Depth of (Understanding of) Field

Aspiring photogs ask me, “how do I focus on two things at once, like that bird in the tree over there and this branch over here?”

The two key phrases in that sentence are “over there” and “over here”. They denote distance. And when distance comes into play, so does “depth of field”.

Depth of field is simply the distance, from front to back, that the image will be in reasonably sharp focus. I say reasonably, because it’s not as if the focus will suddenly be unsharp beyond these points; it’s a gradual rolloff.

And so comes our first reality check. You can only focus on one plane of a scene at a time, not the subject itself. That “plane” (an imaginary “wall” perpendicular to your line of sight) may be a wall 6" thick, three feet thick, or even ten feet thick. And any object falling within the space of that imaginary wall will be in sharp focus.

Finding Your Wall

This is where the advantages of shooting with old-school manual focus lenses becomes most apparent. The hyper-focal distance scale.

Most manual focus lenses have them. It’s a scale centered around the focus index showing aperture values of the lens from the highest on the outside, to the smallest on the inside. You can use this incredibly simple and yet powerful tool several ways.

First, focus on your subject. Then, look on the scale to see the range of sharp focus for the given set of apertures. At the smallest aperture (largest number) you will find the widest spread. The focus index might be on the 10ft mark, but the f/16 hyper focal marks show 2ft and 20ft. So, when you focus on this subject with the lens’ aperture set to f/16, everything 8 ft in front of it and ten feet behind it will be in focus.

Now. We're going to try something a little different. We're going to focus on that bird in the distance, and read that distance off the focus scale. It comes it at 30 ft, pretty close to infinity. Next comes the tree branch. It’s about 15ft. away. So what we’re going to do is center the hyper focal scale as best we can between these two distances. And what we find is that we can line up the f/11 marks with 30ft, and 15ft. Now, when we take the picture, these objects (planes of focus) will be balanced and reasonably sharp. If we wanted them sharper, we might choose f/16 or 22, if those were available and supported by our ISO and shutter speed, and the sharpness not compromised by diffraction.

Who Needs Autofocus?

Armed with this knowledge, you can, to some degree, alleviate the need for autofocus altogether. Set your lens so the f/11 mark is on infinity. (Avoid higher f-stops, as the effects of diffraction can actually reduce the overall sharpness). Now, at f/11, you can shoot from say, 10ft to infinity (depending on the lens) without the need to focus. This is the way fixed-focus cameras work.

If you look closely at several different focal lengths of lenses, you’ll find that wider lenses have proportionally wider hyper focal scales. This is because wider lenses naturally have greater depth of field. This is why wider focal lengths such as the classic 35mm are so popular amongst street photographers and photojournalists. They are the ideal compromise between speed, depth of field, and angle of view. Set 'em up and shoot, no focusing required.