Here is a piece that I wrote for the Seattle Photographic Society's Cable Releases newsletter this month:
The focal length of a lens determines the size of the image projected onto the film/sensor. It also determines the angle of view, the area of the scene which will be recorded. Different focal length lenses are given descriptions like normal, wide, ultra-wide, long, telephoto, etc. These are not absolute designations. For example, a 50mm lens would be considered "normal" for a 35mm camera, but would be a wide angle lens on a 6x6cm (medium format) camera. And 50mm would be considered a moderate telephoto on most (non full-frame) digital SLRs that have a sensor that is smaller than a 35mm fill frame.
The shorter the focal length of a lens, the smaller an object will appear on film (I will use the word film to describe actual film or a digital sensor in this article) when the subject remains at a set distance from the camera. Similarly, a longer lens will make the subject larger without moving the camera position. Focal length and image size are directly proportional. Double the focal length and everything in the image doubles in size.
Different focal lengths present different challenges to the photographer. Short lenses have a wide field of view and often include more of a scene than we really want unless we move in closer (which changes the perspective). Long lenses magnify more than just the subject. They magnify any camera movement causing blurs, leading to the necessity of using a tripod or other stabilization device.
Choosing a lens focal length is pretty easy when you are forced into the selection. If you cannot back up, use a shorter (wider angle) lens. If you cannot get closer, use a longer, narrower angle lens. There! The decision is made. Go take your picture.
But what about the creative side? When you do have options, which lens do you use? Sometimes you have all the room in the world to back up. But you might still want to use a wider lens because you want to get closer to your main subject, yet take in a field of view that shows the surroundings. Examples of this would be coming in close on a rock or plant in the foreground of a landscape scene. By coming in close you alter the perspective of the scene to make the foreground object appear much larger in relationship to the background (or the background much smaller in relationship to the subject). The use of the wide angle lens allows you to capture this. If you used a longer lens from further away you could get the same size for the subject, but the background would show less of the environment, helping isolate the subject. Both are viable treatments, but the looks are very different.
Conversely, you might be in a situation where you are able to walk right up to within inches of your subject. But instead you might select a longer lens to force yourself to move back from your subject, thereby giving you a more normal perspective on the scene. Most of the time we think of this in relationship to pictures of people (portraits). We want to stay a minimum of about 5 or 6 feet back from the subject of the photo so that their facial features are not distorted. We can keep our distance and take the photograph with any lens (the distortion comes from the camera to subject distance, not the lens). Here, the selection of lens focal length here will determine the framing. With a short lens at 6 feet we might get the entire person in the photo. If we want just their head and shoulders we have to crop in and lose quality due to the cropping and enlargement. We cannot move in closer, or things will distort. But if we select a longer lens (maybe 2 times the focal length for the "normal" lens for our film format (35mm, APSc, 6x6cm, etc.) we can maintain the working distance and fill the frame better. As noted above, moving back and narrowing the field of view also helps isolate the subject and make them stand out from the background.
What is not always so obvious is that the same thing applies to landscape photography. In my opinion, too many times a photographer puts on a short lens for a landscape image and is disappointed with the results. There is too much foreground or too much sky, or both. The image can often be improved by using a longer lens and moving back. This is not the same as cropping in. If the photographer kept the camera in the same position and switched to a longer lens it would have the same visual effect as cropping. But what I'm suggesting is moving back so that the perspective changes.
Here is an example. Take a field with a tree in the foreground and a house in the distant background. With a 50mm lens we stand 20 feet from the tree to render it a nice size in the image. But we find the house to be too small in the background. By moving back to 40 feet away and changing to a 100mm lens we find that the tree remains the same size because we balanced the the doubling of the distance with the doubling of the focal length. But the house is now noticeably larger in relation to the tree because the distance to the house has been changed a smaller amount. If the tree was originally 20 feet away and the house was 50 feet away, moving back 20 feet to 40 feet doubled the distance to the tree, but only changed the distance to the house from 50 feet to 70 feet. This is sometimes referred to as "telephoto compression," but is really based on the camera to subject distance, not the lens.
