I've used 4 of the 5 50mm's (50 mk1, 50 mk2, 50 macro, and 50 1.4)
My favorites are the 1.4 and 2.5 macro. I like the gaussian optics and brightness of the 1.4, but love the sharpness of the 2.5 macro.
If you are torn between the 1.8 and 1.4, for the price of a good mk1 50mm, you can spend a little more and get the compact macro.
By the way this forum is amesome!!

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Ivan A.
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my flickr gallery
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"Photographers deal in things which are continually
vanishing and when they have vanished there is no
contrivance on earth which can make them come
back again."
-Henri Cartier Bresson