDHC card or SDXC card
which is not tremendously helpful since it does not even state which physical size card is needed, let alone the speed to go for! I suppose we can assume (rightly as it turns out) that a standard sized card (32x24mm) is expected unless it says otherwise. If you search the support site, you can find "FAQ: What are the Compatible Memory Cards? (EOS 60D" (dated 4 Oct 2010), which says the following:
- The camera does not come with a memory card for recording images. Please purchase it separately.
- When shooting movies, use a large-capacity SD card rated SD Speed Class 6 "" or higher.
The capacity you need comes down to the image size and quality you select (RAW and/or JPEG) and how many shots you think you may need to store before you get back to base and can download the images. I bought Sandisk extreme 16Gb cards which are rated class 10. I normally shoot RAW files and get around 540 photos on one of these card. I have very rarely filled up a card in a single day! These cards have met my needs admirably so far.
PostscriptNow I come to write about it, these numbers don't really add up! With an empty, formatted card, the camera shows a capacity of 539 shots. If I put the card in the card reader in my computer, go to Explorer, right-click on the card's drive letter and check its properties, it shows 15,911,354,386 bytes of frees space.
Here is the first, and well known rip off! In a computer's memory and filing system, a Kb is 1024 bytes. A Mb is 1024 Kb (1,048,576 bytes) and a Gb is 1024 Mb (1,073,741,824 bytes). But people who sell us storage, hard disks, storage cards, etc., use 1Mb = 1,000,000 bytes and 1Gb = 1,000,000,000 bytes. So a 16Gb card is offers 16,000,000,000 bytes of storage (not 16 x 1024 x 1024 x 1024 as you might hope!).
But I seem to be missing 16,000,000,000 - 15,911,354,386 bytes on the empty card - very close to 84Mb. This is presumably taken up the directory structures that formatting the card created. I am not too surprised by this, though it is perhaps a bit more than I would have anticipated.
I can easily see how big I expect a RAW file to be. I have a photo archive with several thousand of these stashed away in it (files with a .CR2 extension). If I shift-right click on my PhotoArchive directory and choose "Open command window here" (I am using Windows 8), I get a command prompt where I can enter the command:
dir *.cr2 /s
This will show me the RAW files it finds in that directory and all its subdirectories (the /S switch) and, at the end, a summary including the number of files and their total size in bytes. Dividing one by the other, I get the average size of a .CR2 file across this very large sample - which comes out at 24,209,182 bytes = 23Mb.
If I divide the free space on the SD card by the average size of a .CR2 file, I get an answer of 657. This is the approx number of files I would expect to be able to store. So why does the camera report only 539? That is rather a big difference. And I know from experience, when that displayed total decrements to zero it will say "Card full". Next time that happens, I must remember to check whether the computer still sees lots of free space.