The aperture is found in the lens, and helps control two aspects, a) how much light enters through the lens to the sensor (or the film for analog cameras), as well as the depth of the area in focus. The aperture is the actual hole, which size is controlled by the aperture blades. The size of the aperture is defined in what is known as “f-stops”, with the lower the f-stop number defining the wider or more open the aperture is, whereas the higher the f-stop number is, the smaller the aperture is. For example, f/4 is more wide open than f/8. The range of f-stops is typically found in between f/1.4 to f/22, though some lenses are going as wide as f/1.2 or even f/0.95, while others can be closed down as much as f/32.
As said, the more you open the aperture, the more light you allow through the lens. While the amount of light isn’t what defines the f-stop, it is still always the case on each individual lens, that the lower the f-stop, the more you open the aperture and the more light you allow through the lens. This means that if you have set the aperture at f/5.6, and you need more light to reach the sensor, opening the lens up to f/4, f/2.8, or more, will make the image brighter. However, some lenses can be brighter than other lenses, even if the f-stop is higher, for example, f/1.8 can be brighter on one lens, than f/1.7 on another lens. Again, the amount of light allowed through the lens is not defined by the f-stop or the other way around. That will be measured by another value known as t-stops. However, t-stops are only used on cinema lenses, which are made specifically for videography. However, here it will be true that t/4 allows the same amount of light through the lens, no matter which lens you use. That won’t be relevant here though, just be aware that f-stops and t-stops both define how open or closed the aperture is, but in the context of different values. The f-stop refers to the size of the aperture relative to the size of the lens, whereas the t-stop refers to the amount of light. But again, the more open the aperture is, the more light comes through the lens. This is true in all cases. Therefore, if you are in a situation where you need to make the image more bright, but you don’t want to change the shutter speed or ISO, you will have to open the aperture up by lowering the f-stop value (for example from f/5.6 to f/4).
While the lens will allow more light in when the aperture is wide, the fact that the area of focus also will be narrower the wider the aperture is means that it at times will be preferable to have the lens “stopped down” to a narrower aperture. For example, since the area which is in focus, known as “depth of focus” (DoF) will be smaller, the closer the camera is to the subject, the DoF typically is very narrow when doing macro photography, in which case you would prefer to close the aperture as much as possible, in order to have as broad a DoF as possible.
However, in other cases, the effect of background blur, which a wide aperture can create, can create pleasing results for for example portrait photography. In these cases, you might want to blur out the background as much as possible, either to hide it or to make the subject stand out as much as possible. In these cases, you might want to open the aperture wide open.
It is with the DoF you can use the aperture creatively. Take a look at the two following examples and the effect of having a wide open aperture vs. having the aperture stopped down:
You clearly see the difference between the two photos, with the background almost being blurred out completely on the first photo, whereas the second photo seems to have everything in focus.
However, it is important to add that the aperture isn’t the only thing, which affects the DoF. As mentioned previously, the closer the camera is to the subject relative to the background, the narrower the DoF will be, and the more out of focus the background will be. This can be good to know, if you have a lens, which can’t be stopped down very much but you still wish to have a blurry background. Simply move yourself and the subject away from the background, and move yourself closer to the subject.
Another thing that also has an effect on how blurred out the background is, is the focal length of the lens, or rather the angle of view. The longer the focal length – or the narrower the angle of view – the more the background will be compressed and thus become blurred out. However, this is entering more advanced territory. For now, we will stick with the aperture, since there might be times when you can’t move yourself and the subject, and the aperture will be the only factor you can control.
When do you want to use the aperture wide open, besides when you’re in need of light? Often when photographing a subject, you want to stand out from the background. For example when you are doing a portrait, when you are photographing flowers in nature, or if you wish to take a food photograph, where you wish to incorporate the surroundings, but still have the main dish stand out.
On the other hand, there might be times when you instead want the aperture to be stopped/closed down. This is particularly true when you are doing macro photography or landscape photography, or where you want everything to be in focus, even with portrait photography. Personally, when I’m doing black and white portrait photography, I often have an all-black background while having the subject in full focus, which best can be achieved by stopping the lens down and using speed/flashlights. This way I will make sure that the background is dark, while the subject is lit correctly and in full focus.