If you're new to DSLR photography, you're probably familiar with shutter speeds. The shutter stays open for a certain amount of time, which allows a certain amount of light to enter the camera. You might not be quite as familiar with the concept of apertures and f-stops.
A lens aperture is the opening in your lens that the light passes through. An f-stop is a way of measuring the size of that opening. The bigger the opening, the more light that enters the camera. This part makes sense, right? Here's where things get a little confusing with the terminology. The larger the aperture (bigger hole), the smaller the f-stop number. The smaller the aperture (smaller hole), the larger the f-stop number. For example, f/1.4 is a very big aperture (big hole), while f/32 is a very small aperture (small hole). Although it might be a little counter-intuitive, it's just the way it is.
So why would you want to adjust the amount of light coming into your camera? Why not just change the shutter speed? Changing the f-stop can drastically change the look of your photos. Have you ever seen a photo where the subject is in focus but the background is blurry? That was probably taken with a lens with a large aperture. This kind of image is just not possible with a small aperture. At the opposite extreme, almost any landscape image you've seen where the flowers in the foreground are just as in focus as the mountains in the background was probably taken with a very small aperture. This is just one reason to use different apertures in different situations.
Another reason to use different apertures is to allow in more or less light. If you're shooting in low light conditions, the ability to open up your aperture and allow in a lot of light, while still using a reasonable shutter speed, might mean the difference between getting the shot and not getting the shot. At the other extreme, if you're shooting in full sunlight, you might want to use a very small aperture and allow a lot less light to enter your camera.
Aperture and shutter speed are directly related. An increase in shutter speed (faster shutter=less light) can be offset by an increase in aperture size (bigger opening=more light). A decrease in shutter speed (slower shutter=more light) can be offset by a smaller aperture (smaller opening=less light).
To wrap up this discussion, let me list a few situations where you might want to use different f-stops:
Large aperture (f/1.4, f/2.8, f/4) - Low light conditions, isolate subject from background, artistic blurring effect.
Small aperture (f/11, f/16, f/32) - Bright light conditions, keep everything in focus, landscapes.
Be aware that shooting at your lens' extreme apertures typically will somewhat degrade image quality. If you're shooting at f/1.4, all of your image will appear a little "soft", meaning that even the point of focus will not be as sharp as if you'd shot it at, say, f/5.6. Also, if you shoot at f/32, you'll encounter more diffraction, which will lead to a decrease in image quality. This doesn't mean you shouldn't use these extreme apertures, but only that you should be aware of these potential effects.
You should always experiment with your lenses to see what effect different f-stops have at different focal lengths. For example, f/4 will look a lot different on a 12mm lens than it will on a 200mm lens. Once you're familiar shooting with your different lenses at different f-stops, you can start to create the images you want to see.
Here are a few examples to illustrate this last point:
|This image was shot at f/4 with a 12mm lens. Notice how everything is pretty well focused.|
I shot this at f/4 because that's as wide as this lens would open and it was getting dark.
|This image was shot at f/4 with a 50mm lens. Notice how the background retains some detail but isn't very focused.|
I wanted to make sure all of the family members were in focus, but everything else was out of focus.
|This image was shot at f/4 with a 135mm lens. Notice how the background has gone completely out of focus.|
Even the grooms glasses are starting to go out of focus.
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