Talking about SD, CF, SDHC, What's the difference?

SD, CF,  SDHC, What's the difference?

A 64 MiB CompactFlash Type I card
Media type	Mass storage device format
Encoding	Various file systems
Capacity	2 MiB to 184 GiB
Developed by	SanDisk
Dimensions	43×36×3.3 mm (Type I) 43×36×5 mm (Type II)
Weight	10 grams (typical)
Usage	Digital cameras and other mass storage devices
Extended from	PCMCIA / PC

Pentax
Pentax *istDS K10D (and the Samsung GX-10)
Pentax K100D
Pentax K110D
Pentax K20D (and the Samsung GX-20)
Pentax K200D

CF cards also have an onboard microcontroller for the memory which takes some load off the host, but this is currently more of theoretical interest than practical significance. The "Secure" in Secure Digital comes from the card's origin which was concerned with digital rights managements schemes to prevent copying of music! SD cards contain encryption hardware, but it's not used in 99% of all applications and it looks just like flash memory to a digital camera. MultiMediaCards (MMC) are essentially the same as SD cards, but without the encryption hardware and with a few other technical differences.

Which Cards are Fastest Update: Sandisk released their extreme IV series in July of 2006. These cards have a data transfer rate of up to 40 MB/s, which is twice that of the extreme III cards. There are available in 2GB ($89.99), 4GB ($149.95) and 8GB ($299) versions. They share the same rugged construction and wide temperature range as the Extreme III series. For more information see the article on Sandisk Extreme IV cards.

In February 2007 Lexar introduced Professional UDMA (Ultra Direct Memory Access) memory cards. Speed-rated at 300x, capable of a minimum sustained write speed of 45MB per second, and available in capacities of 2GB, 4GB, and 8GB, new Lexar Professional UDMA cards deliver a 125% performance improvement over Lexar's previous generation 133x CompactFlash® product line when the card is used with a UDMA-enabled device.

The second fastest current CF cards seem to be the Sandisk Extreme III series, although the Sandisk Extreme, Sandisk Ultra II and Lexar 80x series cards are close, at least on most DSLRs and certainly on the EOS 20D/30D/40D and Digital Rebel XT/XTi/XS/XSi. The speed difference from the fastest to the slowest in this group is probably about 5%, nor really enough to make much of a difference. The 1GB Sandisk Extreme III card which sells for around $43 on ebay. It also comes with a lifetime warranty and free data recovery software. Even slow 1GB cards don't sell for much under $20, so it's not all that much more for the fastest, most durable card.

The Extreme III series cards have a native write speed of 20 MB/sec, though camera limitations usually drop this to around 5-6 MB/sec. They are also tested at temperatures from -13°F to +185°F (-25°C to 85°C) , which should cover most shooting conditions(!) and Sandisk say that they are better protected against shock. The standard Extreme series are tested over the same temperature range, but have a write speed of 9 MB/s. Though this sounds (and is) slower, in actual use there is very little difference in speed in most cameras. Both the Digital Rebel XT/XTi and EOS 20D/30D top out at about 5-6 MB/sec and even the EOS 1Ds Mk II can't write much faster than about 7-8 MB/s.

The Sandisk Ultra II cards are the same speed as the Extreme cards (9MB/sec write speed), but aren't specified for use over such a wide temperature range. The Sandisk Ultra II 1GB sells for around $25 on ebay.

RiData also have some very fast cards. They are labeled as 150x (22MB/s), but that's their read speed. The specs indicate that they can write at 15 MB/s (100x), still very fast but not quite as fast as the Sandisk Extreme III series cards. They have an operating temperature range of 0°C to 70°C, slightly less than the -25°C to 85°C range of the ruggedized Extreme III cards.

The fastest cards come into their own when transfering data to a PC via an optimized card reader. For example the Sandisk Extreme IV cards when coupled to a fast PC via the Sandisk Firewire 800 card reader can hit transfer speeds close to 40 MB/sec, but with a generic USB 2.0 card reader that transfer rate drops to around 6-7 MB/sec, no faster than a much cheaper 60x card. If you want the ultimate performance out of your ultimate speed memory card, you must be careful which card reader you use.

