Depth of field is, like many things in photography, a subjective thing. What's beautiful to one is horrible to another. But to get to the point where you can create art the way you want you need to have full control over your depth of field.
For those of you who are a bit shaky on the term, depth of field basically means what is sharp and what is blurry in the picture. For example in this image both the palm leaf and the Thai fishing boat are in focus. That is what we call a large depth of field. If either the palm tree or boat had been blurry we call that a shallow depth of field. If both are blurry we call that a crappy photographer! :)
Just how blurry or sharp you want to make foreground and background objects in your picture is controlled by your aperture. When you have a large aperture (f2.8, f3.5 etc) you will get a very shallow depth of field. You know the portraits where the eyes are in focus and everything else is blurry. That kind of effect.
When you want everything in focus, say in landscape photography, you would use a small aperture (f16, f22 etc). But what if you want something in between these two extremes?
Just what aperture exactly do you need to get the right amount of depth of field, or is it all a bit hit and miss? Well I'm happy to say it's definitely not hit and miss but you'll need a couple of features on your camera to help you.
When you look through the viewfinder of your camera it is always at its most wide open aperture. This is so the image you're looking at is nice and bright. If you have your aperture set at anything other than its widest setting, when you push the shutter button, while the viewfinder is blacked out, the aperture closes down to take the photo and then opens back up again. So looking through your viewfinder is no indication whatsoever of what the depth of field in your final picture is going to look like because you're always looking at an aperture of f3.5 say, or f2.8. Whatever your widest aperture is.
To see what your picture will look like at a smaller aperture you need to push a little button just underneath your lens. This button is called the depth of field preview button. Not all cameras have them but a lot of them do these days. When you push this button your lens aperture will suddenly close down and the viewfinder will get very dark indeed! It takes a little while for your eyes to get used to this new dark viewfinder but after you get used to it you will be able to see how much is blurry at that particular aperture setting.
To fine tune your depth of field you then open or close your aperture whilst pushing your depth of field preview button and you can see your background gradually get clearer or blurrier. At least this is how it works on the Canons. Some cameras don't let you change aperture whilst holding the depth of field preview button down. So you'll have to let go of the button, change your aperture and then push the button again. This is a great tool to really fine tune your depth of field and is invaluable in landscape photography. In fact every time I buy a new camera this is one feature that is a must on my list.
If you have Live View on your camera you can also use the depth of field preview button, which saves you looking through the viewfinder. The same principle applies. Change your aperture and then push the button and look at the differences.
If you don't have a depth of field preview button then the only other way is through trial and error. But at least with digital it's an instantaneous experiment. Take a shot and look at it on your LCD. Not enough depth of field? Close your aperture down more. Too much depth of field? Open your aperture up a bit more? It's a bit of a fiddly way of doing it but might be your only option.
In this day of auto-exposure, auto-focus, auto bloody everything cameras depth of field is one area where you can really take control of your images. So take yourself out of automatic mode and stick it into Aperture Priority and then play around with your depth of field button until you get to the stage where you really are controlling exactly what parts of your image are sharp and what parts are blurry.