If you use a Holga or Diana or any of those other inexpensive plastic cameras, you will get darkening on the edges (this is called vignetting). It happens when the lens on the camera does not project an image that is big enough to fit on the film. The size of the image a lens projects is referred to as the covering power of the lens.
Originally Posted by mporter012
However, Michael Kenna, like many (I almost said "all" but that's not true) great black and white film photographers, is a master of printing, so whenever you see one of his images you can be sure he has manipulated it in the printing stage to make the picture look exactly how he wants it to look. The two most basic forms of printing manipulation are dodging (witholding light from the paper so that area of the print becomes lighter) and burning (adding extra light to the paper so that area becomes darker). If you want to know more about it there are many great books out there, including a very in depth book by Ansel Adams called "The Print".
If you want to see what is possible with this kind of creative manipulationlook at this page (Rolfe Horn is a former assistant of Michael Kenna). The results of masterful printing technique are quite remarkable, as you can see:
Edit: Looks like John beat me to the Rofle Horn Link