Seems like the [url="http://www.lensrentals.com/rent/sigma-50mm-f1.4-hsm/for-canon]comments by Roger at lensrentals.com[/url] might be relevant:


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Quote Originally Posted by Roger
Roger&rsquo;s take[/b]Its become apparent through many tests and online forums that the Sigma 50 is a &lsquo;special&rsquo; lens. I was glad to find this out because it was driving me nuts. Its a very nice lens when you use it just right, with superb sharpness and smooth bokeh. But here&rsquo;s the summary:


1) It works very well on crop frame cameras and at middle (5 to 15 feet) distance, so its a wonderful indoor lens.
2) On full frame cameras and to a slight extent on crop frame cameras the lens exhibits what I will term &lsquo;schizophrenic autofocus&rsquo;: Closer than 5 feet it will front focus, further than 20 feet it will backfocus. This is not a calibration issue, its just how it is.
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