The best way is with a lens capable of a very narrow depth of field and a wide-open aperture. This is usually how macro images are done. The smaller the aperture, the deeper the depth of field, and vice versa. But mainly, this is handled in the lens, and if you are using a macro lens or extension tubes, you'll see this in action better thn it can be described. (Ideally, you'd set up a coin or something and play with the same image on all different aperture settings to get a feel for it.)
Another way is to use a center spot filter, which is a kind of a diffuser designed to do this specific thing. You can make a diffuser by spreading vaseline thickly around the edges of a UV or polarizing filter, making it lighter toward the center, but this is messy and hard to control. Typically, a center spot filter has a hole in the middle (http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/filter/filter-cspot-hoya-icon.jpg), and the filter radiates out from the hole with increasing blur to the edge. A diffusion filter may do the same, but with no hole /focus spot, or else with a more subtle one.
Yet another way is with a lensbaby (http://www.lensbaby.com/), which is a cheap lens (relatively) that creates extreme sweetspots of focus and extreme "fall away" or focus. You can bend them, too, which is fun -- it's like a lens on an old bellows, and you can bend them around to move the sweet spot and change the fall-away.
You can also do it in photoshop, using a layermask, described here: http://www.photoshop911.com/tricks/focus_falloff/
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Another way is to use a center spot filter, which is a kind of a diffuser designed to do this specific thing. You can make a diffuser by spreading vaseline thickly around the edges of a UV or polarizing filter, making it lighter toward the center, but this is messy and hard to control. Typically, a center spot filter has a hole in the middle (http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/filter/filter-cspot-hoya-icon.jpg), and the filter radiates out from the hole with increasing blur to the edge. A diffusion filter may do the same, but with no hole /focus spot, or else with a more subtle one.
Yet another way is with a lensbaby (http://www.lensbaby.com/), which is a cheap lens (relatively) that creates extreme sweetspots of focus and extreme "fall away" or focus. You can bend them, too, which is fun -- it's like a lens on an old bellows, and you can bend them around to move the sweet spot and change the fall-away.
You can also do it in photoshop, using a layermask, described here:
http://www.photoshop911.com/tricks/focus_falloff/