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Editor’s Note: From time to time we open these pages to product and technical managers within various companies to help explain matters that we feel need clarification, whether it be the explanation of marketing jargon or an inside look at new technological advances. This month we take a look at speed and class ratings in SD cards.

Eric Bone
Vice President of Retail
Product Marketing
SanDisk Corporation

It starts out as a simple errand: buying a memory card for your new digital camera. Yet there are many matters to take into consideration for D-SLR owners, chief among which is how to match a particular card to your camera in order to max out its performance and speed capabilities.

This short guide is intended to describe common card performance measurements and explain how these metrics give you the ability to make the most of your D-SLR. Many users mistakenly assume that their camera is the only reason for slow performance when it’s actually the combination of camera and card that dictates performance speed.

SD flash memory card performance is measured in two ways: speed rating and class rating. Only SD cards (including SDHC, microSD, and microSDHC cards) provide class and speed measurements on the card face. Understanding the differences between these ratings is the first step in selecting the card that will best suit your needs.

Speed Rating: Burst Rate And Speed
The speed rating (e.g., 20MB/s) measures the maximum sequential speed that data can be written to the memory card and transferred to a host device. Speed rating is also referred to as “read/write speed” or “data-transfer rate.”

Speed rating says “this card will read/write to but no faster than xMB/s.”

Cards that write faster minimize lag time between shots when utilizing continuous shooting (Burst mode) on advanced camera models.

Class Rating: Video Use
The class rating (e.g., C2, C4, etc.) measures the minimum sustained speed required for recording a constant rate of video onto the card. A card’s class rating number corresponds to the minimum guaranteed data transfer rate measured in megabytes per second. Class 2 cards are designed for a minimum sustained transfer rate of 2MB/s, while Class 10 cards are designed for a minimum sustained transfer rate of 10MB/s, though the performance might be lower depending on the camera used.

Class rating guarantees “this card will record video content no slower than xMB/s.”

Taken together, class and speed ratings provide you with an SD card’s overall speed range, which is crucial in selecting a card that meets your specific usage requirements. For example, the SanDisk Extreme SDHC card has a Class 10 rating and a maximum read/write speed of 30MB/s. These two performance measurements tell you that the card should record a video stream no slower than 10MB/s and a data burst of photos at up to 30MB/s.

Matching Card And Camera
For the purposes of this discussion, flash memory imaging cards are used in two ways: capturing still images or recording video. Speed and class ratings tell how well a card performs one or both of those tasks. Speed ratings are used to indicate photographic performance, while class ratings are used to indicate video-recording performance.

Camcorders and video-capable D-SLRs record video at a constant, sustained rate onto the memory card in order to avoid dropped frames that typically result in lost data and choppy playback. A camera’s class rating requirement (determined by the manufacturer) indicates the minimum speed needed to capture error-free video.

If a camera’s class rating requirement is 4, then a Class 4 memory card is needed in order for the device to record video properly. If a Class 2 card is used instead, then the device will likely display an error message indicating that video can only be recorded at a lower definition setting or that you need to get a higher class card.

If you want to record video, then you should select a card with an appropriate class rating. A video-recording device will penalize you for using a memory card that falls below the required class rating, but it won’t reward you if the card exceeds it. Read your device’s user manual to determine the manufacturer’s class rating recommendation. If the manual says that the device needs a Class 4 card, then buy a card with that class rating—no more, no less.

While it is important to understand the basics of class and speed ratings, some memory card makers simplify the process for you by including all of the necessary details on their packaging.

For example, the SanDisk Video HD card’s packaging clearly states the card’s class rating, the types of video it can record (Standard Definition, High Definition, Super High Definition, etc.), and the recording time for each of the various video formats.