We've talked about aperture before but today I really want to focus on and maybe even pressure you a little into trying some things out. Opening your aperture up wide means moving your f/stops to a low number. The lower the number the wider the aperture...which then means a more shallow depth of field aka whats in focus. Unlike a small aperture (larger number) where the depth of field is deep and there is lots of room to work on you focus. With a wide aperture focus is sort of a must. When your aperture is small so much is already in the focus field that what your really wanting to focus on sort of just blends in...

Taken with the aperture set and f/8 it didn't really matter what kid I focused on the  aperture was set small enough that everything was already in the depth of field to begin with. Now you should still always make sure what you are really wanting to focus on is in focus, I'm not saying that I'm just saying with a small aperture there's room to mess up.

Now here the aperture is wider f/3.2 this is a pretty wide aperture and there is little room for my focus to move of for my eyes to mess up. Had I been focused on the back of her seleve instead of her fingers I would have mess this whole shot up with just those few cenitmeters of diffence.

With an aperture so wide as say f/1.8 which is as wide as this 50mm lens would go there is no room for error. This one long grass stem is the only thing my lens sensor has to focus on. Everything behind it and in front of it, is completely out of focus. THERE IS NO ROOM FOR ERROR. Shooting at the aperture setting takes lots and lots of practice but I assure you it is so worth it. With an aperture set so wide everything becomes bokeh and bokeh just has a special place in my heart. Just take a look at some of these great flickr groups.
Bokehwhores
For the love of bokeh
Bokehpalooza