One of the more confusing topics with photography is Aperture. Aperture is the opening in the lens diaphragm that allows light to come in. As with everything else in photography, it is measured in F-Stops like 2.8. 5.6, 8, 11, 16, 22.
Now comes the more confusing part, Aperture is backwards, f2.8 is larger than f22.
So why would you use f2.8 or f11, or f22, etc? Well, f2.8 is excellent for night photography because it lets in a lot of light. If you are shooting the milky way, it allows you to have a lower ISO or shorter shutter speed. We will get into ISO in a different blog. However, f2.8 has a very shallow depth of field. This means that your subject will be in focus but anything in front or behind the subject will be out of focus. In this photo, you can see how the backpack is in focus and the surrounding area. This photo was taken at f2.8 causing the background to be out of focus. It is easy for the viewer to know the pack is the subject.
The next photo was taken at f11. Most lenses are the sharpest at f8 or f11 but it also depends on how much of the photo needs to be in focus. If the entire photo is in focus, it is hard for viewers to determine the subject of the photo. As you can as the trees become more in focus, the pack is no longer the subject.
This final image was taken at f22. Almost the entire photo is in focus and the pack really loses the ability to be the subject. It is similar to the f11 photo but the trees are slightly more in focus.
So if you want the entire photo in focus, why not just shoot at f22. Well, like I said earlier, most lenses are sharpest at f8 or f11 and sharpness is what we want. Another problem arises with f22 and that is diffraction. Diffraction happens when the light hitting the pixels on the sensor starts to overlap with other pixels causing your photo to look slightly out of focus.
You also need to think about what shutter speed you want to use. If you can't get everything in focus using f8 and you can using f16 and the correct shutter speed is 1/60 sec, you have to slow down your shutter. 1/60 sec can be a sharp photo hand holding but since you stopped down your aperture by 2 stops you now have to decrease your shutter speed by 2 stops. 2 stops decrease on your shutter speed goes from 1/60 sec to 1/15 sec. 1/15 sec is too slow to hand hold for a sharp image unless your lens has incredible image stabilization. Each stop is a doubling or halving of your aperture, ISO, or shutter speed.
What helps me remember how aperture works to to think about how much of the photo I want out of focus. If I want all but the subject out of focus, think more on the lines of f2.8. If I want the whole photo in focus, I will move to f11 or smaller.
Now that you have a basic idea of what changing your aperture does to your photo, we will discuss ISO and shutter speed in the next couple posts. Once you understand all 3, you will understand the exposure triangle.