Although technically I could shoot with a 35mm wide angle lens, this is a bad idea. I’d be getting so close to the subjects that their heads would appear distorted and large, and because of the wide angle lens’ greater angle of view, a whole lot of background detail would remain discernible. Such a wide angle of view is impractical for this assignment.
But a telephoto lens is impractical too. With, say, a 200mm focal length, I would have to get so far away from my subjects in order to crop from waist to head, I couldn’t communicate with them easily at that point. So it’s clear, the happy medium for this kind of shoot is likely somewhere between a 50mm and a 100mm lens. It just so happens I have both.
So what’s the difference between shooting with a 50mm lens and shooting with a 100mm lens? If you stand closer with the 50, and a little farther away with the 100, it’s easy enough to make sure the subject remains the same size. You’ll notice subtle changes in the subject’s features between the normal and telephoto lens, but these differences aren’t likely to make or break the shot. So do the other differences that come with these different focal lengths really matter either? Absolutely. And it all comes down to angle of view.
The 50mm has a greater angle of view than the 100mm lens—somewhere in the neighborhood of 40 degrees, compared to the 20-degree angle of view from the 100mm telephoto. That means that to render a subject the same size on the sensor as a 100mm lens, the 50mm lens must be half as far away—from 10 feet with the 100mm lens to 5 feet with the 50mm. The farther from the lens, the wider the area of the scene that makes it into the frame. That means that while the subject might be about the same size, the background is considerably different. With the wider angle of view, there’s a lot more area visible in the background. And it appears in sharper focus as well.
Given that my subject wanted simple backgrounds, the 100mm lens is most definitely the way to go. As evidenced by the samples here, the change from a 50mm to a 100mm lens makes a huge impact on the scene, eliminating distracting background details. That’s largely thanks to the much smaller 20-degree angle of view.
When multiple lenses will work to get the job done for the subject, it pays to think one step farther to what the background should look like, and use that to determine if one lens is actually preferable over another. There’s nothing inherently better about a smaller angle of view; it’s just about matching the effect with the right lens. If you want to see more area in the background, and with greater detail, the shorter lens is the better choice. If you want to simplify the background and compress the scene into a tighter window, a longer telephoto lens is likely right for you. Even though both lenses can render the subject the same size, the effects on the background are dramatically different.