Most photographers would say their lens is their most critical piece of equipment. It is used for many vital aspects that make an excellent photo. These include focusing on the subject, the amount of scene captured and passing light to the camera's sensor. Therefore, choosing the right lens for the type of picture you are taking is crucial to the success of your final image. Lenses are divided into groups, depending on the field of view they give. When following a photography course online, it helps to know which lens to use for each situation, or assignment.Wide-angle lenses cover a field of view from approximately 110 degrees to 60 degrees. You can get some distortion when using extremely wide-angle lenses, noticeable around the edges of the frame. Any lens of 35mm or smaller would be considered wide-angled. Wide-angle lenses normally display a large depth of field, which keeps the foreground and background in focus. This is ideal for landscape photography. They also work well indoors, if space is tight, as the lens will be able to capture more of the scene. Wide-angles work well for shots of large groups, but would not be recommended for close-up portraits.A normal lens has a field of view of about 45 degrees - roughly the same as the human eye. The normal lens for full-frame or 35mm cameras is 50mm. Normal lenses are ideal for photographing people, architecture, and most general needs of a photography course online. There is little distortion and medium-range depth of field.Longer focal range lenses are referred to as telephoto. They vary in length from 135mm to 800mm plus. Their field of view is 35mm or under. These lenses allow you to capture distant objects in detail, although the angle of view is reduced to the magnification of the scene. You will achieve a much narrower depth of field. Telephotos are useful for sports photography, or for a situation where you need to get up close to your subject.A zoom lens can be a fine compromise, saving the need to carry a collection of fixed focal length lenses (also known as prime lenses). They cover a wide range of lenses, because of the configuration of their optics. This configuration can compromise the image sharpness as the light has to pass through more glass before reaching the image sensor. However, this can be hardly noticeable to the naked eye. Zoom lenses also typically have smaller maximum apertures than prime lenses. This influences the amount of depth of field possible in low-light settings - although many zooms have 'image stabilisation' built in to counter this.A Digital SLR will normally be available to purchase with a kit lens - the most popular include 28-135mm, 17-85mm and 18-200mm. I currently use a Canon50D SLR, and although I have a range of prime lenses, I find the 18-200mm covers the majority of situations. I found this particularly helpful when I was on the move carrying out my own photography course online assignments, negating the need to carry around a whole array of lenses.