Identifying the speeds of SD cards is confusing. The card to the left is marked Class 4, as indicated by the number 4 with a big ‘C’ around it. Until today, ‘Class 4′ meant nothing. It was one more element to confuse with all the other statements of speed, like Mbit/s and MBytes/s and 133x.
The table below shows the actual speed of different Classes of cards.
SDHC Speed Class Rating
Class 2: 16 Mbit/s (2 MByte/s)
Class 4: 32 Mbit/s (4 MByte/s)
Class 6: 48 Mbit/s (6 MByte/s)
Class 10: 80 Mbit/s (10 MByte/s)
Note these are straight conversions: 32 Mbit/s = 4 MByte/s
(much like measuring temperature: 32F = 0C)
Since the Canon T2i/550D records 5.5MBytes/s, a Class 6MB/s card is required, as recommended by Canon in the manual.
However, card speeds are confused by all kinds of crazy nonsense. I accidentally bought a Class 4 card for the Canon T2i/550D which incidentally, works perfectly well after a low level format by the camera. (It didn’t work at all before a low level format.) It’s a SanDisk Class 4. It says 15MB/s on it.
WTF? If it were 15MB – according to the table above – it would be Class 15. If there was a typo, ie. 15Mb/s was meant instead of 15MB/s, then it would be Class 2. No matter how you look at it, the card has 2 wildly contradictory speeds written on it.
Extensive Googling eventually spits up a plausible explanation: when it says Class 6, it means the CERTIFIED MINIMUM speed is 6MB per second. It is certified by the SD Association. Any other mention of Mb/s or MB/s or 133x on cards are manufacturer’s claims. We can practically ignore them.
So my SanDisk card pictured above is certified Class 4 – it has a certified minimum speed of 4MB/s, and an uncertified speed of 15MB/s.
Sensible course of action:
Buy based on the Class number. Treat any other speed related information as part of the brand. Refer to speed by class number.
