One of the greatest benefits in moving from a compact digital point-and-shoot camera to a DSLR is the increase in flexibility you have over depth of field. Depth of field, or DOF, is a measure of how much of a scene is in focus in an image. Sometimes, you want as much of the scene to be in focus as possible, but there are many times when you want to direct the viewer’s attention to the subject, and leave the background as a nice, non-distracting blur. DSLRs give you far more ability to control DOF than a compact point-and-shoot.
Why is this the case? There are several reasons for this. The first is sensor size. DOF is inversely proportional to sensor size: the smaller the sensor, the greater the DOF you will generally have. Image sensors in cell phones or compact digital P&S cameras are quite tiny, and hence have a lot of DOF compared to the APS-C (1.6x crop) sensors of a camera like a digital Canon Rebel or 40D or 50D. Full-frame DSLR cameras, like a Canon 5D or 1D have sensors that are as large as a 35mm film frame, and thus the effect is even more magnified. Incidentally, this is one example of how the raw number of mexapixels in a camera is not even close to the full story in trying to determine which camera is “better”: the 8 megapixels in a Canon 1DMarkII camera is going to give you a vastly superior image than an 8 megapixel P&S camera.
DOF depends on the aperture setting of the lens (how much light it allows in), and here again P&S cameras are lacking. To get the most blur, the lens has to be set at a large aperture, which means a low f-number. While lenses for a DSLRs can have maximum apertures of f/2.0 or lower, many compact P&S cameras are limited to f/3.5 or f/4. A higher f-number means a narrower aperture, and hence more DOF and less blur. Finally, lens focal length plays a role in DOF, and again compact P&S cameras are limited in their options.
For all of these reasons, images from compact P&S cameras seem a bit flat and busy: they have a large DOF, so everything is more or less in focus. It can be next to impossible to get a pleasing level of background blur (which, by the way, is called “bokeh”, and it has to be one of my favorite words to say). A DSLR, however, gives you much more latitude…
This photo, taken of a bee in a garden along Summit Avenue in Saint Paul, shows a relatively limited DOF. I took this photo with a 50mm lens at an aperture of f/3.2, which gives a fairly shallow DOF. This lens, the “nifty fifty”, opens up all the way to f/1.8. At such an aperture, the DOF is incredibly narrow (sometimes on the order of a couple millimeters) and so it can be hard to focus correctly.
This picture, of a lizard hanging from a tree in the J.R. O’Neal Botanical Garden in Tortola, British Virgin Islands, shows how a longer focal length leads to a shallower DOF. This was taken at 135mm at f/5.6, which, given the distance from the subject, created a fairly shallow DOF. The background is completely blurred, and even the entire tree is not in focus. A similar shot with a P&S camera probably would have had a much larger DOF.
Flash photography often requires large apertures due to the lack of light, and hence there is a tradeoff with DOF. In this photo of Миша sitting on a blanket, taken with a 50mm lens at f/3.5 and a 430EX II flash, the back of his body blurs out, directing attention to his menacing expression.
As you can see, a DSLR camera gives wide latitude over controlling DOF, just one of the many benefits they have.