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Deciding Which Aperture is Best
When shooting in aperture priority or full manual mode with your camera, you need to decide what aperture will best give you the results you want. Although each aperture has a corresponding shutter speed that will result in a correct exposure (provided the required speed isn't too short or too long for your camera), changing the aperture will result in very different photos.
We're going to take a look at what results you get from large or small apertures, and when you'd want to use them. As you will see, the f-stop in photography is very important to creating photos that will evoke your desired reaction.
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What is an F-Stop?
Before we get started, let's review exactly what an f-stop is (if you're brand new to photography, you might want to start by reading one of our free digital photography tutorials).
Aperture refers to the opening at the front of your lens that allows light into the camera. The larger the aperture (opening), the more light is able to enter, and the less time you need to capture a properly exposed photograph.
Aperture is expressed in terms of f-stops. Here's where it gets slightly technical: the f-stop is the ratio of the focal length to the aperture, where focal length is simply the distance from the camera's sensor to the focusing element in the lens. For example, suppose you're using the nifty fifty, which is the nickname for a 50mm f/1.4 lens. The 1.4 means that if the lens is wide open (at its greatest aperture), the focal distance is 1.4 times as much as the opening in the lens. If you stepped down to f/32, however, the opening is now only 1/32 of the focal distance, so it lets in a lot less light.
Lenses with larger apertures (particularly zoom lenses that keep the same maximum aperture throughout their zoom range) are more expensive than lenses with smaller apertures and variable apertures, which is why entry-level lenses will often have maximum apertures like 3.5-5.6, which means that it can open as wide as f/3.5 at one end, but only to f/5.6 at the other end. A pro lens, by contrast, will often have a maximum aperture of f/2.8 for the entire zoom range.
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Large vs. Small Apertures
A small aperture (that is, one with a large f-number, such as f/22) is useful when there is a lot of light (for example, shooting outside on a bright, sunny day), when you need to use a slow shutter speed, or when you want the entire picture to be in focus. For example, if you're shooting landscapes, you probably want the entire photo to be clear. Because the light is coming in through a small hole, everything will be in focus.
Large apertures are naturally useful when you don't have enough light and don't want to use a flash (insider tip: a 50mm f/1.4 is a great lens for taking Halloween photos, where the flash would ruin the ambiance; once you have one you'll definitely want to try out night photography) but they also have another popular effect: they reduce the depth of field. This means that when you focus the camera on your subject, the background will be out of focus; this allows you to put the viewer's attention exactly where you want it. For example, this tends to be very useful for portraits; you focus on people's eyes and let the background fade to irrelevance.
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Choosing Your Aperture
Assuming that your camera and the situation allow it, you can get a correct exposure with any aperture by setting the corresponding shutter speed. Obviously, this has its limits; the camera will only fire so fast, and your subject is often moving. This means that if you're looking for a particular effect, you can often set the aperture for that effect and let the camera choose an appropriate shutter speed. Grab your camera and do some experimental photography; you might be surprised at what works out!
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Article written based on author's own experience.