Apertures & F Numbers

What are F/numbers? The aperture is an adjustable "hole" or iris that lets light through the lens to the cameras sensor & together with the shutter speed, creates the exposure. The size of the aperture is denoted as an f/number & you see this in most cameras viewfinders & on the rear screen. The f/number is one of the most important settings on your camera & you should keep an eye on which value you are using. Once you set the f/number on your camera it will not change, even when your power off or on your camera but you must change it according to conditions at the time or time of day.

How does changing the f/number affect the aperture size? One of the most confusing things about the f/number & aperture is that smaller f/numbers indicate larger apertures & vice versa, e.g.  f/1.4 is the largest aperture that any camera lens can be set to.  f/22 is usually the largest aperture that most cameras can be set to but there are specialist cameras that will go to f/32 & even f/45.

Where do f/numbers come from? The available f/numbers form a standard scale, with each value referred to as an f/stop. At first the f/numbers seem like a random sequence of numbers, but the scale has a mathematical basis. Each full stop lets in half as much light as the one above, & twice as much light as the one below. Thus f/8 lets in twice as much light as f/11 but only half the light of f/5.6 - the bigger the hole the greater the amount of light is allowed to pass into the camera.

What does this mean for my exposures? An exposure is basically a balancing act between aperture (the amount of light coming through the lens aperture) & shutter speed (the amount of time the sensor is allowed to receive the light coming through the lens aperture). The bigger the "hole" or iris, the greater the amount of light coming through the lens, & that allows a shorter or faster shutter speed. If the iris is set to a higher value, thus giving a smaller "hole", a longer shutter speed is needed to compensate for the lower level of light coming through the lens.

What are the full f/numbers? f/1.4, f/2, f2.8, f/4, f5.6, f/9, f/11, f/16 & f/22

What about the numbers in between the full f/numbers? On lenses for old cameras, the aperture control was less advanced & only full f/stop values were possible. On today's cameras, you can set the f/number to values in between the full stops. These are partial f/stops that divide each full f/stop into thirds, & they simply have one third of the effect. The third stops are: f/1.6, f/1.8, f/2.2, f/2.5, f/3.2, f/3.2, f/4.5, f/5, f/6.3, f/7.1, f/9, f/10, f/13, f/14, f/18 & f/20.

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What other effects does changing the f/number have? Opening & closing the aperture not only changes the amount of light that passes through, but it affects something called depth-of-field. Depth-of-field (also known as DOF) is the amount of back-to-front sharpness in a scene. Controlling the f/number can therefore be used for creative effects.

What is depth-of-field? With a shallow depth-of-field sharpness falls off quickly on either side of your focal point so that only a small amount of the scene is in sharp focus. a large depth-of-field keeps everything from front to back sharp.

How can I use depth-of-field creatively? Using a shallow depth-of-field lets you direct attention by picking out your subject from its surroundings. For example, blurring the background with a shallow depth-of-field makes a portrait subject stand out, or in macro photography it highlights specific details. But keeping everything sharp with a large depth-of-field is what you want in landscapes or when capturing as scene as a whole.

How does aperture affect Depth-of-Field? Increase the size of the aperture by selecting a lower f/number, & the depth-of-field becomes shallower. for example, if everything else is kept the same, switching from f/11 to f5.6 creates a shallower depth-of-field. By setting your camera to aperture priority mode, you can change the aperture to control the depth-of-field, & the camera adjusts the shutter speed automatically for a good exposure.

What else affects depth-of-field? It is not just the aperture that determines the depth-of-field. Getting closer to your subject, without changing the aperture or focal length reduces the depth-of-field, as does increasing the focal length without changing the aperture or distance from your subject.

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