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Focal Length Focal length, in its simplest and most practical interpretation, is a measure of how close a lens makes an object appear in a photo. If you are using a zoom lens, you are changing the focal length when you zoom in and out. The focal length is expressed in millimeters (mm), but be advised that focal length values are only comparable between two different cameras if those cameras have the same film or sensor size. A few years ago, when film photography was more prominent, the almost ubiquitous film size was 35mm. This means that the length of an individual film frame is 35mm. (Actually, for some reason it's 36mm, and the height is 24mm.) Higher end digital SLRs use a sensor size roughly the same size as a 35mm film frame, but most dSLRs use something smaller. Even so, the 35mm 'format' is still often used as a standard of comparison. You will frequently see the 'equivalent 35mm focal length' mentioned in discussions of lenses compatible with other film or sensor sizes. The remainder of this discussion will use equivalent 35mm focal length values for all examples.
Magnification and Focal Length For an object at a fixed distance from the camera, a lens with a long focal length will give a greater magnification than a lens with a shorter focal length. Magnification can also be increased by moving the camera closer to the object. There is a limit to how close a lens can be to an object and still focus on that object. The minimum focus distance tends to be a great deal shorter on shorter focal length lenses. Consequently, it is not necessarily the case that longer focal length lenses have a higher maximum possible magnification than shorter focal length lenses. If we confine ourselves to objects that are relatively distant from the camera, then it is the case that the longer the focal length the higher the magnification. The relationship between focal length and magnification is ambiguous at best. Time for something unambiguous—angle of view.
Angle of View The angle of view decreases as the focal length increases. For a given film or sensor size, this is always the case. End of story. Now for the sequel...The angle of view has ramifications that may not be immediately apparent. Let's consider two types of lenses that are familiar to most people, one with a very wide angle of view (the front door peep hole) and another with a very narrow angle of view (the telescope). With these examples in mind, imagine how lenses with different angles of view might be useful in photography. Wide angle (short focal length) lenses are useful when you want to take a picture of something close and you can't get farther away. Narrow angle (long focal length) lenses are useful when you want to take a picture of something far away and you can't get closer. Sometimes even when you can get farther away, you may still wish to use a short focal length lens to include more in the background. Sometimes even when you can get closer, you may still wish to use a long focal length lens to magnify the background.
Zoom vs. Prime Zoom lenses have the ability to be used throughout a range of focal lengths. Removable zoom lenses have an extra adjustment ring that controls the focal length. There will be a numeric scale on which the selected focal length will be indicated. The range of focal lengths shown on the scale is the zoom range of the lens. While the ability to zoom can be a tremendous advantage, the ability to change focal lengths usually comes at the cost of optical quality and speed. Optically, zoom lenses tend to have more distortion than prime lenses. Zoom lenses are also slower than prime lenses, meaning they have a smaller maximum aperture. Prime lenses have only one focal length, but they have the advantage of simplicity of design. This simplicity allows for higher optical quality and speed. Higher optical quality means more realistic images in terms of sharpness and color. Higher speed means faster, easier focusing and faster usable shutter speeds when needed. Prime lenses have one other thing to offer, and that is a depth of field scale. A depth of field scale can be used to set the hyperfocal distance.
Wide, Normal, and Telephoto The focal length of lenses fall into three main categories: wide, normal, and telephoto. By now, you know what a wide angle lens is. Thus far we have used the terms narrow angle and long focal length to refer to telephoto lenses. Normal lenses haven't been mentioned. They are a special case in that they are said to reproduce the field of view of the human eye, resulting in a more realistic perspective. Not everyone agrees with this, but it's worth mentioning. For the 35mm film or sensor format, the normal focal length is generally considered to be 50mm.
Conclusion Please don't confuse focal length with focus distance. Recall that focal length is given in millimeters. Most removable lenses have a scale on them labeled in feet or meters (perhaps both). This scale is the focus distance scale and usually ends with the symbol for infinity (∞). When you focus your lens on an object at a certain distance, that distance should be indicated on the focus distance scale. Focus distance is unrelated to focal length, magnification, or angle of view.
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