I have discovered an easy way to estimate the depth of field (DOF) when shooting portraits – a rule of thumb which I want to share. It is even slightly easier if you are thinking in centimeters, not inches.
I shot these portraits with a shallow DOF, meaning that only a shallow area appears sharp. DSLR cameras have a DOF preview button but when it comes to very shallow depth of field you may not even see the true DOF in the viewfinder (in particular with apertures wider than f/2.8 the viewfinder becomes a limiting factor and you would have to use Live View mode together with DOF Preview to see the depth of field). Yet DOF is a very important factor in portraiture. And I believe that camera settings should be determined by the desired DOF, rather than DOF determined by the settings.
DOF depends on the aperture, the camera to subject distance and the focal length. Apparently things get more complicated when using a digital camera with a crop sensor as the effective focal length changes. In the past lenses had DOF scales which were useful but are rarely found on current models. To know the DOF you would have to use a special chart or a DOF calculator program, or just make a guess based on experience. Right? Wrong! At least not any more…
Say I am shooting a full body portrait with a 70-200mm lens set to 100mm, at f/2.8 on a full-frame camera. I immediately know the DOF is 60 cm! I now changed the aperture to f/4. I instantly know the DOF is 80 cm! I now throw my 50mm f/1.2 on the camera and shoot wide open… the DOF is 25 cm! How do I know that? Do I carry DOF charts with me or maybe use an app on my iPhone? Not when shooting portraits! Do I love learning tables by heart? Nope. Do I have a mathematical brain? By no means.
I just simplify things. Let us forget that DOF depends on the focal length or the sensor size. Why? Because from our point of view it does not. Imagine that you just changed your normal 50mm lens to a telephoto lens (which gives a more shallow DOF) and still want to shoot a full body portrait. You must get further away from the model (and will thus increase the DOF cancelling out the more shallow DOF effect!). So assuming that you are shooting the same portrait type (e.g. a full body portrait) the focal length will not really affect the magnitude of DOF.
Of course it does not mean that the lens choice is not important – a telephoto lens will give a more flattering portrait, a more compressed perspective and more “zoomed-in” and creamier bokeh. But the focal length will not affect the DOF! Even with a wide-angle lens, which in theory gives a large DOF, if you get close enough to the subject you will throw the background out of focus.
With our assumption the DOF depends on just two factors: 1. How much you are filling the frame (e.g. a full body portrait or a head and shoulders shot) and 2. The aperture.
Below are some general DOF values for head and shoulder vertical portraits. I determined them by making life measurements of camera to subject distance with different lenses and using the excellent DOFMaster Depth of Field Calculator (for a fixed f-stop the DOF is basically constant for equivalent focal length and distance combinations, e.g. 100 mm lens from 6 m, and 200mm lens from 12 m).
f/1.4 – 2 cm
f/2.0 – 2 cm
f/2.8 – 3 cm
f/4.0 – 5 cm
f/5.6 – 6 cm
f/8.0 – 9 cm
f/11 – 13 cm
So for head and shoulder vertical portraits DOF ≈ 1 cm × f-stop
(or 0.4 inch × f-stop). This is simple to remember.
For full body vertical portraits the depth of field values were:
f/1.4 – 0.27 m
f/2.0 – 0.39 m
f/2.8 – 0.55 m
f/4.0 – 0.8 m
f/5.6 – 1.1 m
f/8.0 – 1.5 m
f/11 – 2.2 m
So for full body vertical shots DOF ≈ 20 cm × f-stop
(or 8 inches × f-stop). This is also simple to remember.
For horizontal portraits the DOF is approximately 2-fold wider, as you will have to stand further away. And for 3/4 shots it is in between the head and shoulders and the full body shots. It is as simple as that. Some of the DOF will extend in front of the focus point (towards the nose assuming you are focusing on the eyes) and more than half will fall behind the focus point.
Of course you may disregard the numbers and just count on your experience and luck with the aperture choice. But the above simple rules should help you better guesstimate the depth of field.