How to control aperture
Manual exposure gives you total control over both aperture and shutter speed, but it can be a fiddly system when you’re just starting out.
Aperture Priority (A or Av) is a semi-automatic exposure mode that, as the name suggests, enables you to set the aperture as a priority.
The easy way to control aperture yourself is by selecting A/Av mode on the Mode dial. Then simply rotate your camera’s main input dial to increase or decrease the aperture setting. The camera will automatically adjust the shutter speed as you do so.
Digital SLRs offer a choice of up to three aperture scales, with full-stop, half-stop or third-stop increments (head to your camera’s Custom Functions menu to choose your preference). We prefer third stops as it enables fine-tuning of exposure.
There are three ways to keep track of the aperture setting – in the viewfinder, on the rear LCD screen and, in the case of high-end DSLRs, on the small top-plate LCD screen. We like the clarity that a top screen and the viewfinder provide.
Many cameras have a Depth of Field Preview button that closes the aperture down to the setting you’ve chosen, enabling you to gauge the depth of field through the viewfinder. However, Live View offers a more effective representation of the aperture setting.
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Why a small aperture isn’t always best
Most lenses have a minimum aperture of f/22, although some (such as macro lenses) offer an even smaller setting of, say, f/32.
But why is the minimum aperture rarely listed alongside the focal length, as its maximum aperture is?
This is because the smallest aperture is rarely recommended to be used, as this setting leads to softer, lower contrast images caused by an optical phenomenon known as diffraction.
Diffraction occurs when light waves entering the lens are ‘bent’ by the hard edges of the aperture blades.
Every aperture setting causes this, but the bending is generally minimal. However, as the apertures get smaller, the effect becomes more significant.
At the smallest aperture, the light waves are bent and spread out so much by the aperture blades that the image appears fuzzy, even though it’s focused correctly.
So, while a lens’s smallest aperture might enable you to maximise the depth of field when you’re photographing landscapes, the resolution will deteriorate.
Try shooting at f/16 instead to improve overall picture quality.
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What are f stops?
Dial through the f-stops in Aperture Priority mode and the sequence of numbers that appears in the viewfinder can appear quite random.
Why does f/5.6 jump to f/8 when you close the aperture by one full stop, then from f/8 to f/11 to f/16 and so on? There seems to be no logic to it.
How these numbers are calculated is actually pretty complicated, but the thing to bear in mind is that an f-stop isn’t a measurement of the diameter of the aperture, but an expression of the ratio between the diameter of the entrance pupil and the focal length.
This means that aperture values are constant no matter what lens is in use.
For instance, an 80mm lens with an aperture setting of f/4 will have a pupil diameter of 20mm (80÷4), whereas a 400mm lens with an aperture setting of f/4 will have a pupil diameter of 100mm (400÷4).
While the 400mm has a considerably larger pupil, it lets in exactly the same amount of light as the same f/4 aperture does on the 80mm lens, because the light is much dimmer by the time it has travelled the length of the lens.
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