In photography, there are wide angle lenses and telephoto lenses. But there is also the "normal lens." If you ask different photographers to define what a normal lens is, you may get more than one answer.
Many photographers will tell you that a normal lens is one that has a 50mm focal length. In the day of 35mm film cameras (hey, for some photographers, we are still in "that day"), that's what it was.
If you ask other photographers, they will tell you that a normal lens is the one that will produce an image that is what your eye can see in a scene. This means that the image would show the width that is visible to the human eye without scanning from side to side.
Normal Lens - The "Real" Definition
As far as a technical definition, normal will be different for each digital camera if the size of the image sensor is different. The way to calculate the focal length that would be considered "normal" is to measure the diagonal length of the image sensor, and normal will be equal to this measurement. According to this technical definition, professional full-frame cameras such as the Canon 5d Mark II and the Nikon D300 have a sensor with a diagonal measurement of about 43mm, therefore, they would still have a normal lens length of close to 50mm, which is similar to a 35mm film camera.
As you move down the line of digital SLR cameras, you have entry-level and mid-range digital SLRs with smaller sensors. These are commonly called "crop" sensors because the images produced by them are cropped; they look like full-frame images produced by longer length lenses. These crop sensors have an approximate diagonal length of 27mm. According to the definition, that would be normal, however, most photographers consider a 35mm lens to be normal for this group of digital cameras rather than a 27mm lens. The resulting images have the same perspective as those produced by full-frame digital SLRs outfitted with a 50mm lens.
The Benefits Of A Normal Lens
While a normal lens does not usually come as part of the package when you buy your digital SLR camera, many photographers consider a 50mm lens to be their most valuable asset. There are a few different versions of 50mm lenses, but even the least expensive ones have gained favor in photography circles. This is because 50mm lenses usually have a very large aperture, and they produce exceptional quality images. The wide aperture allows much faster shutter speeds, which is quite helpful when dealing with low light situations. It also gives the photographer the ability to take pictures with sharp focus on the subject with a blurry background. This feature makes 50mm lenses excellent choices for portrait photography.
In addition to the wide aperture, the images from a normal lens have a natural perspective, which is sometimes not true of telephoto or wide-angle images.
Since the 50mm lens has been the "go-to" lens for professionals since the advent of 35mm film cameras, the optics have been developed and maintained as some of the best in any photography lens.