This post was updated on November 19th, 2018
The Definition of the Aperture
The aperture is the opening in the lens, which is responsible for part of the exposition, as it decides how much light at a given time can pass through the lens.1 It is defined as the focal length divided through the absolute diameter of the lens.
So the aperture decides how far open the lens is in relation to the focal length you are using, as seen in figure 2.
But the aperture does not only have an influence on the amount of light that can pass through the lens but on the depth of field. The depth of field means the area in the photo that is sharp while its surroundings are not. So a photo of, say, a blossom, where the blossom is tack sharp and the field that surrounds the flower is out of focus, has a small depth of field.
A landscape photo where basically everything in the photo appears in focus, has a large depth of field.
To get a smaller depth of field, you need to use a wider aperture–which means a smaller aperture value. For a stunning landscape with a large depth of field, you need a narrower aperture–which means a bigger aperture value. And at this very moment you are very confused, because how can a wider aperture mean a narrower aperture value and vice versa?
Remember the definition of the aperture–the first formula I gave you?
The aperture is defined as a fraction. The aperture value, which you see in your camera display, is the denominator of the fraction, while the numerator–the focal length–is 1. So if your camera display shows you an F4, you are in fact using an aperture of F/4; and if you are using F16, you are using F/16. So a lower F-number (4) means a higher F-value and vice versa, because F/4 > F/16. Or simplified: you will get more cake (1/4) if you have to share it with only four people, than if you have to share the same cake with sixteen people (1/16). (More cake = more background blur.)
For a landscape photo, you would want to have as much of the scenery in focus (less background blur) as possible–so you would therefore use a small(er) aperture, while for a photo where the subject is supposed to stand out against its surroundings (more background blur) by being the one thing in focus, you would use a narrow(er) aperture.
If you are using a compact camera for which you cannot change the aperture directly, you can achieve a small depth of field by using the macro mode, usually a menu setting with a flower icon. For a large depth of field you would use the landscape program, usually a setting with a landscape icon, and for a depth of field in between those two you would use the portrait mode.
Why are my aperture values so strange?
If you have a look at the full-stop apertures that your camera offers2, you will soon realize that those values look a little strange. Not only is one stop3 wider than 8 not 7 or 4 but 5.6…one stop narrower than 8 is not 9 or 16 (that’s actually two stops) but 11. So let us have a look why that is the case. The explanation lies in the shape of your lens. It is a cylinder, based on stacking circles on top of each other, and not a cube. So the amount of light that can pass through that “aperture-circle” in a very short timeframe–imagine super thin round slices of light–would equal the area of the circle. To calculate the area of that circle, you would use the following formula:
Let us try an example calculation and assume that the radius of the circle is 1, which means the area of the circle would be calculated as:
smaller aperture (√2)
So now you know where those numbers come from 🙂 In the next article of this series, which will be published here on February 24, 2017, we will have a look into how the aperture, shutter speed, and ISO work together in creating the right exposure for your photo.