Let’s talk about focal length, depth of field and angle of view. How do they change based on lens selection? A wide-angle lens up close to a subject and a telephoto lens from far away may render something the same size on the sensor, but still, the images will look very different. Why?
Depth of field is the area of the frame that’s in sharp focus. A shallow depth of field may have a sharp center of interest with a blurry, out of focus foreground and background, while a deep depth of field can show both the subject and background sharp. Learning to control depth of field is essential as it helps to determine the center of interest—whether it’s broad or isolated.
Every photographer learns early on that aperture is the way we alter depth of field. We choose a large aperture (with a small ƒ/ number like ƒ/1.4 or ƒ/2) when we want a shallow depth of field. And we choose a small aperture with a large number like ƒ/16 or ƒ/22 when we want greater depth of field. And that’s the end of the story, right? Not so fast.
It turns out that the focal length of the lens also impacts the depth of field—or at least the appearance of it. When we change lenses from a wide angle (a 24mm, for instance) to a telephoto lens (70mm, let’s say) without changing the aperture, it appears that the depth of field decreases with the telephoto lens. In shorthand, telephoto lenses appear to create shallower depth of field and therefore do a better job of separating subject from background.
This is true in the practical sense, although the optical physics are a bit more complicated. In fact, focal length in and of itself has no impact on depth of field if the framing is recomposed to maintain the same subject size. In practice, that would entail moving farther away when switching to a telephoto lens or moving closer when changing to a wide-angle lens. At the extremes, this kind of movement is so impractical as to be all but impossible.
Overzealous photo geeks (myself included) will sometimes rush to say, “Focal length doesn’t impact depth of field!” But this is being a bit pedantic because in practice most photographers are changing lenses to change the angle of view and we’re not usually repositioning ourselves to maintain the same subject size. In fact, it’s this change in subject size that we’re often after!
So the truth is that in practice, focal length does change the appearance of depth of field because of the change in the field of view. A telephoto lens’s narrower angle of view makes distant areas of the background larger in the frame, while a wide-angle lens makes those background elements smaller in the frame. This is also why wide-angle lenses are more forgiving of focus mistakes and why it’s harder to create the dramatically shallow depth of field that isolates a subject from the background while using a wide-angle lens.
More importantly, changing lenses also changes the angle of view and what’s included or excluded from the frame. You can move to a wider lens and get closer to the subject, maintaining that subject at the same size on the sensor, but the photograph will still change significantly. With a wider lens and its greater angle of view, scenes will include more of the background whether you want it or not. Moving back and switching to a telephoto lens—as with the orchids shown here—may maintain the subject size but the background becomes much simplified. This is a function of the narrower angle of view having the effect of compressing the scene, even without a change to the aperture.
So what’s the takeaway? I find that, in practice, I use this knowledge to impact my lens choices based on whether I want more or less depth of field in a scene—or its appearance, anyway—and whether I want to incorporate more background for context or less to isolate the subject. If I’m trying to drive the focus solely to the center of interest and throw out all ales, I’m going to choose a longer focal length every time—not just because of the appearance of shallower depth of field, but because of the tighter angle of view.