How to Buy an SD Card for Your Digital Camera


There’s possibly nothing more confusing than trying to buy a new SDHC card. Do you buy Class 2 or Class 6. Do you care about the “X” rating and should you pay for spring for a premium card? Frankly, even geeks can get confused when faced with a selection of 14 different SDHC cards of varying sizes and ratings – none of which readily make sense. Fear not, we waded through the specs and grabbed a selection of cards for testing to see what really matters.

Classless Society

First up is the most confusing and possibly useless rating system I’ve seen in a while. You’ll recognize these on the face of the SDHC card with the typical Class 2, Class 4, or Class 6 designation. This is nothing more than a designation for what the minimum write speed is for a card. A Class 2 card guarantees a minimum write speed of at least 2MB/s. A Class 4 guarantees 4MB/s and Class 6 means writes of 6MB/s a second. The new Class 10 cards (which are not quite on the market yet) would support minimum writes of 10MB/s.

The class rating was created to help the camcorder makers shoot for a minimum speed for their AVCHD cams.  The current AVCHD specs maxes out at H264 High-Profile at 24Mb/s. That pretty much makes anything beyond a Class 4 (4MB/s or 32Mb/s) overkill for any current AVCHD cam. In fact, many AVCHD cameras still haven’t even adopted the higher quality profiles and are still down at 17Mb/s  bit rates -- something most Class 2 cards (16Mb/s) can support.

This is my primary problem (and Sandisk’s which has recently been going public with its displeasure over the Class rating). It doesn’t serve the primary audience of SD card users: still photographers. Since it’s pretty clear that anything above Class 4 is overkill for AVCHD, what about still photographers?

That’s where the X-rating or pure write speeds can make the difference between sitting there waiting for the light to stop blinking and getting the shot of a lifetime.


The X-rating is nothing more than a write-speed strangely based on the old read speed of CD-ROM drives. So, 1X is 150KB/s. A Class 2 card equals writes of 13x, a Class 4 equals 26x and Class 6 is 40x. Some manufacturers such as Sandisk, simply skip the X rating, and express write speed in the most plain language that nerds can understand: MB/s. The company’s Extreme III SDHC cards, for example, are rated at 30MB/s. Again, that’s the disconnect that will confuse buyers: a high-performance card that writes at 30MB/s will carry the same Class 6 designation as a card that writes at possibly only 6MB/s.

The Tests

For our tests, we obtained three 8GB Kingston SDHC cards each rated at either 2, 4 or 6. We also looked at a Class 6 Sandisk Extreme III 8GB card as well a Verbatim 16GB Premium Class 6 card that’s rated at 60x writes or roughly 9MB/s. Our first test involved using a Canon Rebel T1i to first shoot video at 1080P and 720P modes. The camera does not write to AVCHD, instead it writes H.264 in .MOV container files with a monotrack audio. It’s no surprise, but every card had no issues with the video mode.

For our still image tests, we had no such expectation. To maximize the file size, we shot at the highest rated ISO of the body (12800) and in RAW mode. We also shot with a fixed white-balance, shutter speed, aperture and with auto focus off. We shot until the camera’s buffer was full and then timed how long it took to write those nine 22.7MB files to the memory card.
The results were not exactly what we expected. First, on the predictable front – the Sandisk Extreme III premium pricing bought us premium performance. The Rebel T1i’s Digic IV CPU took but 10 seconds to write the roughly 159MB of data to the card. That’s half the time of the majority of the other cards here such as the 16GB Verbatim Class 6 card and the 8GB Kingston Class 6 card.

Now for the interesting part. The worst performance was not the Class 2-rated 8GB Kingston card, it was the Class 4-rated 8GB Kingston card. The lowly Class 2 Kingston card turned in the same write performance as the Class 6 8GB Kingston card and the 16GB Verbatim Class 6 card. That dog slow Class 4 8GB card though, took 30 seconds to clear the buffer. Woof.

Our second round of tests measured how long it takes to copy the file from the card to a backup device. For that, we used the highly rated and incredibly fast Nexto Extreme portable backup device that we reviewed in our September issue. To hopefully increase the performance of the Nexto Extreme even more, we removed the 5,400 RPM Fujitsu 160GB hard drive and installed a Corsair P256 256GB SSD. This SSD features write speeds in excess of 100MB/s. The test file was a single 4GB video file captured on the Rebel T1i.

The results were again quite interesting. For the most part, we saw very little difference between all five cards. The previously dog-slow Class 4 Kingston card was actually the fastest by a hair. The Sandisk Extreme III was a close second with the 16GB Verbatim  and the Class 2 and Class 6 Kingston cards coming in next. The takeaway here is not to worry too much about reading from the cards as there seems to be fairly insignificant differences between them to care. Certainly, card readers can impact the but we’re not too worried about how long it takes you sitting at your desk to copy the photos.
A final test was performed with the slowest card and the fastest card using old equipment. We took a Canon PowerShot SD950 IS, set it to its highest ISO and shot continuous frames and measured how long it would take to record the images.

The Sandisk Extreme III could shoot 15 images at 1600 ISO in 31 seconds. That previously dog slow Class 4 Kingston card? Just 34 seconds. At lower ISOs of 100 with a resulting smaller file, it took the Sandisk Extreme III 17 seconds to shoot 25 images while the Class 4 Kingston took 21 seconds.  The upshot is don’t pay for pricey performance cards if your camera can’t use it. It is no different than putting super unleaded into your Toyota Yaris – you’re just giving Exxon mo’ money.

Extreme III
Verbatim Premium
SDHC 60x
Capacity 8GB
Class 2 4 6 6 6
X-Rating n/a n/a n/a n/a 60x write/133x read
Speed Rating n/a n/a n/a 30MB/s n/a
Time to Write 9 22.7MB RAW Files 20 30 20 10 19
Time to Copy 4GB.MOV file to
Nexto Extreme w/ Corsair P256
4:08 3:47 4:09 3:49 3:57

Buying an SD Card for Video

If your primary focus is video for AVCHD you don’t need to pay for anything more than Class 4. Again, any Class 4 card will easily surpass a 32Mb/s write speed. Since AVCHD tops out at 24Mb/s, anything else is wasting your money. It’s better to have a bigger, slower card (but more than fast enough for your video camera), than a faster, smaller card for consumer AVCHD.

Buying an SD Card for Still Imaging

Buy what your camera can shoot or what you intend to upgrade to. It’s clear that the high performance Sandisk Extreme III SDHC is the king of the hill for performance and if you have a modern DSLR such as Canon’s Digital Rebel T1i or Nikon’s D5000 or even the older Nikon D90, paying for the fastest card you can afford will pay dividends in how fast you can shoot images, especially if you shoot in RAW mode.

However, that doesn’t mean everyone with a digital camera should pay the premium pricing for a high speed card. Our tests with the two-year old PowerShot  SD950 SI bear this out: even the slowest card in the round up turned in reasonable performance compared to the premium card. Only those with the newest cameras with faster CPUs (or those with an eye toward soon getting one) need invest in premium cards. For those, a card in the 60x to 100x will find reasonable performance for the price.

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