The aperture range of a lens is the another factor to consider. The aperture is the iris built in the lens, which controls the amount of light allowed to enter the lens. The aperture also controls depth of field. Depth of field is the term which describes how deep into the picture the focus goes. On SLR lenses, focus depth can range from a couple inches to infinity. When an aperture closes to a smaller hole you get a higher number (example: f/16, f/22). This creates deeper focus. This also decreases the amount of light able to enter the camera. If you want deep focus in an image, there must be enough light to support a slower shutter speed. In bright sunlight, this may not be as much of an issue, but in most shooting conditions a tripod would be helpful. The opposite is also true. When you have a bigger hole, meaning more light and a lower f/- number, you get shorter focus. More light and faster shutter speeds will give you the ability to photograph in more situations without having to use a flash.
Many SLRs have a depth of field preview button. This button stops down the aperture without taking a picture. This will give you an idea of how deep the focus looks without taking a picture. Since this button does stop down the aperture, your viewfinder will get dimmer.
Depth of field and using the aperture can really help your photography as well as helping you decide what lenses you may want with your camera. SLRs have an exposure mode, aperture priority, which will allow you to choose the aperture, and the camera will choose the shutter speed.