I found an easy way to estimate the depth of field (DOF) when shooting portraits – a useful rule of thumb which I would like to share.
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 it may not be even possible to 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 in combination with the aperture closed down in real time). Yet DOF is a very important factor in portraiture.
For a particular camera DOF depends on the aperture, the subject distance and the focal length. In the past lenses had DOF scales which were useful but are rarely found on the current models, perhaps because many lenses can be mounted on cameras with different sensor sizes. Telephoto lenses give more flattering portraits, a more compressed perspective and more “zoomed-in” and creamier bokeh, especially on a full-frame sensor. But even with a wide-angle lens, which gives a large DOF, if you get close enough to the subject you can throw the background out of focus.
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. When shooting with a particular camera and fixed focal length combination the DOF depends on 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 approximate total DOF values when shooting vertical portraits with a full-frame camera. 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 head and shoulder portraits the depth of field values were:
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 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 larger, as you will have to stand further away (for 3/4 shots it is in between the head and shoulders and the full body shots). And for APS-C sensors you would still need to multiply the DOF by the crop factor (e.g. by 1.6). These simple rules help me easily guesstimate the depth of field when shooting portraits.