Making sense of aperture f-stops
Your at-a-glance guide to aperture scales and what settings mean
Altering the aperture is one of your most potent weapons, but this simple control can lead to confusion. The aperture used can create varied, seemingly contradictory, effects. Then there is the number scale in the wrong order…
To simplify things, just think of the aperture as an opening that can be varied in size to control the amount of light reaching the camera sensor. Used in conjunction with shutter speed (the length of time the sensor is exposed to light), the aperture enables you to match the exposure to the brightness of the scene. The wider the aperture, the more light that is let in – helping you to compensate for darker conditions, or enabling you to use a faster shutter speed.
The aperture isn’t in your camera; it’s inside the lens. As Canon EOS DSLR and EOS R mirrorless lenses are removeable, the range of aperture settings varies on your lens. Expensive lenses tend to have significantly wider maximum apertures than budget zooms – eg f/1.2 or f/2.8 compared to f/4 or f/5.6.
What might frustrate those new to photography is the f-stop scale used for the size of each aperture. However, aperture numbers are just fractions. So, f/4 is not 4 but a quarter (1/4). This is why f/16 is a smaller aperture than f/4. So, f/4 means the aperture diameter is the focal length of the lens (f) divided by 4. And a setting of f/16 means the diameter of the aperture is a 16th of the focal length.
Altering the f-number does not just alter the amount of light reaching the sensor. As we explain on the left (and further on page 36), the aperture is one of the main factors affecting the depth of field, and how much of the scene actually appears sharp.
Finally, a lens also performs better at some apertures than others, and it doesn’t give its best resolution when set at extreme apertures, such as f/4 and f/22. Resolution and quality improve gradually as the aperture is made smaller, but as it gets even smaller, image resolution and contrast deteriorate again – due to an optical phenomenon known as diffraction. This is why optimal apertures for the best image quality across the frame are usually mid-range, around f/8 to f/11 on most lenses.