The lens aperture is an adjustable hole created by a diaphragm within the camera lens. When you change the size of this hole, you change the amount of light allowed to pass through the lens – in other words you change the brightness of the image and the exposure level.
Lens aperture and exposure
Like other exposure settings, the aperture setting works by doubling or halving the exposure with each full step. Like shutter speed and ISO settings, aperture settings follow a specific sequence:
This f no. is sometimes called the ‘f-stop’, an old photographic term still in use today. The unusual units come from the way it’s calculated. The ‘f’ represents the focal length of the lens and the number after is the size of the aperture hole relative to that.
The technical explanation for the way these numbers are derived isn’t particularly important. The key thing is that the lower the f-number, the wider the hole and the greater the exposure. For example, f/2 gives twice the exposure of f/2.8.
To look at it another way, the wider the lens aperture, the more light is let through and the faster the shutter speed you can use (or you can use a lower ISO setting and the same shutter speed – all part of the different combinations that go to make up the exposure triangle).
Lens aperture and depth of field
Changing the lens aperture has another effect. A wide lens aperture (a low f no.) creates shallow depth of field, so that only your main subject is sharp and everything in front or behind is blurred. A small lens aperture creates deep depth of field so that everything looks sharp from right up close to the camera too far away.
In practice, it’s not quite that simple and there are many other factors that affect depth of field, but the lens aperture does have a strong effect.
Shallow depth of field effects are very popular right now, and this makes lenses with a wide maximum aperture very popular. The other advantage of a lens with a wide maximum aperture is that it allows faster shutter speeds or lower ISO settings, as we’ve seen.