With a sensor that's larger than those found in most digital cameras, the Canon 5D Mark II can take sharper pictures in low-light situations than most DSLRs (and can make better use of any old Canon film lenses you may have lying around). As a pro-level camera, it has a steeper learning curve than lower-end DSLRs, and you'll have to shell out extra cash for such niceties as a flash. With a successor model (the Mark III) recently launched, prices have dropped on the Mark II, though it will never be a bargain.
How It Stacks Up
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