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There are two things that are important to keep in mind when shopping for lenses for an EOS digital camera, such as the EOS 30D, rather than an EOS film camera.
First, EOS digitals are still not that cheap, so don't just try to find the cheapest lens you can. And if you're going to be shelling out this much money for the camera body, buying the cheapest lens you can find is not merely false economy, it's downright foolish, if you ask me.
It's like spending massive amounts of money on a high-end CD player and amplifier and then plugging them into a pair of toy loudspeakers. Just as the final sound of your music will be hobbled by the toy loudspeakers the final quality of your pictures will be hobbled by a cheap lens. So avoid lenses in the cheapie category (see below).
Second, with the exception of the cropping factor the selection criteria for buying a lens for a digital camera really don't differ in any way from the criteria for buying a lens for a film camera. What type of photography do you want to do? Do you need a wide angle for landscapes? A fast-focussing telephoto for sports? A sharp and short telephoto for portraits? How much weight do you want to carry around? These questions are the same regardless of whether you want to shoot film or digital - check the list above.
But as noted, there is one difference to keep in mind with most digital EOS cameras - the cropping factor, also known as the focal length multiplier. With the exception of the EOS 1Ds, 1Ds mark II and 5D, which have full-size image sensors, most current EOS digital cameras use image sensors smaller than a 35mm film frame. If your camera has a cropping factor of 1.6x it means that, say, a lens with a 50mm focal length will suddenly behave rather like a lens with an 80mm focal length. The focal length of the lens does not change, but its apparent effect does. (since the image sensor is smaller, think of it as simply snipping off all four sides with a pair of scissors, resulting in a smaller picture, and then enlarging the picture on a photocopier)
This issue has two nice advantages. First, you can buy a cheap 50mm lens and use it as a very nice portrait lens. Second, you can attach a telephoto to your camera and it'll behave like a much longer telephoto - you'll have much more reach. The drawback, of course, is that wide angle pictures are harder to achieve as you need much wider lenses for such photos. This can be a significant problem for many people.
So with such digital cameras you will probably want to get a lens that's a bit wider than you would if you were buying for a 35mm film camera. If you use a 28-105mm lens on your 35mm camera, for example, you might get a 24-85mm lens instead.
The one question related to this is whether the EF-S 18-55 lens included with the EOS 300D/Digital Rebel/Kiss Digital kit is worth it. The consensus is that it's definitely worth the small increase in price over the body only. The EF-S 18-55 is no L series lens by a long shot, but it offers surprisingly decent optical quality for the incredibly low price, and it's the only affordable way to get down to moderately wide angles (roughly the same as a 29mm lens on a 35mm film camera) on the 300D.