I introduced the exposure triangle last time and mentioned that there are times when you might want to control aperture and hence adjust ISO with shutter speed to get the correct exposure.

Today I thought I’d explain how the choice of aperture affects a shot.

As well as affecting the amount of light that enters the camera, changing aperture also impacts how much of a shot is in focus – referred to as Depth of Field (DoF).

DoF is actually affected by a number of factors including lens focal length and the relative distance between camera, subject and background elements but use of aperture is perhaps the easiest way to control DoF.

Put simply, a shallow DoF is when only a narrow ‘slice’ of the image is in focus and elements in front and behind this slice are blurred. This is a useful way of isolating a subject from the background. It can also give pleasing blurry background elements known as Bokeh (pronounced bo-ker or bo-key);

In the image below, look at how little is in sharp focus.

Large depth of field is useful if you want to get the majority of a scene in focus, such as in a landscape shot. In fact there is something called the hyperfocal distance where everything from a given point to infinity will be in focus – but I’ll cover that another time.

The key thing to remember is small f/ number (large aperture) = shallow DoF and large f/ number (small aperture) = large DoF.

As Gina Milicia (famous Australian tog and presenter of So You Want To Be A Photographer podcast) says;

“Small f/ number = small amount in focus,
Large f/ number = Large amount in focus”

Calculating DoF is complex, but there are many apps out that will do this – I use DoF Calc by Jesper Svensson. In reality you won’t often want to calculate the DoF, you will just know whether shallow or large DoF is best for your shot and adjust the aperture accordingly.

However it’s worth downloading the app to get an idea of how aperture, lens focal length and distance impact the DoF. The shots below are examples of how the app works.

For example;
At f/2.8 and focus point 5m away;
35mm lens has focus between 3.72m to 7.62m = 3.9m DoF
50mm lens has focus between 4.28m to 6.01m = 1.73m DoF
70mm lens has focus between 4.61m to 5.47m = 0.86m DoF

Repeating this but with a smaller aperture;
At f/11 and focus point 5m away;
35mm lens has focus from 2.10m to Infinity
50mm lens has focus between 2.99m to 15.25m = 12.26m DoF
70mm lens has focus between 3.73m to 7.59m = 3.86m DoF

So now you know a bit about the impact of aperture on your photographs, but in order to control this yourself you need to make sure you shoot in either Aperture Priority mode or fully Manual mode on your camera. If you are shooting in Auto or Program modes, the camera will try and work out what aperture is best – and it won’t get it right too often!

Your challenge for the week is to switch to Aperture priority and take some shots with different focal lengths, adjusting the aperture to see what impact it has on the Depth of Field.