When thinking about focal length, many people only think about longer reach or wider field of view. Remember that you are not static, at the same spot. Good news, you can move your feet (or your wheel chair). Three things are important to understand about focal length and how they affect your lens choice.
- Angle of view
Usually, the wider your lens, the bigger will be the perspective distortion. If you do portrait photography, you don't want that, except if you want to do funny or creative shots. It is not flattering to the model. On the other side, choosing a longer focal length for portraits will help the person look thinner.
In the example below, the two shots were taken at the same distance from the subject. It reduces the perceived distortion. Wide angle lenses can be nice for lifestyle photography. I have a soft spot for the 35mm focal length.
On the other hand, wide angle lenses add a feeling of vastness/immensity to landscape photography. I especially like it when the sky is partly cloudy.
Field of view
The shorter the focal length, the wider the field of view you get (hence the name Wide Angle Lens). In the following photos, I kept the composition the same: the little girl is in the center of the photo and is the same size on every photos. It is easier to see the effects of choosing a lens over the other. The focal lengths used are: 14mm, 35mm, 50mm, 100mm, 140mm and 200mm, all shot at f/2,8.
Understanding this is very important to choose the right lens for the right job. A couple of years ago, I was shooting a wedding at a church that was undergoing repairs. There were trucks and scaffolding on the right side of the church but the front was nice and clean. For the group shot, instead of choosing a wide angle to show the whole church, I used the 70-200mm and crossed the street to take my shot. On the final image, you would never know there was a construction site on the right side.
In the case of the photo of the little girl, I would choose the 200mm lens because it eliminates all the distracting elements from the shot.
A longer focal length will compress the background. The objects will seem closer. If you watch the cedars at the back of the photos, they seem to get closer at 200m than at 14 or 35mm. Once again, here are the six photos side by side so you can see the difference. The background is getting closer when we use a telephoto lens. Since we have to move backward to keep the same composition, the relation between the photographer, subject and background changes. The distance between the photographer and the subject is longer but the one between the subject and the background stays the same. That explain why the background seems closer to the subject.
The perfect lens does not exist. It always depends on the situation. Making the best lens selection for a specific job is a big part of being a photographer. The three things explained here can help you decide which lens to use but you also have to factor in the type of photography session you have (lifestyle, studio, etc.) and your shooting style. Knowing how a lens affect the look of your image is an essential skill to master though.