I've been conversing with a few comrade Photobloggers lately on the topic of aperture. This is probably the most important setting on your camera. If you already know about it, you can tune out here.
For a really good, simple explanation, see the Canon web
site.
To control your camera's aperture setting, you need to either put it into
Manual or
Aperture Priority mode. In the latter, you set the camera, and the electronics work out the best shutter speed. I probably use this mode about 90% of the time, but as I become more confident, I'm using Manual mode more.
In these posting I've used three different settings. Notice how the depth of field (DOF)
increases as aperture
decreases (a smaller number means a larger aperture. My Sigma 24-70mm has an aperture range of f2.9 (wide) to f32(narrow)).
Notice also how the shutter speed get longer. In the final one, the shutter speed was going to be too long, so I increased the camera's sensitivity by increasing the ISO. Had I not done so, the shutter would've been open too long, and I would've had a blurry picture.
#1 - f4.5 - 1/200 sec - ISO100
#2 - f18 - 1/13 sec - ISO100
#3 - f32 - 1/13 sec - ISO400
On a SLR cameras it's very quick and simple to change these settings. On other cameras you can often do it, but it's generally more fiddly. Sometimes it might help to put the camera into 'night mode' (which doesn't mean tuck it into bed with a hot toddy).
Incidentally, the fancy term for background blur is
bokeh.
