With its Pro X2 card, Eye-Fi finally makes good (mostly) on its concept’s Jetsonesque promise: wireless, seamless, and fussless transmission of your pictures from your camera to your computer, as you shoot.

The idea’s sheer coolness factor has been landing Eye-Fi cards on geek want lists and garnering rave reviews from techie blogs since the cards first began appearing in late 2007.

For many serious photographers, though, the first Eye-Fi cards had one deal-killer limitation: they worked only with JPEG files. If you used raw format, Eye-Fi wasn’t for you; your only options for save-as-you-go shooting were to tether your camera via a USB cable (assuming your camera allowed it) or opt for an expensive, hard-to-configure external transmitter such as my Nikon WT-4A.

By 2009, Eye-Fi had introduced a 4-gb “Pro” card that promised to handle raw files. But after reading a fairly scathing PDN review – which uncovered such issues as janky reliability and transmit times of up to 10 minutes per file – I for one decided to keep my money in my pocket.

Now, though, Eye-Fi has introduced a new line of cards with a new “X2″ hardware architecture that, the company claims, improves speed and reliability. The claims made me hopeful enough to spend my own $150 to buy one. And, I’m relieved to say, my money didn’t go to waste. Sure, it has its limitations – but in my tests, the Eye-Fi Pro X2 delivers.

Eye-Fi: What and Why?

So, what is it? An Eye-Fi card is an SDHC memory card – an 8 gb card, in the case of the Pro X2 – that has a built-in WiFi transmitter and antenna. It works in many, but not all, SDHC cameras; Eye-Fi has a compatibility chart you should check before ordering.

While you’re cruising the Eye-Fi website, you’re bound to notice that they make several other models of wireless card, all of them considerably less expensive than the Pro X2. You may be tempted to wonder: Couldn’t I just use one of the less-expensive models instead?

For me, the answer is no. Of all the Eye-Fi cards, only the Pro X2 has two features I think are critical for serious photographers: it’s the only one that can handle raw files, and it’s the only one that allows an “ad-hoc” connection to your computer’s WiFi.

All the other Eye-Fi cards require an “infrastructure” connection – in other words, they can connect only to a separate WiFi router, not directly to a WiFi-equipped computer. That’s fine for casual shooters who will use their Eye-Fi cards mostly around the house.

But I think serious shooters will want the freedom to use the Eye-Fi on location. Sure, it would be possible to carry a wireless router around and plug your laptop into it – but that’s a complication that negates the fuss-free promise. My advice: Spend the extra grickles for the Pro X2 so you can connect ad-hoc.

All the Eye-Fi cards, including the Pro X2, have various other features. The selling point for most casual snapshooters is probably the “instant-share” option: You can configure the card so that it automatically snorks your JPEG photos out of the camera and uploads them to your accounts on popular photo-sharing sites.

That’s great for the casual user: you can come home from a party, set the camera down near your wireless router, and have your pix on Facebook before you’ve brushed the confetti out of your hair.

But I think serious photographers will be more interested in using the Pro X2 in conjunction with a laptop for acquiring and reviewing photos as they shoot – so that’s the kind of usage on which I’ll concentrate here.

Getting Set – Internet Required!

Once you’ve got your Pro X2 card, the next step is to plug the card into your computer and configure some settings. Plugging in requires nothing but a card reader; Eye-Fi supplies one in the package, but my other card readers worked fine too. The setup software your computer needs is stored right on the Eye-Fi card, and you’ll be prompted to install it once you plug the card in.

The software – Eye-Fi Center – is attractive and reasonably slick, but I found the configuration process arcane at first. I finally got my head around it when I realized that setting up Eye-Fi is a three-way conversation: it involves the card, your computer, and Eye-Fi’s online servers, which store some of the key setup info.

The need for Eye-Fi’s servers to be in the loop means that you’ll need a stable Internet connection, both for the initial setup and whenever you change card settings afterward. Make sure you understand this: You can use the card to send photos to your computer whether the Internet is available or not, but you can’t change some important settings (such as the destination folder for your images) unless you can get online. That may be a train-wreck caveat for off-the-grid hardcores; for the rest of us, it just means you’ll need to plan ahead.

Shooting With the Pro X2

Once it’s configured, there isn’t really much to shooting with the Pro X2. As soon as you power up the camera, the card connects to your network and transmits your files according to your pre-configured settings.

You can set it to transmit either all the files, or only the ones you mark with your camera’s “Lock” setting, but other than that there are no controls – not surprising, since the all-on-a-card design means there can’t be any user interface (except on a few cameras that have a special Eye-Fi menu.)

That means there’s no way to be sure it’s really working except to look at the computer. On a Mac (sorry, I didn’t have a Windows PC to try) the software installs a small orange icon in your menu bar; this icon pulses gently whenever you’re receiving Eye-Fi files – an inconspicuous but reassuring reminder.

As an option, you can see a preview thumbnail of each incoming image – unless you’re shooting raw files, that is, in which case the preview just shows a generic icon. I found this distracting as well as useless, so I leave the thumbnail preview off.

