WIDE ANGLE LENS
A good wide angle lens is essential. Any focal length less than 50mm is considered wide. The wider the lens, the more dramatic will be your images. For example, a 16mm lens will produce pictures that have a more exaggerated perspective than a 24mm wide angle. Keep in mine that if you have smaller sensor camera, the focal length of a wide angle is multiplied by the magnification factor and therefore you won’t be getting as much drama out of your lens as you would otherwise see. A typical kit lens, such as the 18-55mm lens, is magnified by 1.6x with Canon and 1.5x with Nikon. The 18mm becomes 27mm with Nikon and 29mm with Canon. These are still wide angle focal lengths, but they don’t have the drama that I like. The two photos below illustrate the kind of exaggeration of perspective I love to create with wide lenses.
I had a Canon 16-35mm lens for years, and while the focal length range is great it just wasn’t sharp at the edges. Then Canon came out with the 16-35mm III, and this is a great lens. Very sharp. I also recently bought the Sigma 14mm f/1.8 for shooting in low light. It's sharp, even at the edges, but it's heavy -- 2 1/2 pounds. I do hand-held HDR sequences with this lens because the large aperture enables me to use faster shutter speeds in low light environments. A favorite lens of mine and one that I use all the time is the Canon 24-105mm. Canon introduced the 11-24mm lens and this is truly incredible. However, it is expensive and very heavy -- heavier than the Sigma. But if you can carry the weight and don't mind the $3000 price tag, it is the primo wide angle lens to own.
On the Nikon side, there is the 14-24mm which is awesome. It’s sharp, and it has a great wide angle range. For those of you who have smaller sensors, I recommend the Canon 10-22mm wide angle lens or the Nikon 12-24mm wide angle lens.