Aperture is often the most difficult concept for people to grasp when they're learning how their camera works, but it's pretty simple once you understand it. If you look at your lens, you can see the opening where light comes through. When you adjust your aperture settings, you'll see that opening get bigger and smaller. The larger the opening, or wider the aperture, the more light you let in with each exposure. The smaller the opening, or narrower the aperture, the less light you let in. Why would you ever want a narrow aperture if a wider one lets in more light? Aside from those situations where you have too much light and want to let less of it in, narrowing the aperture means more of the photograph will appear to be in focus. For example, a narrow aperture is great for landscapes. A wider aperture means less of the photograph will be in focus, which is something that's generally visually pleasing and isn't seen as a downside. If you've seen photographs with a subject in focus and beautiful blurred backgrounds, this is often the effect of a wide aperture (although that's not the only contributing factor—remember, telephoto lenses decrease depth of field as well). Using a wide aperture is generally considered the best method for taking in more light because the downside—less of the photograph being in focus—is often a desired result.
Shutter Speed
When you press the shutter button on your
camera and take a picture, the aperture blades take a specific amount of time
to close. This amount of time is known as your shutter speed. Generally it is a
fraction of a second, and if you're capturing fast motion it needs to be at
most 1/300th of a second. If you're not capturing any motion, you can sometimes
get away with as long of an exposure as 1/30th of a second. When you increase
your shutter speed—the length of time where the sensor is exposed to light—two
important things happen.
First, the sensor is exposed to more light because it's been
given more time. This is useful in low light situations. Second, the sensor is
subject to more motion which causes motion blur. This can happen either because
your subject is in motion or because you cannot hold the camera still. This is
fine if you're photographing a landscape at night and the camera is placed on a
tripod, as neither the camera nor your subject is going to move. On the other
hand, slow shutter speeds pose a problem when you're shooting handheld and/or
your subject is moving. This is why you wouldn't want a shutter speed any
slower than 1/30th of a second when photographing handheld (unless you're known
for your remarkably still hands).
ISO
ISO is the digital equivalent (or approximation) of film speed. If you remember
buying film for a regular camera, you'd get 100 or 200 for outdoors and 400 or
800 for indoors. The faster the film speed the more sensitive it is to light.
All of this still applies to digital photography, but it's called an ISO rating
instead.
The advantage of a low ISO is that the light
in a given exposure is more accurately represented. If you've seen photos at
night, the lights often look like they're much brighter and bleeding into other
areas of the photo. This is the result of a high ISO—a greater sensitivity to
light. High ISOs are particularly useful for picking up more detail in a dark
photograph without reducing the shutter speed or widening the aperture more
than you want to, but it comes at a cost. In addition to lights being overly
and unrealistically bright in your photos, high ISO settings are the biggest contributors
to photographic noise. High-end cameras will pick up less noise at higher ISOs
than low-end cameras, but the rule is always the same: the higher you increase
your ISO, the more noise you get.
Most cameras will set the ISO automatically,
even in manual mode. Generally you can stick with the same ISO setting if your
lighting situation doesn't change, so it's good to get used to setting it
yourself. That said, sometimes lighting changes enough in dark, indoor settings
that letting the camera set it for you automatically can be helpful—even when
shooting manually.
