What kind of performance improvement should I expect on a Canon 500D with a class 6? Will a class 4 slow the writes/reads noticeably? I mostly take (low shutter speed) photos and occasional short videos.


See http://en.wikipedia.org/wiki/SDHC#.C3.97_rating

Class 4 is 32MBit/s (4MByte/s) and Class 6 is 48MBit/s (6MByte/s). You probably shouldn't worry about this if you're just shooting JPEGs, but if you're going to take RAW shots in quick succession, or, as you said, take video, it may make a difference. I can't speak for all camera manufacturers, but if the write speed isn't high enough on the card, you may not be able to take the highest quality video your camera can offer.

So I would personally recommend you get the Class 6 SDHC card. You'll be able to shoot pictures faster, and you won't limit yourself in terms of video. In short, "better safe than sorry".

  • When lookign at the speed of the card make sure to note the write speed because the limiting factor will be your camera's ability to write the image to the card.
    – kacalapy
    Feb 1 '11 at 21:05
  • 3
    -1 You're confusing MB /s with Mbit /s. Class 4 is 4MB/s and class 6 is 6MB/s. 4MB is 32 Mbit. Feb 4 '11 at 4:24
  • 2
    Technically the answer wasn't really wrong, just missleading, as Mb/s is often used to refer to mbit/s while MB/s is used to refer to mbyte/s...easy to mix up. I've edited the answer so that it shows both rates, and the reader can pick the one that is most meaningful to them.
    – jrista
    Feb 10 '11 at 1:32
  • As @kacalapy stated, class usually refers to the write speed. It is entirely possible for a card to have a label that states 15MByte/s speed...however that speed usually refers to the READ speed, not the write speed.
    – jrista
    Feb 10 '11 at 1:39
  • I appreciate the corrections though one more slight nitpick I'm afraid - usually by convention Mb or Mbit as in megabits has a lowercase "b" whereas MB as in megabytes has an uppercase "B". Quote wikipedia page on megabytes: "It is commonly abbreviated as Mbyte or MB (compare Mb, for the megabit)." Feb 10 '11 at 4:13

Cards of a particular class are not all made the same. Hence the preferred cards are the more expensive because they produce the best results in terms of speed and reliability.

Look at the sandisk models. They are very popular with professionals for the reasons mentioned above. Personally I took a chance on the Transcend model to get more bang for my bucks. It provides 16 Gigs on a class 10 card and have no problems to report with it after almost a year of usage.


I have a Canon 500D and own a few Sandisk Extreme III's which are extremely fast and I'd generally recommend them... but essentially they outperform the camera.

Most of the time I use a couple of PNY 16gb Class 4's simply because of the convenient size when taken RAW's. The only slowdown I ever notice is that which is from the camera's buffer size and write speeds itself that is documented in the many reviews/specs rather than from the card itself. I've never had any slowdown issues from pictures or video on the camera from the card, but there may well be not so good quality Class 4's out there that will effect the use of the camera


The Sports Photo Guy regularly reviews different SD (and CF) cards and posts the real world results. I'd highly recommend reading his reviews since you'll find that there are some class 4 cards that are faster than supposed class 6 cards.


I am unable to shoot HD video on my T2i with class 4 and even class 6 SDHC cards. Class 10 seems to work fine.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.