Have you ever noticed those black blades within your lenses? In optics, an aperture is simply the hole through which light travels. As you can imagine, changing the size of that hole can make a huge difference in the look and feel of your photographs. There's an exhaustive definition of the topic over at Wikipedia if you're interested, but we're assuming you stopped here because you're just looking for the long and short of it. Here are a few general rules to understanding aperture:

• The lower the f/stop, the more light is allowed in.
• Exceptionally low f/stops (f/1.2 through f/2, for example) are only found on a handful of lenses, primarily professional DSLR lenses.
• Most point-and-shoot cameras only stop as low as f/3.5 (at best), limiting the amount of light you can fetch when shooting in dimly lit scenarios.
• You'll pay dearly for exceptionally low f/stops. A Nikkor 50mm f/1.8 can be found for ~$100; the same lens with an f/1.4 rating (just one step lower) is three times more expensive at ~$300.
• Lower f/stops narrow your depth of field; a shot at f/2 will have a very tight focal point, with a tremendously blurred background, whereas a shot at f/14 (as an example) will focus on the foreground and background with essentially no 'bokeh' to speak of.

Now that you've got a grip on that, we're going to break down the most common uses of aperture when it comes time to compose a shot.

• A lowered f/stop can be artisically chosen if you want to focus in tight on a foreground subject while introducing a silky, beautiful blur (that's the 'bokeh' we mentioned above) around the subject. This is great for focusing on a person with a less-than-exciting backdrop.
• A higher f/stop is useful for capturing vast groups, where you want the persons on the edges to be just as sharp and in-focus as the person in the center of the image.
• A lower f/stop is very useful for letting more light enter an image during dimly lit or dark situations; this prevents you from having to boost your ISO (and thus, inject noise and grain) or dramatically slow your shutter speed (and thus, potentially introduce unwanted blur from hand-shake).

Let's look at an example of how lowering your f/stop can be beneficial at night and in situations where you want oodles of bokeh surrounding the subject. The image below shows an identical shot at f/1.4 and at f/8, both taken in a dimly lit room with very little ambient light around. Lowering the f/stop allows a tremendous amount of light to flood in, in turn giving us a useful image without resorting to firing a flash. The moral of this story? Lower your f/stop when you're in dimly lit areas -- your images will thank you!

Gallery: Aperture and light, and how they interact

Now, let's look at an example of injecting bokeh into a shot. These two pictures were taken with a f/1.4 (left) and f/16 (right) aperture. You'll notice the shot on the left has a soft, silky, progressive blur surrounding the focal point. This highlights the subject and simultaneously hides the ho hum background. The f/16 shot has most of the background in focus, effectively destroying your ability to focus only on the foreground subject and disregard the lackluster backdrop. On the flipside, your backdrop is in focus, so if that is your goal for a shot, now you know how to accomplish it. The moral of this story? Lower your f/stop if you want to introduce bokeh, bring out the foreground subject and blur the background; raising the f/stop will help you to focus on a larger image, such as capturing an entire soccer team.

Gallery: Aperture and bokeh, and how they interact

Our suggestion now is to give it a try! If you have a camera where you can adjust the aperture manually, try placing your camera in Aperture Priority (the "A" mode on the dial) and stopping it completely down as low as it'll go. This will vary based on the lens, but toggle the f/stop and lower it to the smallest number allowed by whatever lens you are using. Focus close on a foreground object, and snap the shot. Check out that bokeh! If you're having a hard time getting the bokeh effect, try holding an object out in your hand and focusing; that's an easy way to get the background to blur nicely. Now, try that same shot with an aperture of f/8 or greater in order to see how wide your focal range becomes.

Keep in mind that this is just a basic explanation of aperture to get you started. In future articles, we will cover tips on how to use changes in aperture for creative effects in scenarios related to travel. For example, using the aperture to help you focus on your kids while blurring crowds behind them, ensure that your entire background is in focus in self-portraits, and more. Hopefully with the pointers listed here and in our previous articles on ISO and shutter speed, you'll be three steps closer to understanding your camera's 'Manual' mode.

Let's recap:

• The lower you set your f/stop, the more light you'll have access to. This allows you to rely less on a boosted ISO and a sluggish shutter speed to still get a usable image in low-light situations.
• If you need to focus on a large group of people, or you want the ocean behind you to be sharp, use a higher f/stop.
• If you want to introduce artistic blur (or 'bokeh') into your images, use a lower f/stop.

Stay tuned for more tips on understanding metering, white balance and more! Our basic guide to understanding ISO and shutter speed can be seen here.