If the speed of transfer from card to PC isn't important to you, buying the very fastest cards won't be cost effective since they will be no faster in the camera. Just go have lunch while your card transfers data! For comparision, with everything fully optimized you could download the fastest 2GB card to a fast PC via the fastest Firewire 800 card reader in under a minute. A slower card and slower USB 2.0 reader might take 6 minutes. I have an old card reader that would take close to 30 minutes! However if I'm at lunch while the downloading is done, I don't really care how long it takes.

Which Cards are the Cheapest This is a more difficult question since prices can vary all over the place on older, slower cards. It used to be possible to make significant savings by using such cards, but today the faster cards aren't really all that much more expensive in many cases, so going for the "rock bottom" price isn't always the best course of action. The lowest prices are often the result of mail-in rebates or other discount offers which come and go. You just have to keep your eyes open for such offers.

So which card to buy? A few years ago you might pay two or three times as much for a fast card, and large capacity fast cards cost hundreds of dollars (as much as a thousand dollars for some high capacity cards). Back them it didn't make much sense to spend extra on a card that wasn't going to do much for you, so slower cards often made sense. Today fast cards don't cost that much more than slow cards and you can get a fairly fast 1GB card for under $30, so the decision is a bit easier to make. A few years ago you'd might have paid several hundred dollars for a 1GB card, and it would have been slower!

For most cameras probably the best value are the 1GB and 2GB cards with speed ratings around 50x. The slower cards aren't much cheaper and the faster cards don't give all that much extra speed. While your current camera may not benefit from a fast card (if you have a 300D or 10D for example), it's very likely that your next one will!

The highest capacity cards available at reasonable prices are probably the Seagate 8GB Microdrive CF card (type II) which currently sells for around $148, and the Transcend 120x 8GB CF card which sells for around $91, which seems like quite a bargain. There's a compatibility matrix for the 8GB Seagate microdrive on the Seagate Website which lists the cameras with which the 8GB drive has been tested. Personally, I'd be inclined to go with the solid state flash memory rather than the microdrive.

IMPORTANT NOTE:In order to use drives with a capacity greater than 2GB (either solid state or microdrive), the camera must support the use of the FAT32 file system. Most current higher end digital cameras (including the EOS 10D, EOS 20D and Digital Rebel XT as well as the Powershot G6, S50, S60 and S70) do support FAT32. Older cameras (including the EOS D30 and D60) may be limited to the use of 2GB microdrives and memory cards. If you are in any doubt about whether your camera supports FAT32 and can use 4GB and 6BG microdrives, Hitachi have a compatibility matrix chart on their website where you can check. The current 4GB drive is designated "3K6-4". "3K4-4" is an earlier vesion of the 4GB drive. The 6GB drive is the "3K6-6". Drives with the "-2" suffix are 2GB.

Bigger isn't always better Cards are being made in larger and larger capacities. 4GB CF cards are now available for around $70 and 8GB cards for under $150 and if you really want the biggest card around you can get a Sandisk Extreme III 16GB card, though the suggested retail price is $1049.99. You can get one from ebay for only $560, though you can certainly find buy four 4GB cards or even two 8MB cards for less.

Obviously high capacity cards and microdrives have advantages in that you don't need to keep swapping cards when shooting. However there is one disadvantage and that's having "all your eggs in one basket". If the card should fail (which is quite rare, but which can happen), you've lost 4GB worth of images. If you were shooting with 4 x 1GB cards you'd only lose 1GB worth of images if one card failed. This is a factor some photographers take into account when deciding what size memory to use. Of course if failure was equally likely, given long enough both systems would lose the same amount of data, but it would be in smaller chunks with the 1GB cards and it wouldn't all happen on the same shoot! However there's no denying that it's more convenient to have one card than to have to swap between cards.

Cards to buy and Cards to avoid Most of the major CF cards are reliable and most have a good warranty. There are good reports about Viking, Kingston, Lexar and SanDisk.

Although Ebay may be an excellent source for many items, be very careful when buying memory cards though them. There are many fake memory cards being on Ebay. The cards may not be made by the manufacturer advertised, they may not have the advertised capacity or they may not have the advertised speed. There are companies in China and other places who will put any label on a card you want if you buy from them in bulk. Some resellers do just that, buying off-brand, low capacity, low speed cards and putting name-brand, high capacity, high speed labels on them. You may be OK buying from major US retailers, but if you buy a cheap card from a dealer in the far east, you're gambling on getting what you expect, and the odds aren't particularly in your favor. I'd stick with Kingston, Lexar, SanDisk Transcend or Viking.