So you want to bring your subject into sharp focus while keeping the background blurry, eh? This is a function of your "depth of field" (DOF) and is often desirable for portraiture and macro photography. There are two main variables in this equation:
A) Your subject
B) Your camera
Let's first focus on the former, as it's a little more straight forward. The best way to achieve a shallow DOF (as described above) is to separate your subject from the background as much as possible. The closer a subject is to a camera (and subsequently the further the background is from the subject) the shallower the depth of field. It pretty much works like the human eye. So let's say...
1....Your friend is standing 6 feet from you (the photographer) and a tree is 100 feet behind your friend. In this instance, you have a good opportunity to achieve a shallow depth of field (focused friend, blurry tree)
2.....Your friend is standing 20 feet from you (the photographer) and 2 feet behind her is a brick wall. Due to the lack of separation between your desired subject and the background, the DOF will appear deep (focused friend and focused wall [or if you're a crappy photographer, blurry both!])
Simple, right?
Unfortunately, the variables of the camera are a bit more nuanced, and get quite technical, so let's focus on the "what", and save the "how" for wikipedia. You may hear the terms aperture, DOF, AV, f-stop, etc, but they are are all essentially referring to a single camera function - aperture. Here's the gist - on most newer cameras and virtually all SLR's (you know, the big fancy ones with the detachable lenses) you'll see a setting for "AV". This refers to the len's aperture and determines how wide the lens opens when the shutter snaps. Because the number is fractional (ie - f/1.4) the lower you set the denominator on the bottom, the shallower the depth of field, and the greater your chances of getting that blurry background you're seeking. Simply put:
f/2.8 = blurry background, sharp subject
f/14 = sharp background and subject
As often the case, it isn't quite so simple (it never is, is it?). The aperture of the camera is tied to the shutter speed and the focal length. As for the shutter speed, suffice to say that the longer the shutter speed and the wider the aperture, the more light you let in, so you'll need to watch out for over-exposure. As for the focal length, here's the scoop: The wider the angle, the more you are essentially "flattening" the image, thus compressing the foreground and background, negating the separation you've created between the two. The longer the focal length (the more "zoomed in" you are) the more you are optically expanding the space between your friend and that tree, and the better depth of field.
All of this is a merely anecdotal compared to the vast science behind these statements, but in the spirit of brevity, here's the skinny:
1) Separate your subject from the background, and get as close as possible to the subject.
2) Crank up the aperture as wide as possible (low AV number).
3) Zoom in!
Thanks for stopping by, and as a token of my appreciation, here is a picture of a duck.
Cheers.
