If there’s one technical aspect of photography that causes more confusion for beginners than any other, it’s probably aperture. I feel like I explain it a lot, and just this week a friend was asking about a lens purchase and the subject came up again; I thought it was about time to write a quick post on the subject.

The aperture affects how much light is able to reach the camera’s sensor. There’s a bladed diaphragm inside the lens that expands and contracts, depending on how you’re using the camera and what settings you’ve chosen, to allow more or less light in. The aperture is expressed as “f/2.8″ or “f/4″.

What do those numbers mean? The smaller the number, the larger the aperture (meaning more light gets in). That may seem confusing until you think of the expression as a fraction. 1/2.8 is greater than 1/4 is greater than 1/8. Lenses specifications are described in the maximum aperture the lens can achieve — e.g., the Nikon 50mm AF-S f/1.4 can “open up” to f/1.4, which is very wide. Minimum apertures are rarely a concern; that 50mm Nikon can “stop down” to f/16. Cheaper zoom lenses, like the kit lens that came with my Nikon D40, sometimes have variable minimum apertures. For example, my 18-55mm lens is rated as “f/3.5-5.6″. That means that at 18mm it can open up to f/3.5, and as you extend through the zoom range up to 55mm you eventually can’t open it any wider than f/5.6.

So what good is having a wide f/2.8 or f/1.4 aperture? The first and most obvious is the advantage of having more light reach your camera sensor. That means you can use faster shutter speeds than at a lesser aperture in the same lighting conditions; with a smaller aperture, the camera has to hold the shutter open longer in order to gather enough light to make a meaningful image, and/or boost the ISO sensitivity.

Neither of these options are ideal in most situations, since longer shutter speed will blur any motion in your photo and higher ISOs typically mean higher noise (more grain in the photo). Being able to use a quicker shutter speed depending on aperture is why you sometimes hear lenses described as “fast.” A 70-200mm f/2.8 lens (constant f/2.8 throughout the zoom range) would be considered a “fast” zoom; conversely, the 55-200mm f/4-5.6 is considered “slow.” See the diagram for just how much difference there is between f/2.8 and f/4 or f/5.6.

Then there’s bokeh, the other major effect of aperture. Bokeh is the blurring of the background in your photo. There are varying degrees and quality of bokeh depending on the lens used and other factors, but one of the main ways to get a richer, more blurry bokeh is to use a wider aperture. The larger the aperture, the more narrow your depth of field. The DoF is the range at which your subject is in focus; for example, if your aperture and focus is set such that anything between 12 and 14 feet away from you is more or less in focus, then your DoF is 2 feet. The cutoff isn’t instant, of course, but falls off into blur the farther you go outside the DoF area. So with a f/1.4 lens opened all the way up, your DoF is extremely narrow; if you try to take a picture of someone’s face, you could potentially have their eyes in focus but not the tip of their nose or their ears.

So aperture affects light-gathering ability and your depth-of-field. What else? The price! Yes, it’s not technically a photographic trait, but maximum aperture more than anything else seems to affect the price of a lens. Take the two zooms I cited above, for example. The “consumer” zoom, the Nikon 55-200mm f/4-5.6 VR, costs about $200. Works great as long as there’s plenty of light. The “pro” zoom, however — the 70-200mm f/2.8 — is around $1600. Yes, it also has some higher quality glass and is weather-sealed, but the biggest spec difference you’re going to see when going from consumer to pro gear is a wider maximum aperture.

Building wide, constant-aperture zooms is an expensive endeavor. As such, many amateurs or casual photographers will instead buy a fast prime — fixed focal length — lens. There are numerous popular 50mm f/1.4 and f/1.8 lenses available for every SLR platform. I’m personally looking forward to the new high-end Nikon 50mm AF-S f/1.4, which with its 8 elements and 9 blades should be the best-performing “nifty fifty” ever made. It will also, being an AF-S lens, autofocus on the affordable Nikon D40, D40x, and D60 cameras. Having f/1.4 available to me should allow me to photograph in all but the darkest of situations without using any flash.

Make sense? Right. As clear as mud.