So all lenses aren't made the same. You can get a model that back focuses, focus ring is loose, or is below standard sharpness for it's model. You can also get an anomaly that's better than its peers. In my case I got a copy that was unbelievable sharp. How sharp?
Comparison Part I
Here's a comparison of the image quality between Canon 50mm f/1.2L, Canon 50mm f/1.4, and Sigma EX DG 50mm f/1.4, (left to right) at the 1.4 aperture. ( I didn't take these three, they're from The Digital Picture. Very thorough reviews of camera equipment. Visit it!)
Comparison Part II
I started by grabbing the good' ol test chart and shooting both lenses at f/1.4. I took a photo of the Canon 50mmm f/1.0L wide open with some pretty amazing results. I mounted the camera on a tripod, took three shots for each frame below and picked the best from each one. I actually had to mov the Sigma in closer because it was slightly wider than the Canon. For some reason the definition of "50mm" varies slightly between manufacturers. I then cropped the center of the frame so you can see the differences better.
Looking below you can see the Sigma 50mm at 1.4 first, then the Canon 50mm at 1.4, last the Canon 50mm at 1.0. The Canon is actually sharper at 1.4 then the Sigma, it also has less chromatic aberration (the Sigma shows a lot of green fringing), What's mind blowing is that you can clearly see that the Canon is actually sharper at 1.0 then at 1.4!?! It does show a lot of chromatic aberration and does loose contrast. The performance wide open is strangely stellar.
- have a good chunk of change they can spare
- are collectors (because this lens is no longer made, with a predicted price of $15k by 2030)
- really want to set their work apart
- shoot in extreme low light (it's the great equalizer at weddings)
- want the depth and bokeh of the 85mm f/1.2L II USM while maintaing the perceptive of a 50mm
- want bragging rights
Do I recommend buying this lens? If I had received a standard copy, probably not. If you have the extra money go for it. You might get lucky and get a phenomenal copy! I think it should be standard practice for ISO crops to be pictured with auctions on eBay. It allows proper pricing for the seller and more confidence for the buyer. I'm fairly sure that the guy who I bought mine from was unaware of what he had.