A question about memory cards.

[Netskimmer]

I picked up a new 32GB SanDisk Ultra 30MB/s SDHC card from Walmart the other day but haven't opened it yet. I was looking at the package a while ago and noticed it's only a class 6 card and the fine print says 30MB/s transfer, write speed slower! After cursing Sandisk for being deceptive little *****s I tried to look up the spec for the card and found the exact same card in a class 10 that has the exact same specs. What am I missing here? I was under the impression that the whole point of the card class was to indicate write speed. Now I have to dig up the receipt and take the card back on my next trip.

 

[iresq]

Why do you have to take it back? I've been using class 6 without issues. My experience is that the data chock point is the camera, not the card.

Sent from my RAZR

 

[Dao]

I do not think it is write speed as far as the class concern.

I have a Nook color which has a microSD slot. I can boot the Nook Color with the Android OS in the microSD card. So basically running the Andrioid from the microSD. In the beginning, I tried couple class 10 and 6 microSD card and they were not working right. It was because the write speed was too slow.

At the end, after I saw the thread in xda-developer site and learned that most of the card were only fast on read, not write. So I end up getting the Sandisk class 4 instead. After that, zero problem what so ever. So even the class 4 Sandisk blow away many class 10 microSD cards in terms of write speed (Especial random write speed)


Link to the SD card test.
32GB microsd cards - xda-developers

 

[Netskimmer]

That link was an interesting read, thank you for posting it. I wonder if the results are the same for standard SDHC cards or if that is just for the microSD? The initialization time discussed on the class 6 and 10 cards could be a serious performance killer if the card does it every time the camera tries to write from the buffer to the card.

 

[Rwsphotos]

That link was an interesting read, thank you for posting it. I wonder if the results are the same for standard SDHC cards or if that is just for the microSD? The initialization time discussed on the class 6 and 10 cards could be a serious performance killer if the card does it every time the camera tries to write from the buffer to the card.

Agreed wonder the same myself now Net my hubby bought class 10 for me due to they were suppose to be better. Going to be a seriously un happy camper if I run into buffering issues on my Nikon during a wedding.

 

[EDL]

It all depends on the write speed of the camera. My Canon T3i, according to Canon, can write up to 45 mega bits per second. That's about 5.5 mega bytes per second. Class 6 is supposed to be able to be written to at a sustained rate of 6 mega bytes per second for a fragmented state. So, theoretically, a class 6 card will work fine in my T3i (and it does). Read speeds can be higher, and often is in the case of memory in general. So, a card claiming "transfer" speeds of 30MB/s might be correct, but they would be referring to the read speed. If it's class 6, then do not expect to write any faster than 6 MB/s.

 

[korn_2956]

I can benchmark a write speed of 11 to 13 and a read speed in the low 20s with my class 10 micro and could only get around 4 to 6 write and 13 to 15 read with my class 6. The class of a card is supposed to be the minimum write speed.

 

[EDL]

Yes, the "Class" rating is supposed to indicate the minimum sustained write speed. They will go faster on the write speeds, but this is most likely only in bursts and not sustained.

Some of the cards are still rated under the older "X" speed rating which is multiples of the CD-ROM speed standards (1x being 150 kilo bits per second). A Class 6 card would be a 40X under the X rating. The difference is that the "X" speed rating is the theoretical maximum speed. It tells us nothing about minimums, which is more useful info from the DSLR perspective. Also, that X rating is going to indicate the maximum read speed as writing to memory is always slower than reading.

Some manufacturers will include the write speed along with their X rating, but not all do. As with all things speed rated, higher numbers make for better marketing, so some will simply be labelled as 133X cards.

The Class rating came along because minimums are more important info, as mentioned. Class 2,4 and 6 support the respective number of MB/sec as a minimum sustained write speed for a card in a fragmented state. Things change however when you hit Class 10. Although it is still a Class rating, the rating is based on a slightly different set of standards. Starting at Class 10, the card supports 10 MB/s as a minimum non-fragmented sequential write speed.