Attached Link: http://www.renderosity.com/mod/gallery/index.php?image_id=1402723

In real-world photography, depth of field refers to a "zone of sharpness" that extends in front of and behind the point you have focussed on and can be very narrow, where only your subject, or part of it, is in focus, or very wide where everything in the picture is sharp and in focus.  The closer you get to a subject the narrower this zone gets too.

The two most obvious controls you have over depth of field are lens focal length and aperture.

The focal length of the lens, briefly, can be termed the "size" of the lens, i.e. (say) 28mm or 500mm; 28mm is a wide angle lens, 500mm is a telephoto lens.  It refers to the distance behind the lens (in mm) at which the lens focuses the image it is being pointed at.  The longer the lens, the less depth of field you get, conversely, the wider the lens, the more depth of field you get..
The aperture is what you control with the aperture ring and is usually referred to by it's "f-stop"...an f-stop of 1.2 is wide open, and one of 64 is tiny.  The aperture's primary function is to control how much light gets through the lens.  The wider your f-stop, the less depth of field you have ; conversley, the smaller the f-stop, the greater the depth of field..
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Bryce attempts so simulate this with it's Lens Radius setting; the larger the number, the more blurred your background will become.

The picture on the left is an unretouched render done with Premium Antialiasing and depth of field (and soft shadows!) - link to the full-sized image is above.

The settings were:

Lens Radius 0.07
Focal Length 56.50

The one on the right has a lens radius of 1.0, just to show the effect (couldn't wait for it anti-alias, though!)

Hope this helps; I used to be a photographer and darkroom technician.

My self-build system - Vista 64 on a Kingston 240GB SSD, Asus P5Q Pro MB, Quad 6600 CPU, 8 Gb Geil Black Dragon Ram, CoolerMaster HAF932 full tower chassis, EVGA Geforce GTX 750Ti Superclocked 2 Gb, Coolermaster V8 CPU aircooler, Enermax 600W Modular PSU, 240Gb SSD, 2Tb HDD storage, 28" LCD monitor, and more red LEDs than a grown man really needs.....I built it in 2008 and can't afford a new one, yet.....!

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