Tech-Talk; The Memory Card Dilemma; Speed And Class, Explained Page 2

Camera Write Speed
Cameras capture images in bursts, flooding the memory card with a large amount of data in a single instant. For optimal performance, these devices require high-performance memory cards featuring fast maximum write speeds.

Today’s D-SLRs can generate massive photographs in either the Raw or JPEG file formats. Typical Raw images often reach 10-20MB in size, quickly filling a memory card’s bandwidth pipe as the data is written from the camera to the card. The larger the stream of data being written, the faster the memory card’s write speed must be in order to handle the increased bandwidth strain.

In addition, a memory card’s write speed plays a crucial role in the overall camera system when taking pictures in rapid succession using Burst mode. When combined with Raw image files, this shooting technique makes fast write speeds a necessity due to the sheer volume of data produced. If a memory card cannot process the data quickly enough then shooting may pause unexpectedly as the card catches up to the camera, potentially causing photographers to miss crucial shots.

If you want to capture high megapixel still images, then you should select a card with a maximum write speed fast enough to meet your usage requirements. If you plan on simply taking single pictures with entry-level point-and-shoot cameras, then you probably don’t need a high-performance card. However, if you intend to use the advanced features of today’s D-SLRs such as Burst mode and Raw format, or if you photograph in any kind of professional capacity, then for best results you will need a memory card that offers fast write speeds.

Both Ratings Matter
Rather than simply choosing the most expensive high-performance memory card on the shelf, users instead should determine their usage requirements and then select the card that is best able to meet those needs. As we have already covered, class ratings are important when recording video, while speed ratings are important when taking photos. However, speed and class ratings do overlap to provide a comprehensive performance indicator.

Not all cards of the same class are equally fast. Class ratings designate a card’s minimum speed, but give no indication of maximum speed. While all Class 4 cards operate at a minimum speed of 4MB/s, their maximum speeds may vary greatly. One Class 4 card may have a maximum speed of 9MB/s while another may write at up to 15MB/s. The difference in speeds is important in two regards.

Although any Class 4 card can record HD video, cards with faster read (or “transfer”) speeds allow the user to spend less time downloading data from the card to a computer. For example, a Class 4 card reading/transferring at 10MB/s will take more than 13 minutes to unload 8GB worth of data. However, it takes less than nine minutes for another Class 4 card to download that same 8GB at 15MB/s.

Increasingly, D-SLRs allow users to record HD video. These devices require a card with appropriate class and speed ratings for either circumstance. If the card offers fast maximum write speeds but only a Class 2 rating, then it will not be fast enough to record in HD. Similarly, if the card has a Class 6 rating but a low maximum write speed, then the use of Burst mode will be impeded, and it will take a long time to offload data once it is written to the camera.

Finally, keep in mind that only SD cards identify both class and speed performance capabilities; CompactFlash and Memory Stick cards only refer to speed.

About the Author:
As Vice President of Retail Product Marketing, Eric Bone directs global product life cycle management for all SanDisk retail products, including flash memory cards for mobile phones, gaming, photography, and camcorder devices, and USB flash storage drives. Bone’s duties include developing the business cases for new retail products, ensuring alignment with the company’s current and future technologies, and executing a complete product road map. Bone brings an intimate knowledge of the retail channel, broad experience in implementing go-to-market strategies, and international exposure to retail customers. Bone holds a B.S. from Vanderbilt University and an MBA from Santa Clara University.


AJNY's picture

I've noticed that when I record movies from my point-and-shoot to the camera's memory card, I lose about a second at the end of each clip. For instance, a 10-second recording produces a 9-second clip; a 30-second recording produces a 29-second clip. And it's always the last second that gets lost.

Is that about the card speed, or is that just a natural lag-time between the camera recording and the card saving?

Obviously, the easy fix is to let the camera record for another second or two before stopping, but I'm curious about the cause of the clipped movie.