Understanding Depth of Field, or DoF, is one of those topics in photography that may not be as intuitive as it may seem.
Most people grasp the concept that the wider your lenses aperture is open, which translates to a lower f-stop number, the less distance is in focus on either side of the focal point. The narrower your lens aperture, which is a higher f-stop value, the greater the distance in focus on either side of the focal point.
That’s the simple explanation, it gets more complicated from there. The reality is the focal distance on either side of the focal point is not a clarity cliff that stays sharp until you hit the edge and suddenly falls off. DoF is actually a continuum where the focus gradually falls off to the point it’s visible with the human eye. There’s no critical point in transition, until you can see it.
There’s even a term for that nebulous spot, caused by the light rays not coming into perfect focus, it’s called the “circle of confusion”, also known as the disk of confusion, and the blur spot.
The standard to determine an acceptably sharp circle of confusion is the distance a single point becomes so out of focus it’s noticeable on a standard 8×10 inch print viewed at a distance of 1 foot.
The circle of confusion changes not only at every f-stop, but it also changes with the lens focal length (how far it’s zoomed) as well. At smaller focal lengths, the DoF tends to increase, all other factors staying equal. Another factor is the distance to the focal point, how far away the subject is.
As you can see there’s a lot of subjective observation in that standard and, indeed, human eye perception is much finer than the standard lens manufacturers use. If that isn’t confusing enough, the circle of confusion is different for every print size and viewing distance. So take the DoF marks on your lens with a grain of salt and understand that near the outside edge, you likely see the difference.
To bring this around to some practical application, if you need depth of field, a good rule of thumb is to use the aperture priority mode on your camera and use at least f/8 or f/11. In most lenses the circle of confusion falls off quickly at f/5.6 or lower. If you’re shooting for bokeh, deliberately throwing the background out of focus, you’ll be shooting at f/5.6 or below.
Ironically filmmakers were drawn to fast lenses for the very capability of throwing the background out of focus! It’s not just still photography where the subject comes up.
DoF is another one of those subjects where it’s important to experiment on your own equipment and understand where your lenses fall in the circle of confusion. Don’t trust the factory marks, get out there and shoot some samples.
There’s no substitute for gaining an intuitive grasp of your lenses DoF and the only way you can do that is get out and practice. Every lens will be different. This should be a ritual the first time you get a new lens.