How Single-Lens-Reflex (SLR) cameras work 
The main difference between point- and-shoot cameras and SLRs is the image which one sees through the viewfinder. SLRs allow the user to see the exact same image that is captured on film (refer to diagram on the left), whereas the viewfinder of point-and-shoot cameras is but a window through the body of the camera, which only gives a rough idea of what is captured. When the shutter button is pressed, the mirror is flipped upwards out of the way so as to direct the image onto the screen. That is why the viewfinder blacks out when you take a photo.

SLRs allow the photographer to adjust aperture and shutter settings whereas not all point and shoot cameras do. This allows for greater creativity in taking photos, as illustrated later as well as the ability to take good photos in different situations. There is also the luxury of choosing from a variety of lenses, which bestows flexibility in taking wide angle, telephoto and zoom lens shots.

Lenses
One distinguishing feature of SLRs is the interchangeability of lenses, which gives the camera a range of focal lengths. Focal length is defined as the distance between the lens and the screen where the image is projected. However, this does not mean that a 500mm lens is 500 mm long! The significance of the focal length is the angle of view obtained. The longer the focal length, the narrower the angle of view, and the more magnified the image is (at a fixed distance) as illustrated by the diagram. below. Wide angle lenses are actually short focal lenses which have a larger angle of view. They have focal lengths ranging from 20 - 35 mm. Standard focal length lenses are around 50mm and telephoto lenses range from 80-300mm. Anything above 300mm is considered a super-telephoto lens.

The picture on the far left was taken with a wide-angle 24mm lens and the one to the right of it was taken with a telephoto 300mm lens. Notice how, in both cases, the telephone boxes fill the frame, but the perspective in each shot is very different. Telephoto lenses tend to compress perspective making objects look closer together, whilst wide-angle lenses distort and can make fairly close objects (such as the furthest telephone box) seem very distant.

Aperture – Brightness and Depth of field
The technical definition of aperture value, (or f-stop), is the ratio of the focal length of the lens to the diameter of the lens opening. The size of this hole, known as the aperture, may be controlled by turning a collar usually located at the base of the lens. Typical values may be f2, f2.8, f5.6 and f16 where the "f" denotes the f-stop, often referred to as f-numbers.

F-stops are calibrated to allow exactly half as much light through as the previous setting and twice as much light through as the next setting (in a given time of course). For example, a lens set at f8 will allow twice as much light through as one set at f11 but only half as much as one set as f5.6.

Smaller f-numbers (f2, f2.8 etc.) represent wider apertures (i.e. more light can pass through the lens in a given time) and higher f-numbers (f11, f16, f22 etc.) give narrower apertures (meaning that less light passes through in a given time). This is due to the fact that the size of the aperture is proportional to 1/f-stop value, therefore the larger the denominator (f-stop), the smaller the aperture.

Depth of Field
Depth of field can be thought of as the amount of the image which has acceptable sharpness. This means that on either side of the selected point of focus, there is a region in which the image remains in focus. Moving outside of this band of focus (towards or away from the lens), the image becomes progressively more out of focus.

The amount of depth of field is controlled solely by magnification and aperture. Since the magnification is normally fixed for a given subject, the depth of field is usually controlled by aperture alone. Wide apertures (such as f2, f2.8) give lesser depth of field whilst narrow apertures (such as f16, f22) give much more depth of field.

This control is very important when deciding how to isolate a subject or where to direct the viewer's attention.

This technique of isolating the subject using depth of field is very effective, particularly for portraits or situations in which the viewer's attention should be directed to the subject and the background left out of focus to avoid clutter.