Understanding Memory Card Speed
Memory Card Speed Ratings
Memory card performance is measured by a class rating as well as speed rating. You'll see often these ratings on the front of SD, Micro SD and Mini SD memory cards. In the image to the right, this Lexar professional SD memory card holds 8GB and is rated Class 6 (see the number "6" enclosed in a "C") and has a read/write speed of 133x.
Class Rating
The number class rating (Class 2, Class 4, Class 6,and Class 10) indicates the minimum sustained speed that the card can continuously record video data onto the card. This is measured in megabytes transferred per second. For instance, a Class 2 card transfers data at a minimum of 2 MB per second and a Class 10 moves data at a minimum of 10MB per second. The class rating is especially key for camcorders and cameras that shoot video clips.
Speed rating
The speed rating is often specified with an "X" rating, such as 133X. The speed rating refers to the maximum continuous speed that images or video can be written to the memory card and moved to a host device. It is sometimes referred to as the" read/write speed." 1X means data can be recorded onto the card at the rate of 0.15MB per second. A 133X card would transfer data 133 x 0.15 = 19.95 or 20 MB per second. So if you have a digital SLR and take photos in "burst" mode, you want as high a speed card as your camera can utilize.
Putting it All Together
For example, a Lexar Class 6 133x speed flash memory card can record video at a minimum of 6 GB/second and a data burst of up to 20MB per second.
The actual speed may be higher or lower than what is stated due to factors such as the camera electronics, the card reader and even your PC.
Can Your Camera Handle the Highest Speed Cards?
Some cameras may not be designed to handle the highest speed cards. The newest and highest speed cards can be expensive, so why pay for something if your camera cannot handle it? You may be able to find your camera's class rating requirement in your manual or the manufacturer's website. So if your camera's recommended class rating is 4, then a Class 4 memory card is what will work best. A Class 6 or 10 will be overkill and a Class 2 may give you an error message telling you that you need a higher class card.
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