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Thread: SD card - rated class and actual speed?

  1. #1

    Default SD card - rated class and actual speed?

    Having picked up a 450D after my beloved 5D was stolen, I am kinda new to SD cards.

    Apparently, a card's minimum speed is indicated by its "class" - Class 4 means 4MB/s and such. However, the two pieces of Class 4 SD cards I have, one Toshiba, the other Sandisk Ultra, have much higher reading and writing speed than 4MB/s, much like 15-20MB/s. Now I wonder, under what circumstances are the minimum speeds derived, and how does one know a card's speed when used normally with cameras/card-readers?

  2. #2

    Default Re: SD card - rated class and actual speed?

    Marketing will promote the peak speed (15MB/s, 30MB/s, etc). Guaranteed minimum speed is the critical part. Real world performance will exist somewhere in between. Depending on the state of fragmentation, I don't think it's meaningful to derive any "average speeds".

    Maybe digging through the official site might yield more results:
    http://www.sdcard.org/developers/tech/speed_class/

    ...
    The SD Association has specified operating conditions for SDHC memory cards to guarantee the minimum speed according to the card's speed class. Class logos are provided to help identify cards that comply with these specifications.
    Apparently Class10 do not exist as an official standard yet.

  3. #3

    Default Re: SD card - rated class and actual speed?

    Quote Originally Posted by CamInit View Post
    Marketing will promote the peak speed (15MB/s, 30MB/s, etc). Guaranteed minimum speed is the critical part. Real world performance will exist somewhere in between. Depending on the state of fragmentation, I don't think it's meaningful to derive any "average speeds".

    Maybe digging through the official site might yield more results:
    http://www.sdcard.org/developers/tech/speed_class/

    Apparently Class10 do not exist as an official standard yet.
    Thanks for the reply and especially the link.
    Will fragmentation have any effect at all if the use pattern is: shoot - download - format - shoot again?
    Last edited by grantyale; 21st December 2009 at 09:17 PM.

  4. #4

    Default Re: SD card - rated class and actual speed?

    Quote Originally Posted by grantyale View Post
    Thanks for the reply and especially the link.
    Will fragmentation have any effect at all if the use pattern is: shoot - download - format - shoot again?
    I wouldn't know the write pattern of SD cards but format will clear everything so no fragmentation unless the memory block is previously marked bad. I think it takes quite a (long) while before a significant number of bad blocks actually starts to have any fragmentation effect.

    Faster minimum speed ensures that the camera buffer is cleared quickly for the successive shots. Useful if you take continuous shots (especially with raw) or HD videos. 3 RAW files if 20MB each would take up 20MB x 3 = 60MB which would take at most 10sec to clear the buffer for class 6 card. At (assuming) peak speed of 30MB/s, it takes minimum 2 sec. Note that doing reading from memory is always faster than writing to it, so there's a high chance 30MB/s more likely happens when you are copying the files out and not during taking the actual photos. This way you can have some gut feeling on how long the transfer takes. This is my understanding of it.

  5. #5

    Default Re: SD card - rated class and actual speed?

    Erm memory cards don't suffer from fragmentation.
    Back to topic, yep, faster memory cards will facilitate faster photo transfer speeds and the ability to shoot multiple images non-stop (IIRC Nikon D60 ADR limits it to 5 frames, regardless fast or slow memory card.)

    Regards,
    gibss.

  6. #6

    Default Re: SD card - rated class and actual speed?

    does anyone wonder why the camera write speed isnt as easy to find as compared to its fps?

    in a sense no point getting 30mbps cards if the camera write speed is only 15mbps max.
    You wont see me much less remember me but i am the guy who makes you look good.

  7. #7

    Default Re: SD card - rated class and actual speed?

    Quote Originally Posted by gibss View Post
    Erm memory cards don't suffer from fragmentation.
    ...

    Regards,
    gibss.
    I bet you didn't even click on the link to www.sdcard.org . Better still, google on flash memory fragmentation.

  8. #8

    Default Re: SD card - rated class and actual speed?

    Quote Originally Posted by Reportage View Post
    does anyone wonder why the camera write speed isnt as easy to find as compared to its fps?

    in a sense no point getting 30mbps cards if the camera write speed is only 15mbps max.
    Hmm... is there any article with regards to camera write speeds? I believe cameras are using RAM(?) for buffers which can easily be in hundreds of MB/s. Memory card i/o and image processing speed should be the main bottlenecks?

  9. #9
    Senior Member giantcanopy's Avatar
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    Default Re: SD card - rated class and actual speed?

    Here is an interesting link on some SD card performances on the 450D

    http://www.robgalbraith.com/bins/cam...?cid=6007-9424

    ryan

  10. #10

    Default Re: SD card - rated class and actual speed?

    Quote Originally Posted by CamInit View Post
    Marketing will promote the peak speed (15MB/s, 30MB/s, etc). Guaranteed minimum speed is the critical part. Real world performance will exist somewhere in between. Depending on the state of fragmentation, I don't think it's meaningful to derive any "average speeds".

    Maybe digging through the official site might yield more results:
    http://www.sdcard.org/developers/tech/speed_class/



    Apparently Class10 do not exist as an official standard yet.
    Fragmentation in flash memory shouldnt be a concern for speed (insignificant) because there is no moving mechanical parts unlike harddisk drive.
    D7100,SB910,17-50/2.8OS,105/2.8VR,85/1.8D,2xE-M1,O60/2.8,12-40/2.8,35-100/2.8,14-42,LX100

  11. #11

    Default Re: SD card - rated class and actual speed?

    Quote Originally Posted by CamInit View Post
    Hmm... is there any article with regards to camera write speeds? I believe cameras are using RAM(?) for buffers which can easily be in hundreds of MB/s. Memory card i/o and image processing speed should be the main bottlenecks?
    The processor, RAM and memory card works hand-in-hand and each could be a bottleneck.
    You are right that RAM are fast, but how much capacity of RAM is given will greatly affect your fps as well, when worked in conjunction with the memory card.
    D7100,SB910,17-50/2.8OS,105/2.8VR,85/1.8D,2xE-M1,O60/2.8,12-40/2.8,35-100/2.8,14-42,LX100

  12. #12

    Default Re: SD card - rated class and actual speed?

    Quote Originally Posted by sin77 View Post
    Fragmentation in flash memory shouldnt be a concern for speed (insignificant) because there is no moving mechanical parts unlike harddisk drive.
    Google turns up this:
    http://www.lagom.nl/misc/flash_fragmentation.html

    I won't be surprise if one could come up with other similar tests but with very different results. Fragmentation does exist but without knowing what write strategies were used and its effectiveness to reduce its effect, I would be wary to claim that the performance hit would be insignificant.

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