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A lens designed for Macro Photography, with 'Macro Lens' in its name, would be best.
E.g. a USM Macro Lens, or an HSM Macro Lens.
(Of course the lens should match the lens mount on your Canon EOS 50D.)
Some say that a 1x maximum magnification, for such a lens, is ideal.
If you need to include anything much in the background, short focal length is ideal, also. A longer focal length will limit background. Also, a longer focal length will blur the background more.
If you need specific names of lenses that would do for your needs and your camera model, as opposed to the above specifications, I will advise you further, if you let me know, please.