EssPee mad a comment on my previous post about aperture and said "Thought F numbers basically changed depth of field!"...

Which is funny because that was my next planned post :D

As well as deciding how much light is let into the lens aperture also changes the depth of field of a picture, or in plain English... how much of the pic is in focus.

It does this because... because... OK, it's some scientific reason I'm not smart enough to understand or even research, so lets just call it magic ;)

Basically, the smaller the aperture the more of the picture will actually be in focus.

Earlier today I went out with my camera, popped it on a tripod with my 50mm lens and snapped a shot for every aperture range I have on the lens...

If you want to see them all (you strange person...) then you can go HERE but to save you all from the tedium, lets pick a few...

These were taken on a tripod, focussed on the branch and as close together as they could be, I snapped, turned the wheel, snapped... and repeat.

Taken at f/1.8 (smallest available aperture)
f/1.8

Taken at f/7.1
f/7.1

Taken at f/13
f/13

Taken at f/22 (widest available aperture)
f/22

As you can see, at f/1.8 anything not the branch I focussed on is bokeh (fancy word for out-of-focus blur) there's a distinct item in focus with everything else fuzzing out.

As we slip through the aperture numbers the pot it's in and the background start looking much clearer until finally, at f/22, the entire scene is in some kind of focus.

Obviously this means you can't simply aim for the biggest aperture all the time, if you want more than your main subject in focus you'll need to 'stop down' a bit.

One use for this in the opposite direction is to use a large aperture for portraits as you can isolate your subject by removing the background.