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Conclusions
The relatively recent SanDisk Extreme IV shows a very slight advantage over the III on the E-System cameras and none on the older ones. Depending on the price difference, it may or may not be worth the extra expense. Probably not.
The writing speed in the E-510 has been visibly improved since the E-500 and E-300. While it performs best with the Extreme IV, even with slower cards it writes faster than its older siblings (which, in turn, were improved compared to the preceding models, in spite of files being almost twice as large).
Interestingly the last three cameras are not really faster when saving JPEG files (for the '510, writing times 1:2.7 and 1:4 JPEGs are, within measurement errors, the same as for raw files). This indicates that the compression time is becoming a bottleneck, not the writing speed.
The SmartMedia format, now obsolete, is no match for the CompactFlash.
When it was introduced by Toshiba (late Nineties), it was as fast or even a bit faster than CF, and at some moment it had about 50% of the memory card market. Lacking the internal controller, SM cards suffered notorious compatibility problems: with almost every new capacity introduced, the camera firmware needed to be updated to handle it. The largest SM cards have capacity of 128 MB.
A few years later the same error was repeated by the same manufacturers. Yet another semi-proprietary format, xD-Picture, not offering any advantages (except lower production costs).
The original xD cards are the slowest ones of all types tested. I am unable to verify another claim, about lower power consumption compared to other types.
The new "H" variety of xD-Picture cards, introduced in 2005 (?) is, indeed, faster than the "plain" XD. On the C-5060WZ this is the fastest card I've tested (although the combination is still quite slow in absolute terms). On the E-500, however, the xD "H" still trails behind CompactFlash, even behind some older ones.
I haven't checked the xD-Picture "M" type, but it is specified to be slower than "H". I see no reason to invest into xD-M, being able to get an xD-H on a sale for $30 or so.
The E-20 is clearly showing its age, at least in terms of writing speeds, with the camera, not the card, being the writing speed bottleneck. If you are using one, assign to it the oldest and slowest cards you may have, including any SmartMedia ones found in your drawers.
The C-5050Z is not only faster than its younger sibling, the '5060 (almost by a factor of two with most CF cards), but also takes better advantage of the SanDisk write acceleration technology used in the Extreme III and Extreme IV lines. With the raw writing time of three seconds it may be the speed champion of the baby boomer generation.
The C-5060WZ, contrary to manufacturer's claims, is not a speed demon at all. (I haven't clocked the C-7070WZ.)
xD-Picture card compatibility: Because the xD-Picture cards are missing a built-in controller, with every new generation compatibility problems may arise. Unfortunately, most of cameras built two or three years ago will not work with the recent cards, a real mess.
For example, 1 GB or larger xD-H cards will not work with the C-5050Z; smaller ones of that type will (according to Olympus; I haven't checked this myself). For other Olympus cameras, refer to the official card compatibility chart.
Note of August, 2008: Even now I keep getting email messages asking if Olympus provides a firmware upgrade addressing the M- and H-type xD Picture cards. Sorry, the answer is no, they don't. With the "standard" cards almost off the market, looks like the C-60 users are left out in the cold.
Last but not least, speed is not the only issue. Flash memory suffers from endurance problems — individual, random cells die after a number of uses. This is not something card makers advertise, but, still, a fact of life. While the CF controllers work around the problem by "spreading the load" (i.e., trying to use all cells equally often), this is only a partial solution. And some brands have better record in this aspect than others, reducing the chance that you will loose a picture without an apparent reason.
More, some test results indicate that the writing speed and card endurance may be inversely correlated: faster cards, on average, have shorter life spans. Some write acceleration technologies (Lexar) may affect card longevity quite a bit. For more on this subject, have a look at the AnandTech article, published in December, 2005.
I wouldn't worry much about this, though: it is highly unlikely a card will become affected by endurance problems in the two or five years before it becomes obsolete: too slow or too small. After all: how many times do you overwrite a card over that time: a thousand?
What cards to buy?
Most cameras don't give us much choice about the card standard: you are limited to the card type which your camera accepts. However, if your camera gives you a choice, then my recommendation would be:
For the E-510 (and E-410) I would recommend a large (as of July, 2007, perhaps 2 GB) and fast CF card, and a one- or two-gigabyte xD-Picture. This may be less expensive and safer than a 4 GB Compact Flash.
For a fast camera not performing too well with XD (like the E-300, E-500, or C-5050Z), use a also large-capacity, fast CF and the least expensive (per megabyte) xD as a backup;
For a slow-writing camera (like the E-10 or E-20) the card speed does not matter. Use whatever is on sale or too slow for a faster camera.
The C-5060WZ seems to work best (if not by a large margin) with xD "H" cards and this would be my recommendation, in addition to a generic, inexpensive CompactFlash; the camera does not make much use of the Extreme write acceleration. The C-7070WZ (which I haven't clocked) behaves, most probably, the same way.
Keep in mind that the fastest card will probably not last as long as slower ones.
Actually, if you store your images in the JPEG format, you will not notice speed differences between most of the recent-vintage cards. One more reason not to get overly excited with writing speed alone.
Web references
Alfred Molon of myolympus.org maintains a number of lists, comparing CompactFlash writing speeds for various Olympus cameras:
E-500: SanDisk Extreme III is the speed king, being twice as fast as most of the competition, including Lexar;
E-300: similar results, with SanDisk Ultra II also clocked at half of the speed of the Extreme III;
C-7070WZ and C-5060WZ: no surprises, but, generally, these cameras are twice slower than the E-System SLRs;
C-5050Z, compiled by Jens Birch, and based on his list mentioned below; limited to older card types.
The original '5050 comparison by Jens (updated August, 2003) can be found here. The measurements quoted by Jens have been performed using the same technique as those described in the present article; they were done by various authors, therefore there may be some extra variation from author to author.
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