To put it simply, focal length is similar to a magnification factor. The longer the focal length (the larger the number in millimeters), the closer it will appear that you are to your subjects, and the narrower the view will be. A short focal length will allow you to see a widerangle view and make it appear that you are further away from your subject. Focal length, in technical terms, is the distance in millimeters between the surface of the image sensor and the center, or emergent nodal point, of the lens. It is also a number that specifies the amount of magnification you get when you look through your camera. If you have used a 35mm film camera, you may be familiar with the 28mm, 50mm, 100mm, or possibly even the 400mm lenses. If you have used several lenses with different focal lengths with your 35mm film camera, you may have developed a quite good sense of the angle of view or the degree of magnification you get with each lens. You use this sense to pick lenses and to pick the place where you want to stand to shoot each photo.
Unfortunately, digital cameras have image sensors that are usually smaller than 35mm film. In order to project an image on a smaller area, the lens focal length must be shortened by the same proportion. This means that you will have to get familiar with an entirely new set of focal lengths—maybe even a different set for each digital camera you use!
For instance, the Canon PowerShot G2 has a zoom lens with a focal length ranging from 7mm to 21mm. Canon publishes the 35mm equivalent as being 34mm to 102mm. If you divide 34mm by 7mm, or 102mm by 21mm, you get a multiplier of 4.85. Why do you care? You will only care if you are used to thinking in terms of the equivalent 35mm focal lengths and you want to know what they are. When you read the EXIF data found in the image files, the listed focal length is the actual focal length—not the 35mm equivalent. After you have
computed the multiplier for your camera, you can easily convert these actual focal lengths to the 35mm equivalent focal length if they are more meaningful to you.
Depending on your digital camera, you may have one or more ways to change focal length.
All compact digital cameras today come with a built-in zoom lens, which allows you to change the focal length between a minimum and maximum length. An increasing number of manufacturers also make supplemental lenses. Supplemental lenses are “add-on” lenses that you add to an existing fixed or zoom lens. You find out more about using supplemental
lenses some other posts. The last way to change focal length is to use a digital zoom feature. Okay, so much for definitions. The purpose of this technique is to get you familiar with the view you get with your digital camera and any accessory lenses you may have. This familiarity is essential if you want to know where to stand and what lens or accessory lens to use to get the shot you want. It is also important to learn about the implications of using different focal lengths.
See also: Controlling Exposure, Understanding Exposure.
And of course the Photography tips page with a list of articles that might help you improve your photography skills.