Memory Card prices and storage capacity (PDF) as at August 2008
Purchasing a digital camera normally locks you to a specific card type, with Secure Digital (SD), Memory Stick (MS) and xD-Picture Cards most popular among compact digicams and CompactFlash (CF) cards for enthusiast digicams and DSLRs. A few cameras have two card slots, although this situation is becoming rare. Many cameras come with a card slot plus an internal memory, the latter providing a low-capacity back-up when card space runs out. This article covers the important factors affecting card capacity and shows you how much memory is needed for your camera.
The storage capacities of the internal memories and cards supplied with most cameras are woefully inadequate and an increasing number of cameras, especially high-resolution models, are now sold without any cards. Consequently, most new camera purchasers must buy a memory card with their cameras. In this feature we’ll outline the data transfer speed and storage capacity issues you should consider.
Factors Affecting Card Capacity
The actual capacity of a memory card can depend on several factors:
1. How the computer operating system sees it. Most operating systems define a megabyte as 1,024 KB (1.024 MB), which means a 512MB card has a total capacity of 524.288MB and a 1GB card’s capacity is 1,048.576MB.
2. How the card manufacturer defines a megabyte. For many manufacturers (and most disk drives), a megabyte is exactly one million bytes.
3. How formatting the card changes its capacity. This varies, but typically around 1% of total capacity is allocated to a ‘security area’.
The last two factors together explain why the actual capacity of most memory cards is slightly lower than it might appear.
How Much Memory Do You Need?
Assuming you always shoot with your camera’s highest resolution and quality settings, it’s easy to calculate the size of the memory card you require. When we used film, most amateur photographers would shoot up to two 36-exposure films in any one day; the equivalent of 72-75 shots. With digital, we’re less conservative, so a better estimate for a day’s ‘normal’ shooting would be roughly 100 shots. In photographically exciting places, this figure could safely be doubled – and if you plan to shoot video as well as stills, you will probably need at least four times more memory capacity.
The table below shows the approximate number of pictures popular memory cards can store for digital cameras of different resolution levels. The figures provided are based on JPEG images at the highest resolution and quality settings. For the approximate number of uncompressed RAW files or TIFF images, divide the number of shots per capacity by three. Where RAW files are compressed, compression rates are normally low, so divide the number of shots per capacity by two.
Note: The above figures represent JPEG images recorded at the highest resolution and quality with average compression levels. Card capacities will be higher when lower resolution/quality settings are used.
When memory cards are used for storing video clips, capacities are measured in time, rather than number of image files. The table below shows the video capacities for two of the most popular video formats.
How Many Minutes of Video Can You Store?
The top card manufacturers include ‘high-speed’ cards in their CF, SD and MS ranges. The most common speeds offered are ’40x’ and ’80x’, where the ‘x’ represents 150 kilobytes per second. Lexar Media’s 133x CF and SD cards and SanDisk’s Extreme III cards support transfer speeds up to 20 MB/second. In most recently-released DSLR cameras, a standard card will have a data transfer rate of around 1MB/second, while a 40x card should support a rate of 4-6MB/second and an 80x card 9-12MB/second.
Is it worth paying more for a high-speed card? It is if you shoot lots of photos each day, particularly with burst mode. High-speed cards can also be worthwhile if the camera is unable to shoot and write simultaneously, or if your camera’s innate write speed is slow and its buffer memory is small. Otherwise, you may not notice much difference between a high-speed card and a standard one.
Card manufacturer have also begun to include additional features, such as security, speed and connectivity enhancements and environmental protection into some cards, CF cards in particular. Most of these facilities have been designed to appeal to professional photographers and the cards cost more.
Lexar Media’s Write Acceleration Technology is a speed enhancement tool that claims to boost write speeds by up to 23% when the cards are used in suitably-enabled cameras (a list is available at www.lexar.com). SanDisk’s Extreme III memory cards combine fast write speeds with protection against extreme environments. They claim to work in temperatures from -25ø‚ º C to 85ø‚ º C and are shock and vibration resistant. Lexar also manufactures USB-enabled CF cards that integrate USB functionality into the controller chip, making it easier (and faster) to transfer data from card to PC via a card reader.
The major card manufacturers supply image rescue software with their professional-standard cards, allowing photographers to recover images and other files from cards that have been accidentally damaged or erased. In most cases, deleted files and files on cards that have been re-formatted can be recovered. Where the card’s file system has been corrupted, the chance of recovering images is less certain. Some manufacturers provide hotline product support – but it is seldom worldwide.
Lexar also sells CF cards with ‘Lock Tight’ password-based security protection, which controls access to the card. Only a few professional DSLRs support this system; details can be found on the Lexar website (www.lexar.com.). SD cards come with Content Protection for Recordable Media (CPRM) copyright protection technology, which provides a high level of protection against illegal copying. The SD Memory Card copyright protection function has the following features:
Memory Card Tips
Make sure you have at least two cards for every important shoot. That way, if you think the card may be filled part-way through a burst or sequence of shots, you can replace it with the back-up card and avoid facing a Full Card error in the middle of the action. If you start running low on card space towards the end of the shoot, you can use up the space that remains on the earlier card.
By default, Windows XP will format any CF card with a capacity greater than 32MB with the FAT32 format. Some digital cameras can’t operate with a FAT32 formatted card so you should always format your CF card in your camera before shooting if you plan to download images to a Windows XP computer. Don’t expect a camera to display images captured on a card by a different camera – even if it comes from the same manufacturer.