I decided to try my cheap Sigma 18-50DC on my 6D before I returned it, the results didn't seem any worse (And much wider) then on my T2i after I cropped out the vignetting. I zoomed the lens fully and focused to both stops before putting it on and the rear element ever protrudes past the mount... so I'm assuming this lens is safe to use on the 6D? I don't think I'll ever feel the need to since I'm getting the 24-105L now, but I'm curious :P
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Sigma self portrait by EverydayGetaway, on Flickr
I think you need a new pair of socks.
There appears to be a hole in it.
Such a sweet and beautiful cat. <3
He/she is probably thinking: you can photograph me as much as you want, as long as you give me my food now.