If you own a standard compact, the word “aperture” probably does not come up very often. You’ll probably have “portrait” and “landscape” modes. Fancy compacts and DSLRs, however, have a “PASM” program dial. The “A” stands for aperture
Aperture is simply the size of the hole through which light passes in the lens before it is saved as an image. Every camera (except pinholes) allows you to change the size of that hole. Adjusting the aperture does two things: it varies the amount of light when taking the picture and changes how much of the scene behind and in front of your subject is in focus, which is known as depth of field
When shooting a portrait, it’s usually a good idea for the subject to stand out sharply against a less sharp background (i.e., a narrow depth of field). If you accidentally shoot a portrait in landscape mode, the subject tends to be lost in an image where everything is equally sharp, as the depth of field is much wider. The important thing is to switch off the “auto intelligent / camera decides everything” feature when you shoot portraits.
If the hole is bigger, more light enters the camera when taking the shot, which means you need less time to take it: a wider aperture means a shorter exposure time
. This is particularly useful when taking pictures of moving objects (sports, children, pets, etc.). If your compact has one, try the ‘sports’ or ‘portrait’ mode in these situations. With a DSLR, set the mode dial to “A” and dial the f/ number to its lowest value.
Conversely, when the hole is small, less light enters when taking the shot, which means you need more time. A narrower aperture requires a longer exposure time, but more of the scene is in focus. This is useful for landscapes and interior/exterior architecture. Remember that moving objects (trees on a windy day, for example, or people) may be blurry. You’ll probably need a tripod or some sort of solid support.
When shooting “wide open”, especially outdoors, you can easily burn out the sky or the brightest areas because there is too much light coming in. There is usually a lot of “vignetting” a.k.a. “light fall-off” (corners are darker than the centre) when shooting at the lowest aperture value. Geometric distortion (buildings bent, people oddly shaped) occurs more often at maximum aperture.
At the other end of the aperture scale, images can become slightly blurry (referred to as “soft”) owing to a phenomenon known as diffraction. In general, it’s better to avoid the highest aperture value.
Slightly techie stuff
(which you can probably ignore)
In photography, aperture is the size of the hole divided by the focal length of the lens. If the focal length is 50 mm and the maximum diameter of the hole is 25 mm, the aperture is stated as f/2. This is why the larger the f/number, the smaller the hole. The hole at f/2 is bigger than at f/2.8 or f/5.6. Think of “f” as “fraction” or as “1 divided by…” if that helps.
Lenses are rated by focal length and by maximum and minimum aperture. If you have a camera where you can change the lens, chances are the lens is a zoom; you can change the focal length. Unfortunately, in all but the most expensive lenses, the maximum aperture (lowest f/number) also increases when you zoom in. One of my lenses is a 15-85 mm, 1: 3.5-5.6, in which the maximum aperture ranges from f/3.5 at 15 mm to f/5.6 at 85 mm. In practical terms, this means that you need more time at the long (tele) end of the zoom because the hole is smaller and therefore lets in less light.
You can counteract (slightly) the need for slower exposure by increasing the camera’s ISO sensitivity. Here, the camera sensor records more light, so the exposure time can be shorter, but also adds more “noise” (grain) to the picture. You can also use a tripod, but this won’t help much with moving subjects.
Below are two captures of the same subject taken at different apertures.
In the first, taken at f/2.8 (the widest aperture available with that lens), notice how everything behind
and in front of
the spout is out of focus. This is an example of a narrow depth of field.
In this one, taken at f/13, both the foreground and the background are still in focus along with the spout. This is an example of a wide depth of field.
for the technical definition of aperture.
I adapted this post from something I was teaching kids here and updated it with recent photos. I hope this helps people experiment beyond the “full auto” mode, but it certainly isn’t intended as authoritative. Feel free to add to / correct this thread.