I have a basic understanding about depth of field, but when composing a photo, how can I determine what will be in focus and what will not? That is not something I can see through my Digital SLR’s viewfinder or in Live View, right? Also, how can I get photos where the background is nicely blurred like the ones that professionals take?
It’s impossible to fully cover depth of field (the range of acceptably sharp focus behind the subject and in front of it) in less than 2000 words Dani, but in a nutshell, depth of field is shallow at a wide aperture (small f/number) so the background will be blurred to some extent. But if you use a small aperture (large f/number) depth of field will be more extensive so the background will be more distinct.
To answer the primary question, most DSLR cameras are equipped with a depth of field preview button that allows you to view a scene at the actual aperture that will be used to take a photo: whether f/1.4, f/5.6 or f/22. You must press the button to view the actual range of acceptably sharp focus because by default, the viewfinder always shows the scene at a lens’s maximum aperture (this feature works only when you use the viewfinder and not when using Live View).
If you set f/16 for example, pressing the button allows you to view the scene at this very small aperture; you’ll be able to see that the range of acceptably sharp focus is extensive. The viewing screen will get dimmer since less light enters the lens when it’s set to f/16. That should not be a problem in bright light; even in a dark location, your eye should soon adjust to the dimmer view. View the scene at several f/stops, using depth of field preview, and use the one that provides the effect that you want.
And here’s an answer to your second question. Depth of field is shallow at high magnification. Hence, it’s easy to blur the background when you use a long telephoto lens or in extremely close focusing with a Macro lens. (With a wide-angle lens, we rarely move close enough to the subject for shallow depth of field.) So if you really want to blur the background in a photo, use a 200-mm or especially a 300-mm focal length at a wide aperture such as f/5.6. You can achieve that effect even at 55 mm but to do so, you would need to move very close to the subject and that’s much less convenient.