Aperture: getting to know your DoF

Beautiful view in the Alps, Switzerland
(Aperture f/20, Shutter Speed 1/125 sec, ISO 200) with 16mm lens; DOF is infinite
The Aperture determines the amount of light that goes to the sensor. At the same time it also determines which area of the picture is sharp. A wide open Aperture has a much smaller Depth of Field (DoF) then an almost closed Aperture. For example, for a 50mm lens the DoF at 2 meter is:

  • 26.6cm for an Aperture of f/2.8, and
  • 100.1cm for f/11.0.

This is quite a difference. Also, the DoF decreases for longer lenses. For example, for a 200mm lens the DoF at 2 meter is:

    • 1,5cm for an Aperture of f/2.8, and
    • 6.1cm for f/11.0

(all these values were computed with an iPhone app called Simple DoF of Dennis van den Berg for a full frame camera).
So, the question we should ask ourselves is what type of picture do we want to take. If, on the one hand, we want to highlight our subject it is nice to have a small DoF, because it leaves the background blurry. Our eyes tend to go to sharp areas in the picture and to avoid the blurry areas. Exactly what we want. This is often used for portrait photography. There are a couple of pitfalls to be aware of:

  • a DoF that is too small may unintentionally leave part of the person unsharp;
  • when shooting several people they may not be at the same distance from the camera, so make your DoF a bit bigger to make sure everybody is sharp (unless you don’t want to).

On the other hand, for landscapes and storytelling portrait pictures everything has to be sharp, requiring a large DoF. A common rule of thumb is: if you need a large DoF use a wide angle lens and a small Aperture and for a small DoF use a telelens with a large Aperture. For portrait photography and pictures of details I use my Nikkor 135mm and Nikkor 70-200mm lenses, and for land- and cityscapes I use my Nikkor 16-35mm lens.
Female student working on maquette in design lab
(Aperture f/3.5, Shutter Speed 1/80, ISO 100) with 135mm lens; DOF is 4.3cm