The most common measure of a lens is the focal length – designated by a length in millimeters. The focal length refers to the magnification capability of the lens. The image circle projected onto the focal plane varies in direct proportion to the focal length of the lens. The longer the focal length of the lens, the more it magnifies a subject at a given distance.
To understand the concept of focal length, we need only look at a simple double convex lens, more commonly called a magnifying glass. In your younger years, you may have taken a magnifying glass outside on a bright sunny day and tried to burn a hole into a piece of paper. You would hold the lens, perpendicular to the sun and adjust it until you had a very bright spot focused onto the paper. In a few moments, the focused energy would cause the paper to burn. With this simple activity you calculated the focal length of the magnifying glass. If you were to measure the distance between the center of the lens and the focused image of the sun on the paper, you would have the focal length of the lens.
Measuring the focal length of today’s compound lenses is far more complex and you can have a lens that is physically shorter than the designated focal length. Even so, lenses of like focal lengths will produce images of like magnifications.
We classify the focal length of lenses into three major groupings, wide angle, normal and telephoto. Lenses can’t be classified, however, without knowing on what camera body a lens will be mounted.
For most photographers, when we talk about focal length we talk in terms of using a lens on a 35mm film camera, but the magnification of a lens changes depending upon the digital sensor size in the camera. For example, look at the images of Poseidon. The image at left illustrates the magnification of a lens using a full frame sensor or 35mm film.
Many digital cameras have sensors that are actually smaller than a piece of film so the resulting image is magnified even more. See for example the image below. It represents the same scene shot with a smaller sensor and the image of Poseidon is magnified.
You’ll find magnification factors listed for many digital cameras. In Canon’s marketing material for the new Canon Rebel XSi, it states “(35mm-equivalent focal length is approx.1.6x the lens focal length).” This is because the Rebel XSi uses a CMOS sensor that measures 22.2 x 14.8mm where a 35mm frame is 36 x 24mm.
Don’t get hung up on the math, but you need to understand why you want to classify lenses in the first place. You’ll find that certain lenses share properties that affect the way your image appears and you may want to avoid or embrace these features.
A normal lens comes closest to our human vision. When you hold the camera up to your eye and look through the viewfinder, it should look like your peering through a window. The subject appears no closer or farther away.
As stated above, the classification of a lens as a normal lens is based upon both the focal length of the lens and on the size of the sensor in the camera. A Normal lens is going to have a focal length that is the same – or close to – the diagonal measurement of the sensor. Remember all that a-square plus b-square stuff in school. Well, there really is a use for it.
For a 35mm film camera, the film is 36mm x 24mm. The diagonal measurement is about 44mm. So, for a 35mm film camera, a normal lens is 44mm, although in reality we classify lenses anywhere from about 40mm to 55mm as normal lenses for such a camera.
What about for the Canon Rebel XSi described above. In this case, the diagonal measurement of the sensor is about 27mm so for this camera, a normal lens is anywhere from about 25mm to 35mm.
Wide Angle Lenses
The easiest way to classify a lens is to forget all the optics and math and to simply hold the camera up to your eye. If subjects in the viewfinder look smaller and farther away, then the lens is a wide-angle lens.
Wide-angle lenses have a broader angle of view showing more of a scene than a normal or telephoto lens from the same location. These lenses also have a considerable depth of field. For example, when I use a 20mm lens on a full-frame camera I’m able to bring subjects into focus from 1-½ feet all the way to infinity.
Wide-angle lenses also show both real and perceived distortions. One such real distortion is the curvilinear distortion that is created with such lenses. You’ve seen this in images of buildings where the straight architectural lines seem to bend inward as you look at the top of the building. To some extent, such distortions can be corrected but this adds to the cost of the lens.
A perceived distortion is the perspective created by a wide-angle lens. Images that are closer to the camera appear larger and since a wide-angle lens can be focused very close to a subject, you can get an exaggerated size relationship between subjects in the image. At times you’ll want to avoid such distortions – portrait subjects seldom want their noses to appear out of proportion. But, you can also take advantage of this effect, creating a sense of depth in a beautiful landscape.
A telephoto lens provides more magnification and a narrower angle of view than that of a normal lens. These lenses are great when you can’t get close to a subject. You can get a wonderful frame-filling image of a venomous snake with no worries.
Telephotos – or long lenses – also have a shallower depth of field, allowing you to isolate your subject against a blurred background or foreground. As such, they are excellent lenses to use for portraits.
Zoom lenses are the most common in today’s camera bags because of their incredible flexibility, combining a wide range of focal lengths into a single lens. For years photographers were wary of such lenses because of perceived lower quality but today’s zoom lenses are incredibly sharp with unparalleled results.