SD cards come in a large variety of capacities, in general the bigger the capacity the more expensive, but there is a second consideration speed. The SD Association have a speed class system that can help you understand the performance. the symbol is a number (the number of Mega Bytes per sec) inside a C, a bit like the @ symbol (see example on right hand side). Whilst most cameras will work the original the original standard SD cards (mainly still available in 2Giga Bytes) many older cameras cannot use the newer SD HC (High capacity) which general come in between 2 GB and 32 GB capacities, These newer card can have speed from 2 MB/s up to 10 MB/s. Understanding the performance is pretty simple if your DSLR camera writes a jpeg or RAW file that is 10MB in size it will take 1 second to write (with a tiny overhead for the cameras software). If you are still using a speed class 2 card that same file will take 5 seconds. A little more in cost of the card may make a big difference, particularly in bust mode.
This was all fine for still cameras but video is even more demanding, so there is now another class the SD XC or UHS (Ultra High Speed), which an come in capacities up to 1 TB (terra byte(more commonly 32GB). Their speed class is shown in a U, The class U 3 is equivalent to 30MB/s (as shown on the left). Some newer cameras and particularly video cameras like the GO Pro actually require these higher speed cards.
So here is a guide of what size/speed card might suit you best
| Type of Photography | Capacity | Speed Class |
| If you just shoot jpeg (or old camera) | 2-4GB | C2-4 |
| If you mainly shoot RAW | 4-8GB | C6 |
| If you shoot jpeg + RAW | 4-16GB | C10 |
| If you shoot video & RAW | 8-32GB | U1-U3 |
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