Focal Length

Focal Length

The focal length of a lens is the distance, in millimeter, from the optical center of the lens to the focal plane when the lens is focused at Infinity. There are two important features to look for in a lens. The first thing is the angle of view. Angle of view tells us how wide a lens can see. A telephoto lens sees a small portion of an area. A wide-angle lens sees a much wider view.

The second thing to look for in a lens is the area of coverage. This is the area the image covers on the film plane. A telephoto lens coverage is much larger than the size of the chip or the film frame, thus it covers only a portion of the full image. A wide-angle lens covers about the same size as the chip, therefore the lens can capture most of the image.

What is wide or a telephoto depends on the format. First, let's see what is normal. A normal lens has an area of coverage that is close to the same size of the diagonal measurement of your chip or film size. On a 35mm camera, a 50mm lens is considered normal. The diagonal measurement of a 35mm piece of film is 44 mm. This tells us that the 44mm is actually a normal lens on the 35mm camera. On a 6x6 square format, an 80mm lens is considered normal. On a 4x5 view camera, 150 mm is considered normal. On most digital cameras, the CCD chip is much smaller than a piece of 35mm. This means a normal lens on the digital camera is smaller than that of 35mm camera. This also tells us that all lenses we use on our 35mm cameras, cover a smaller portion of an image when used on digital cameras.

A normal lens has approximately the same angle of view as your eyes. The circle in the diagram above is the actual coverage. Because the lens is round, the area of coverage is circular. If you place a piece of film or CCD chip inside this circle and measure the area diagonally, you can figure out what would be a normal lens for a given film or CCD size.

Because the CCD chip sizes are different from each manufacturer, a normal lens on one camera may not be normal on another. A 50mm lens on a Cannon EOS D20 with a magnification of 1.6 times compared to that of the 35mm, has an angle of coverage of an 80mm lens on a 35mm camera. The same 50mm lens on Canon EOS 1D Mark II is equivalent to about 65mm since Canon EOS 1D Mark II has a magnification of 1.3 times to that of the 35mm film frame.

Manufacturers use the equivalent-to-35mm method to describe the actual angle of coverage of their lenses when used on digital cameras. Keep in mind that focal length is focal length regardless of the format. Whether it is a digital camera, 35mm camera, or medium format. What you're really concerned about is how much coverage a lens gives you on each given format. Camera manufacturers are now producing extremely wide-angle lenses such as 10mm and 12mm lenses to give wider angle of coverage for use with digital SLRs. A 10mm lens covers the same area on a Cannon EOS D20 as a 16mm lens does on a 35mm camera.

Left: Approximate angle of view of a 200mm lens on 35mm (main image) and that of a digital SLR (inside the box).

You don't need to carry a calculator to figure out which lens you should use to cover a certain subject. I own medium format and large format cameras and can't remember ever having to calculate the millimeters on my lenses and then comparing those figures to my 35mm format before I took a picture. All you need to do is to find out how much coverage each lens gives you. The quickest way is to figure out what is a normal lens on your digital camera. A 30mm lens on a Cannon EOS D20 is equivalent to about 50mm lens in 35mm format. This makes the 30mm lens normal or standard lens on a Cannon EOS D20. Knowing this, you can quickly figure out which other lenses you need to cover different types of subjects. For portrait photography for instance, a focal length of about twice the standard lens gives good working distance for head and shoulder shots . Since a 30mm lens is standard on a Cannon EOS D20, a 60mm lens is a good choice for portrait photography. For landscape photography, a 15mm lens would be adequate.

The only time you ever have to figure out how much each lens covers, is before you purchase a lens. This procedure is the same as before digital cameras were introduced. We did the same thing whenever we purchased a lens for our 35mm or other format cameras. In a short time after using your camera and lenses, you'll know exactly each focal length you'll need for the type of photography you do. Don't think too much about millimeters. Good photography can be done with one or two lenses. I know that because some of my best images were done with my 4x5 view camera and the only two lenses that have for it.