One of the steps in changing from someone who "takes snaps" to one who makes photographs is to create images which guide the viewer to the subject. There are a number of things you can do in this regard, but one is to learn to control depth of field (DoF).
Depth of field (also sometimes called depth of focus) is the depth of the area in focus. For example, while you would typically want deep depth of field in a landscape shot (just about from your toes to the horizon), in a portrait, you may want the background to be pleasantly blurry, and you can achieve that with a shallow DoF.
Of course, this isn't always the case. For environmental portraits, where you may wish the subject's location or workplace to be clearly in view, you would set up for deep depth of field.
Claude in his workshop
Four factors generally dictate depth of field, and ordinarily, only three of these can be controlled by you:
So let's look at some examples - and this is a simple exercise which you could do at home.
Firstly, let's look at the effects of aperture. In these two shots, both the focus point and the focal length of the lens is unchanged. The top image was shot at f/4, while the second image was shot at f/16. You will see that the DoF is much deeper in the second image than the first, which is as one would expect as the smaller the aperture (ie: the higher the f-number), then the deeper the DoF.
Image shot at 50mm and f/4
Image shot at 50mm and f/16
Now lets look at the effect of focal length. The second pair of shots was made with the aperture set at f/8. In the first, the lens was zoomed out to 120mm, while in the second, it was set to a wide 24mm.
Image shot at f/8 and a focal length of 120mm.
Image shot at f/8 and a focal length of 24mm. Note that the shot made with the long 120mm has much more shallow DoF than the image made at 24mm. This is despite the camera being much closer to the subject in the second shot, where you would expect DoF to be more shallow!
So, choosing an appropriate aperture or focal length or carefully choosing where to focus (or all three) will help control depth of field, and guide your viewer's eyes to the key parts of the image.
Ok, so much for test shots. Let's look at some actual photographs and examine how they were created. First, this image of a street in Cambridge:
Cambridge street. f/11 at 38mm focal length. This was shot with a fairly wide 38mm focal length and with an aperture of f/11. The focus point would have been around that black object on the right, to ensure the maximum area in focus.
Both the focal length and the aperture contributed to its deep DoF.
Next, let's look at this image of terns on Montague Island. The settings were f/16 at 200mm:
Terns with chick on Montague Island. f/16 and 200mm. I used a small aperture (f/16), so why was the DoF so shallow? Two reasons: the long focal length of 200mm had a lot to do with it, and the fairly close focus also reduced DoF. If I'd got up close and shot this at say 50mm, that grass and those birds would be quite sharp - but that's not what I wanted.
Finally, let's look at this image of dry grass. I wanted the grass at the front to be sharp, but with a rapid fall-off of sharpness so that the background was quite blurry.
Dry grass seeds, Australian Capital Territory. f/5.6 at 36mm In this image, and despite the focal length being shortish at 36mm, the combination of a reasonably wide f/5.6 and more importantly, the lens being close to the focal point on the nearest grass created the pleasingly shallow depth of field I was looking for.
So that's it - control the aperture of your lens, select where you wish to focus and chose the right focal length of your lens, and you can decide what's in focus - and hence, what viewer looks at in your photographs.
If this isn't all that clear, you can google for other examples, or even better - try different combinations with your own camera. Practice makes perfect!