Aperture is the second element within the exposure triangle and like shutter speed the magic happens inside your lens. Shutter speed is the length of time your shutter is open but aperture is how wide your shutter opens during each exposure, meaning the wider your aperture the more light can hit your sensor during a single exposure giving you a brighter image.
What do these f numbers mean?
The way your camera displays aperture is in “f stops”, beware some maths is coming up but you don’t need to understand this I just wanted to put it in for the geeks.
F-stops are shown as fractions like 1/1.8, 1/2.8 or 1/5.6 which is a ratio between your focal length and aperture (focal length/aperture). This ratio allows you to calculate the diameter of your shutter at a particular aperture and focal, for example for a 50mm lens set to an aperture of f2.8 the diameter is calculated by 50/2.8=17.85mm.
As the f-stop increases with the same 50mm lens from f2.8 to say f16 the diameter decreases to 3.13mm. Due to the diameter of the aperture reducing this restricts the amount of light entering the camera and hitting your sensor, resulting in a darker image. Like I said you don’t need to know the maths but it is nice to know why f-stops are shown as fractions. All you need to know is the smaller the f stop number the wider the aperture and so more light you will get in your camera, and vice versa.
Why do some photos only have one thing in focus?
Like shutter speed there is a secondary effect of changing your aperture other than the amount of light getting into the camera. If you have a wide aperture say f/1.8 your depth of field will be very shallow, meaning the area that is in focus is very narrow (this is useful for macro or detail shoots).
If you look at this image which is taken at f/1.8, Chewie is the only figure in focus with both the foreground and background blown out of focus. Note the shutter speed of this first image is at 1/125th of a second.
If we increase the f-stop to f/4.5 you can see the other figures are starting to come up into focus BUT because the aperture has reduced in diameter less light is getting into the camera so the shutter speed has been slowed down to 1/20th.
If we take the aperture all the way to f/16 almost all four figures are now in focus because the depth of field has got wider or deeper. Again notice how the shutter speed has had to get slower to let more light into the camera.
This is normally the hardest element of the exposure triangle for beginners to get their head round because you automatically think the smaller the f-stop the smaller the aperture but it is the opposite, so be careful.
So that covers the basics of aperture and the three main rules of thumb you need to remember:-
1. Aperture controls how wide your shutter opens during each exposure
2. A smaller f-stop number (1.8 for example) is a wider aperture opening letting more light into your camera and therefore a brighter image
3. The smaller the f-stop number the narrower the depth of field (macro and detail images) and the larger the f-stop number the wider the depth of field (landscape)