Thursday, May 19, 2005

Flash Focus

One measure of just how pervasive flash memory has become is the volume of mail we receive on the topic. The story "Flash Memory: Pick a Card" in our issue of September 2 describes the variety of flash formats available, but digital-camera users wanted to know more. Here are some of the questions we see most often.

What's the best file format for storing photographs on flash cards?

The best format depends on your application. Professional photographers need the highest picture quality possible at a given resolution, and the best way to ensure that is to save photographs in uncompressed TIFF or "RAW" format, which stores full-color information for every captured pixel.

The problem with uncompressed images is the huge amount of space they consume, which can easily exceed 10MB for a single photograph. This significantly reduces the number of images you can squeeze on a flash memory card or CD, makes photos more cumbersome to edit, and increases the time it takes to transfer them to a PC or portable device or to upload them to the Web. Consequently, most people choose to store images in JPEG format, which provides a good compromise between picture quality and file size.

JPEG employs a lossy compression algorithm, which means that every time you save an image in JPEG format, you irrevocably lose at least a little bit of information. But this isn't as big a drawback as you might think, because JPEG supports variable levels of compression. Storing a JPEG image at its highest quality level results in little degradation, but saving it with maximum compression settings can produce a 90 percent reduction in size. Because of this flexibility and the ubiquitous support for the JPEG standard in Microsoft Windows and on the Web, we recommend JPEG for all but the most demanding applications.

How can I tell how many photographs will fit on a flash card?

This one requires some math—and a little guesswork. If you're storing uncompressed TIFF or RAW images, simply multiply each picture's color depth by its resolution (remember that a megabyte consists of 1,048,576 bytes). Most cameras capture images with 24-bit color depth, which means that a 1,600-by-1,200 image would require 1,600 x 1,200 x 24 bits = 46,080,000 bits, which converts to 5,760,000 bytes (or approximately 5.5 megabytes). On most types of flash media, a small amount of storage is consumed by system files, but you should still be able to fit eleven 1,600-by-1,200 TIFF images on a 64MB card.

Predicting the size of a JPEG file is a little trickier, because the exact amount of JPEG compression depends on a camera's image-quality setting, the internal workings of its compression algorithm, and the amount and type of detail in the photograph itself. Despite all this, you can estimate based on typical images and common JPEG settings. The table shows ballpark ranges for the number of 24-bit JPEG photos you can store on a 64MB flash card.

What size flash card should I buy?

As with most other buying decisions, the answer to this question depends on how you use your camera. If you upload your photos to a computer at the end of the day, simply pick a card that can hold a full day's output. If you use a 64MB card with a 3-megapixel camera, for example, you should be able to shoot 80 to 90 JPEG images without having to swap media. Most amateur photographers should consider larger cards only if they plan to shoot uncompressed images or if they'll use their cameras to capture audio annotations and video clips, both of which require large amounts of storage space.

Remember that your requirements may change if you take your camera on an extended vacation without taking your PC. It can make sense in such cases to use a rapid-turnaround photofinisher to dump your daily crop of photographs to CD, but most people find it more convenient and cost-effective to purchase enough storage to last the entire trip.

s it better to buy one large card or several smaller ones?

This is largely a matter of taste, and there are advantages to both options. Many people prefer the convenience of carrying only one card and knowing that it's always stored safely inside the camera. But having multiple cards provides backup ammunition in case your primary card is damaged, lost, or stolen.

A more important buying consideration is cost, according to Gartner analyst Joseph Unsworth. He says that the flash-card market is driven primarily by price, and every flash technology has a capacity "sweet spot" that offers the lowest cost per megabyte. For example, if a 128MB card costs 30 percent more than an equivalent 64MB unit but is one-third the price of a 256MB card, then one 128MB card is obviously a better buy than two 64MB cards.

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