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-   -   SDHC speed/ class confusion (

captbly1 Dec 14, 2008 6:56 PM

From the link below

It seems pretty straight forward. A class 2 card is 2MB/sec a class 4 card is 4MB/s. Armed with this info, I go out today tobuy my SDHC card. What I come home with confuses me.

I got a Lexar 4GB SDHC card. it is labeled as a class 4 card (4 with that c around it) yet it says 60X with 9MB/sec write. If this card is can do 9MB/sec wouldnt this be like a class 9 card ? Why is it still labeled as class 4?

Seems there is some flaws still with the class "standards", or at least my understanding of them.


Sintares Dec 14, 2008 7:22 PM

The speed class is the guaranteed minimum write speed, the x60 would be the top (burst) speed of the card (as determined by the manufacturer)

TCav Dec 14, 2008 7:26 PM

The Speed Class for SD cards is the guaranteed minimum transfer speeds. The specification of, for instance, "9MB/sec" is the maximum transfer speed. The difference is that when you're shooting video, in order to shoot an ininterupted vieo stream, it is the minimum transfer speed that matters.


captbly1 Dec 14, 2008 7:54 PM


The card is speed-rated at 60x, the equivalent of a minimum sustained write speed capability of 9MB per second.
Per the Lexar link (posted above) and the packaging says the 9MB/sec is minimum sustained write speed.

I wonder if this is bad wording on Lexar's part? I'll try and see if I can email them on that. If its true minimum sustained then they wouldnt label it class 4 right?

TCav Dec 14, 2008 8:49 PM

Actually, there are only 3 classes: Class 2, Class 4, and Class 6. A better question might be why it isn't a Class 6 card.

The SD Association SD Card CLasses are determined by tests established by the SD Association , whereas Lexar's speed ratings are determined by tests established by Lexar. I beleive the difference is probably a direct result of the different methods used to test the cards.

captbly1 Dec 14, 2008 9:27 PM

Yeah seems there are only the 3 classes from what I can see also. Seems they need to add a few more as its already an obsolete system of measuring the cards.

Interesting that for the Lexar card marked class 6 added the word "capable" there. Maybe that word was left out on the class 4 cards.


This high-speed card is capable of minimum sustained write speeds of 20MB per second,

I sent them an email to see what they say. Just seems like there should be a better bench mark number/test to compare all these SD cards.

thanks for the input

TCav Dec 14, 2008 9:39 PM

captbly1 wrote:

Just seems like there should be a better bench mark number/test to compare all these SD cards.
There is. It's the SD Association SD Card CLasses. It does not, however, keep companies from advertising their own numbers.

captbly1 Dec 14, 2008 10:21 PM

Seems very flawed too me. Kind of like saying in 1920 that, "hey, we are going to have 3 classifaction ofcars. A can go 50 MPH, B can go 75 MPH and C can go 100 MPH" (no need to go higher than 100 MPH right?)

I think cards are already showing they are running better than class 6.

TCav Dec 15, 2008 7:55 AM

captbly1 wrote:

I think cards are already showing they are running better than class 6.
The manufacturers say their cards are fast, but when subjected to the SD Association's test, they aren't. That's why a card that Lexar says can transfer data at 9MBps is only a Class 4 card. The tests aren't outdated; they just disagree, and the most stringent of the two tests is the one I recommend you base your purchasing decision on.

The cards aren't running better than Class 6. The manufacturers are just quoting specs that make their own products look good.

When Company A says it's products are reliable, but Consumer Reports says they're not, who do you believe?

Fila Mar 25, 2009 6:47 AM

Hi there,
It is an old thread - but is best I've found so far.
I've purchased Canon PowerShot SH1 IS last week choosing over SX10 because of its full HD video.
The shop recommended card with a purchase was SanDisk Ultra II 4GB, and I bumped it up to 16GB one.
I used the camera so far mostly for photos, that's the reason I did not buy HD camcorder, but I did a couple of HD video tests.
It is of course some learning curve, correct use of IS etc., but I became suspicious of SD card performance and I have had another look.
To my big surprise SanDisk Ultra II 16 GB I bought is Class 2 card only! :evil:
The shop I bought everything downplays the importance of Class 2 rating and justifies the performance, or lack thereof, to Canon itself saying that it not really HD camcorder.:P
To this I can only laugh, but I was honest enough to admit that I did not do all the tests (most importantly with various IS setting) and additionally compare them to my another SanDisk SDHC, this time Extreme III which is Class 6 to make sure it is problem of SanDisk Ultra II 16 GB Class 2. So we agreed that I would test for another week, and then we'll see.
At that moment I was more negative about the shop, but further investigation shows that SanDisk is far from innocent.
I own a number of SanDisk SDHC cards, and another SanDisk Ultra II (8 GB) is Class 4?:?
Searching the net I've found a number of references (or rather pictures) to both Class 2 and Class 4 SanDisk Ultra II 16 GB, even if SanDisk declared speed is the same:15 MB/s!
I am writing this to warn others because if I've know before what I know now I would be much more careful.
I've trusted SanDisk before and this trust has blinded me. Not the first time, and alas probably not the last, but what surprised me is missing more information about this on the net. This thread is the only so far being spefic about Lexar cards, and I wanted to add SanDisk to the list.:evil:
Caveat emptor – double check speed ratings.

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