The primary measurement of a lens is its focal length. The focal length of a lens, expressed in millimeters, is the distance from the lens’s optical center (or nodal point) to the image plane in the camera (often illustrated by a "Φ" on the top plate of a camera body) when the lens is focused at infinity. The image plane in the camera is where you will find your digital sensor or film plate. If you are an optical engineer, this is important stuff. For the photographer, however, we do not need to know about nodal points or why the 200mm lens in our closet is only 193mm long, to make great photos. What we need to know, as photographers, is what focal length means to our images. When we talk about lenses, the focal length is not only related to the lenses’ physical length, the linear measurement is representative of an angular field of view.
On a 35mm film camera, it turns out that the angle of view afforded by a 50mm lens provides a field of view that is approximate to the field of view produced by the human eye. (When we say “35mm film camera” we are referring to the size of the frame of film, not the focal length.) We all know that our eyes have a wide field of view and that we also see things off to the periphery of where we are looking—peripheral vision—but, when you look through a 50mm lens on a 35mm camera, what you see is very similar to what your eye sees. Therefore, the 50mm lens, and lenses measured close to 50mm (say 35mm to 70mm, opinions vary) are known, collectively, as “normal” or “standard” lenses.
Before I go on, I need to say a quick word about “crop factor.” Different digital cameras have different-sized sensors. This causes an effective change in the field of view of the camera, but not in the focal length of a given lens. Because the sensor size is independent of focal length, we often speak of the different field of view produced by a smaller sensor as a “35mm equivalent” field of view or focal length. I will be covering crop factor in an upcoming article but, for the purposes of this article, we will be talking about focal length in relation to 35mm film or a full-frame digital sensor, as it is a standard baseline for discussions on focal length.
We already said that the 50mm lens gives us the “normal” field of view perspective. What about lenses of different focal lengths?
If the lens is shorter than 50mm, say, a 24mm lens, then the image produced by that lens will give the photographer a wide-angle perspective of the world before them—wider than your “normal” vision. The field of view of the lens is wider than that of the standard lens.
A lens with a focal length longer than 50mm will give the photographer a telephoto perspective—making it appear that you are closer to your subject by producing a field of view that is narrower than that of a standard lens.
The family of wide-angle lenses includes fisheye lenses that can provide more than a 180-degree field of view; much greater than the human eye, including the periphery, so much that if not conscious of it, you can photograph your feet in the frame when holding the camera to eye level. Telephoto lenses, especially extreme “super telephoto” lenses, can narrow the field of view to where it feels like you are looking through a soda straw, albeit a really big and heavy soda straw!
Prime lenses are those that have fixed focal lengths. Zoom lenses are those that have variable focal lengths. This is accomplished by physically changing the length of the lens, internally or externally.
In relation to focal length, there is not much more you can say about a zoom versus a prime, but it is important to know that there are usually optical tradeoffs for the convenience of a zoom.
One “side effect” of focal length is image, camera, or lens shake. When you handhold a camera, no matter how steady your hands, between your hands and arms and the mechanicals of the camera, things will be moving when you depress the shutter release. This movement causes blur in an image at varying degrees; sometimes not noticeable and other times, ugh.
Unfortunately, when you venture into the telephoto realm of focal lengths, this movement is amplified by the fact that the field of view of the lens is smaller than that of wide-angle or normal lenses. Therefore, it is more difficult to get a sharp image at telephoto focal lengths, especially extreme focal lengths.
To counteract this shake, you can stabilize the camera on a tripod or other support and reduce the duration your shutter is open. The faster the shutter speed, the less movement will be captured. In order to maintain the same exposure, you may need to increase the size of your aperture opening or increase your ISO sensitivity.
The general rule for maintaining sufficient shutter speed for a given focal length, to avoid the appearance of image shake, is to simply use a shutter speed quicker than 1/focal length. Therefore, you should try to shoot a 300mm lens at a shutter speed quicker than 1/300 of a second and adjust aperture and/or ISO to help you achieve that shutter speed.
The other thing that lenses of different focal lengths have an effect on is what is known as “perspective.” To put it very simply, wide-angle lenses distort the scene, and telephoto lenses compress the view.
At first, you might think that to achieve the same field of view with different focal length lenses, all you need to do is move closer or farther from the subject. This is partially true, but the way your image changes will be very obvious, even if the subject is about the same size in an image taken with a wide-angle lens and then a telephoto lens.
If you get close to a subject with a wide-angle lens, the distortion characteristics of that lens will distort the subject. If you don’t believe me, take a portrait of a friend up close with a wide-angle or fisheye lens, and ask them if they like the image. Chances are, they will not.
A standard lens will provide the most normal perspective of a given subject.
When you shoot through a telephoto lens, you will see the image get virtually “flattened.” This means that the image will appear to have less depth—the background behind your subject will appear much closer, and your portrait will be more flattering to the subject.
Again, unless you are designing a lens from scratch, you, as a photographer, are free from knowing the nuances of measuring focal length and you should keep in mind how lenses of different focal lengths affect the way your images look in terms of proximity, distortion, and perspective.