To zoom or not to zoom?
That is the question. While it is nobler in the eyes of some photographers to use prime lenses (fixed focal length, non-zoom lenses), I offer the contradiction that a zoom lens gives you much better control over the final image. Years ago zoom lenses were often of inferior optical quality, so prime lenses were required for the best image. But many of today's zoom lenses offer quality rivaling that of prime lenses.
We've all probably heard the saying, "zoom with your feet, not with your lens." I have to disagree. You cannot zoom with your feet. Zooming implies maintaining the relationship between objects in the frame. Once you start moving the camera, the relationship between objects changes. You set the camera to subject distance to give the overall look of a scene and set that relationship. You then select the focal length lens to fill the frame. With prime lenses you often cannot frame the scene exactly as you want it and have to later crop in post processing (digital or dark room) or move and change the photo. With a zoom lens you have the ability to keep the camera stationary and alter the focal length to get the cropping done in camera. If you zoom with your feet (move in closer or move back) you change the relationship between the objects in the photo (not necessarily a bad thing, but it is a different picture).
Many book chapters have been written about the differences in seeing between eye and lens. I strongly recommend picking up one of the many books by Andreas Feininger written in the late 60s or early 70s for detailed discussions of this. I will offer here some of his observations:
1. While a lens mechanically reproduces everything within its field of view, the eye/brain combination consciously perceives only those aspects that you are momentarily interested in. It disregards and doesn't notice the rest of the scene. The lens is objective, the eye/brain is subjective. The photographer has to learn to pay close attention to everything within the image, no matter how insignificant or dull it might seem. It is always those "unimportant" parts of the image that ruin the photograph.
2. The visual impression is only a part of what we take in. There are sounds, smells, vibrations, and possibly other people around us that all contribute to what the eye/brain sees. The lens only captures the visual part. The photographer must learn to mute senses other than sight and then be satisfied that the visual aspect of the subject is enough to reflect its essence.
3. A photograph presents a subject taken out of context. There is a frame around it. Peripheral vision is cut off by the boundaries. No matter how important those peripheral influences are in real life, they are not in the photo. The beautiful blue sky doesn't help when you come in tight on a clump of flowers, though it might have been part of what drew you in to take the close up photo. The photographer must learn to evaluate his subjects on their own merits and disregard everything outside the scene.
4. Because our eye/brain works so quickly, we sometimes think that we see everything sharp simultaneously, no matter its distance. But the camera lens can only be focused on one specific plane at a time, showing objects in front of or beyond this plane increasingly blurred the further they are from that plane. And at the same time, the eye/brain can be very selective and let you concentrate on just a part of the scene, whereas the camera lens, when stopped down, will give equal sharpness to the entire scene and remove the concentration. The photographer has to learn how and when to control the sharpness.
5. The eye/brain can adjust automatically to brightness levels in a scene as we look around the scene. The lens diaphragm stays the same throughout the exposure, and can lead to both over- and under-exposure in the same scene. The sky blows out white, the trees turn to black and only a little of the mid-tones appear as we expected. The photographer has to learn to recognize excessive subject contrast and control it (fill light, reflectors, or just move on without taking that picture). Similarly, the eye/brain adjusts to changes in the color of light. Whether in daylight, tungsten light, etc. a sheet of white paper appears white to our eyes. Not so to film. The photographer has to learn to control white balance with camera settings, filters, or post processing.
Theory to practice
Of course a lot of this photographic theory goes out the window in the real world. But I think it is important to have the theory buried someplace in the back of your mind. I'm the first person to admit that I often don't see the photo until after I take it--sometimes seconds later, sometimes a month later when looking at a set of images again. Much of the process, for me, is subconscious. Something catches my eye and I react. Other photographers are very methodical and go through pre-visualization processes, knowing everything about the image and its final print before clicking the shutter. As in most things artistic, there isn't a right or wrong way. Find the way that works for you and have fun with it!