Today we are going to talk a bit about depth of field, and I'll give you several visual representations of what it looks like.
Here is an image of what the aperture blades look like in a lens (though they are technically known as the "focal plane diaphragm blades"), the wider they are open the more light comes flooding in and the wider the blades are open the more shallow your "focal plane" becomes (meaning the things in front of and behind the object you are focusing on will be blurry).
The lower your f/stop is set (say f/1.4), the wider those blades are open.
The sample images below were shot with a Nikon 60mm f/2.8G AF-S Micro lens, so the lowest the aperture can be set at is f/2.8 (aka "wide open"). When any lens is wide open (lowest f/value) you won't even see the blades, all you'll see is a round hole.
There is also a direct correlation with your f/stop and shutter speed, a lower f/stop (number) will allow you to shoot at a higher shutter speed. When photographing a wedding (or reception) for example you may have low light and don't want to (or can't) use a flash, by dropping your f/stop you can bump up your shutter speed.
I could ramble on, but I am more of a visual kinda guy, so here are a few examples I shot this morning, the lighting was not changed for any of the shots, the only changes were to the f/stop and shutter speed.
Notice that as the f/stop increases the blades begin to close, and if you want to keep the exposure the same you need to decrease the shutter speed, and as this is going on the depth of field becomes deeper.
(click on any image to view larger)
I am a huge fan of Mark Wallace, in the 7 minute video below he does a very nice job explaining depth of field.
(subscribers will need to return to the blog to watch the video)