What I do instead is to set up Adobe Lightroom’s “watched folder” option to keep an eye on the folder I’ve set up to receive my Eye-Fi files. The files preview in Lightroom a few seconds after they’re received, so I’m sure I’ve got them.

Of course, the files also are being stored on the Eye-Fi’s 8 gb of memory – so even if something horrible happens to my laptop, I’ve still got the backup images in the camera, and vice-versa. For me, this double protection is one of the key benefits of saving files to both camera and computer.

Drinking the Juice

The Pro X2′s WiFi transmitter is powered by the camera’s battery. And according to the instructions, the transmitter is running whenever the camera is powered on; there’s no way to shut it off if you’re out of WiFi range and just want to use the card as storage.

I had to wonder: does the transmitter suck enough juice to kill battery life?

So I made a simple test: I put the Pro X2 into a camera, disabled the camera’s auto-power-off mode, and ran it until the battery died. Then I did the same thing with a normal non-Eye-Fi 8 gb card. I used a little application called Gawker to shoot timestamped time-lapse photos of the camera so I’d know how long it ran before it quit, and I repeated the test a couple of times each with different batteries.

The results show that, at least with the camera I used, there’s not much difference in power consumption between the Eye-Fi and the standard SD card. The Eye-Fi card seemed to reduce battery life by only about 8 percent. While I can’t claim this test is super-scientific, it’s good enough that I’m not going to worry about reduced battery life when using the Pro X2.

How Does It Compare?

Until the Eye-Fi Pro X2 came along, the only way to transmit raw files wirelessly from your DSLR was to use an expensive and cranky external transmitter, like the Nikon WT-4A I own. Can the $150 Eye-Fi really do the same job as the $700 WT-4A?

Somewhat to my surprise, I found that – within its limitations  – the answer is that it can.

Mind you, the performance of WiFi devices is hugely variable, because so many factors – such as interference and signal absorption – affect how well they work. So, I didn’t even try to make technical evaluations.

What I did do was load up the Pro X2 into my Panasonic Lumix G1 (which shoots 12-megapixel raw images) and hook up my WT-4A to my Nikon D300 (ditto.) I tried out both in some simple real-world comparisons.

Not only did their overall performance match up well, but in many ways the Pro X2 was better – more consistent and more dependable. Here’s how they stacked up.

Range: I didn’t try to measure this directly, because so many things can affect it. All I know is that Nikon claims a range of nearly 600 feet for the WT-4A, while Eye-Fi claims a maximum range of about 90 feet for the Pro X2. The difference isn’t surprising, considering that the WT-4A has a big external antenna and runs on its own battery.

How much does this matter? Well, I know from experience using the WT-4A to shoot ballet productions that it can transmit at least a couple of hundred feet across a theater; the Eye-Fi probably wouldn’t be able to do that. On the other hand, in a typical studio-shooting scenario with the laptop near at hand, either should have plenty of range.

Transmission time: This is always a psychological barrier; when you’re staring at the laptop with the model and the director, waiting for the next image to appear, any transmit time longer than “instant” is going to seem like nearly eternity.

Vertical axis: Average transmit time per raw file, in seconds. Horizontal axis: Transmit distance, feet.

I tested this by shooting bursts of 10 raw files each, timing how long it took them to transmit, then dividing to get an average time per file. I tried both transmitters at several distances. Overall, times get longer as distances increase, presumably because the WiFi hardware needs more retries as the signal fades.

You can see on the graph that both the WT-4A and the Pro X2 ramp up times similarly as the distances get longer, suggesting similar overall performance. But notice how much more variable the times are with the WT-4A.

I know that in general, the WT-4A isn’t really as slow as the tests showed at the 1-foot and 10-foot distances. Normally, its times are in the 13-second range, more or less in line with the Pro X2′s. But I’ve also noticed that the WT-4A is somewhat temperamental; sometimes it will slow down or stop transmitting entirely, for no apparent reason.

I don’t have any equipment to measure it, but I strongly suspect that interference is the cause; for example, when I stupidly tried to microwave a frozen meal for a mid-test snack, the WT-4A went right off the network and had to be restarted. I had no such problems with the Pro X2; it seems that you pay for the WT-4A’s longer range with greater susceptibility to electrical “noise.”

What to Pick?

Of course, an external transmitter such as the WT-4A has other advantages that the Pro X2 lacks. Most can transmit over wired Ethernet as well as WiFi, and Ethernet speeds tend to be blazing-fast. With the right software, many transmitters permit remote operation of the camera as well as sending files. If you know your TCP/IP, you usually can configure them without needing an Internet connection, as Eye-Fi does. And if you’re shooting a big job, an external transmitter plus a fistful of generic memory cards may work out to be more cost-effective than a bunch of Eye-Fi cards of the same total capacity.

But if all you need to do is send raw files wirelessly to a nearby laptop as you shoot, the Eye-Fi Pro X2 is a great alternative to an expensive external transmitter. Just make sure that:

• Your camera is on Eye-Fi’s compatibility list;
• An 8-gb card is big enough for most of your needs;
• Range in the 45-to-90-foot range is long enough;
• It’s not a problem that you need an Internet connection when you want to change configuration settings.

If you can live with those caveats, you may be ready to cut the cord with the Pro X2.