The truth about SD and CF memory cards
By Fred Singer
March 16, 2011
There is a lot of misinformation about memory cards out there. Let’s debunk four myths about memory cards.
You might be surprised at how many people have made bad buying decisions based on incorrect information when purchasing memory cards. Let’s look at four key memory card myths so you can make intelligent decisions next time you need to buy a memory card.
Before we get to the myths, let’s look at some memory card basics.
Think about a memory card as if it is a sponge. The data from your camera is a glass of water. Three basic principles govern the relationship between the sponge (card) and the water (data):
Card capacity: How much water (data) the sponge can absorb.
Write speed of the card: How quickly the sponge (card) can absorb the water (data), expressed in MBS (MegaBytes per Second).
Read speed of the card: How quickly I can wring out the sponge (card) once it’s full of water (data); also expressed in MBS (MegaBytes per Second).
While that seems pretty straightforward, it ain't necessarily so. Here are four memory card myths, busted:
1. "All cards are created equal"
False. There are only six prime manufacturers of flash memory wafers in the world. SanDisk was the first company to make a CompactFlash card, for instance. Peter Liebmann, National Strategic Account Manager for SanDisk. SanDisk manufacturers its own flash as well as its own controllers (the “traffic cop” under the hood that determines where data gets written on the flash). Controller technology is important since a well-designed controller reduces the likelihood that any one sector of the flash wears out prematurely. This maximizes the life of the card. Lexar, another key card maker, says they work closely with camera companies to make sure their cards work smoothly with current and future digital cameras.
2. "Class is relevant to all performance in all types of memory cards, both for still and video."
False. There are a few measurements of speed for flash cards. There are two measures of speed for CompactFlash cards – MBS (megabytes per second) and the “X” factor.
Example: SanDisk’s Extreme CompactFlash and Lexar Professional 400X cards are both 60MBS and 400X. These numbers represent the rate at which data can be transferred from/ to the card and host device (camera/ camcorder). If you know the MBS number (in this case 60MBS), you simply divide that number by .15, so in this case: 60MBS/ .15 = 400X
Conversely, if you know the “X factor, you can easily calculate the MBS: 400X x .15 = 60MBS It’s a little more complicated with SDHC cards, where there are three measures of speed. Let’s take a 30MBS, 200X, Class 6 card as an example. The same relationship between MBS (megabytes per second) and the “X” factor applies to SDHC as to CompactFlash: 30MBS/ .15 = 200X
In addition to the MBS and “X” factor, SDHC cards are also designated with a Class rating (typically Class 4, 6, or 10). The Class rating system is ONLY relevant when shooting full 1080P HD video onto SDHC cards. It is not applicable to CompactFlash cards and is NOT relevant to still photography when using SDHC cards.
By definition, Class is the MINIMUM sustained read/ write speed of an SDHC card expressed in MegaBytes per Second (or MBS). The Class system was developed when flash based video came into vogue a few years ago, as a means to ensure the end result would be a drop-out-free video when viewed on your television or PC.
For example: Your camera manual recommends a Class 6 card for shooting 1080P video. This means you need a card with a minimum sustained write speed of 6MBS (megabytes per second) to ensure proper video quality from your camera/ camcorder.
In addition, the Class rating is the minimum video recording speed of the card, not the maximum performance (speed) of the card.
Bottom line: Not all Class 6 cards are created equal, the Class rating is only relevant to SDHC cards, and only applicable to shooting 1080P HD video.
3. "The speed of a card is more important when shooting video than when shooting still photos."
False. When recording video, you are shooting a small, but sustained stream of data onto a card (like a garden hose). When you are shooting RAW files (still photography) at a burst rate of 5 or 6FPS, you are sending a 10- to 20 megabyte file (based on your particular camera brand and model) onto a card at 5 or 6 times per second (like aiming a fire hose at your memory card). High-resolution RAW images shot in rapid sequence are a lot more information that needs to be transferred quickly, and require a larger “hose” than videos.
4. "CompactFlash cards are 'professional' and more durable than SD cards."
False. CompactFlash was the first form factor to the market and was the mainstay in digital photography for about 5 years. SD cards were developed to allow more compact camera designs, but they are actually more durable than CompactFlash cards because they are waterproof, shockproof, magnet- and x-ray proof.
5. "SDXC cards are faster than SDHC cards."
False. SDXC is simply a measure of a card’s capacity, not its speed. Here's how the different SD capacities break down, by card type:
0 – 2 gigabytes = SD Card
2+ - 32 gigabytes = SDHC Card
32+ gigabytes = SDXC Card
Our thanks to Peter Liebmann, National Strategic Account Manager for SanDisk, for the white paper that inspired this article.