Other than asking me what camera they should buy, the question I get most from friends (and students) with DSLR cameras is: What lens should I buy next? Most people buy their DSLR as a kit these days and that usually includes a fairly substantial zoom lens that covers a range from moderately wide angle to moderate telephoto. These lenses are typically lightweight, fast-focusing and, for the optical range they cover, offer a lot of flexibility.
The larger issue for me is that these lenses are restricted to a fairly moderate (i.e., creatively safe) optical zone. You get a moderate wide-angle lens through a moderate telephoto lens range and that's it. I much prefer working with optics that are capable of providing a more extreme view of the world--either extremely wide-angle or extremely telephoto. Let's look at wide-angle lenses first and in the next posting I'll talk about teles.
Most consumer-level DSLR cameras have a DX sensor that is about one-third (in terms of diagonal size) the size of a 35mm frame of film. One of the byproducts of having a digital sensor that is smaller is that, because the sensor is only seeing the center of the lens' coverage, the focal length of whatever lens you're using is effectively longer. This magnification is called the cropping factor. Typically you have to multiply the focal length of a lens designed for 35mm by a factor of 150% (it varies by camera and sensor) to get the effective focal length. (And again, I'm only talking here about cameras that have a smaller sensor; lenses used on digital SLRs that have a full-frame sensor provide the same coverage that they would provide on a 35mm film camera.) This is great if you're a telephoto lens user (if you often shoot wildlife or sports, for example), but not so great if you like using very wide-angle lenses.
If, for example, you're using a 24mm lens--which provides a stunning wide-angle view on 35mm--you're working with what is effectively a 36mm lens on a DSLR. That's only a moderate wide-angle at best and is actually closer to a "normal" lens. In order to get really wide-angle effects with your DX-sensor DSLR you'll need a much wider lens. I recently bought a 10-20mm Sigma ultra wide-angle lens and I love it. In terms of 35mm lenses it provides the angle of view of a 15-30mm lens and so restores the ultra wide-angle view that I'd get with a lens in that focal-length range. Nikon makes a 12-24mm lens that provides coverage equal to an 18-36mm lens on 35mm and Canon's EF-S 10-22 USM is approximately the same.
The beauty of a really wide-angle lens is that it exaggerates spaces like crazy and can elongate shapes and provide a much greater feeling of depth in all kinds of photos from landscapes to still lifes. I shot the photo of the old freight train show here (shot in Greenville, Maine) using the Sigma 10-20mm lens specifically to exaggerate the shapes and length of the train cars. Look at how that near car seems to be looming over the rest of the train and really draws the eye into the composition.
If this type of visual exaggeration appeals to you, you might want to consider adding an extreme wide-angle lens to your DSLR kit. I would suggest getting something that offers a minimum focal length in the range of 15-22mm (in terms of 35mm coverage), so 10-15mm in the DX lens format. (I wouldn't go for a "fisheye" lens as your first ultra-wide lens, but I will talk about those in a future posting.) In the next posting I'll talk about the advantages of adding a more extreme telephoto lens.