First time I have heard someone say Sigma is sharper than Canon. I wonder if others have the same experience?
I am surprised the price difference is so little. It is indeed a very hard sell to get someone to buy third party unless it is significantly less expensive.
The one thing I do like better about Sigma is the black color. I really hate white lenses (even though I bought the Canon version 3 on first release).
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In the centre of the image circle (i.e. 50% of the total image area) , the Sigma and Canon's are basically the same!
It is the OUTER CORNERS where I have consistently noticed the Sigma's are sharper at wider apertures. You can always stop down on a bright day or use a neutral density filter to reduce that difference. BUT... When I'm shooting at F/1.4 to F/2.8, I want the wider open lens to get all the light I can at Sunset or Sunrise or my Night Shots! Sigma lenses have had an on/off quality control issue in that you MUST TEST the actual lens you are going to buy with a 2400 dpi resolution chart so you can compare between individual Sigma lenses. Canon IS VERY CONSISTENT for ALL of it's L-series lenses. Two Canon lenses will be almost identical in sharpness and bokeh, while two Sigmas will vary, sometimes by quite a lot in my opinion!
However, the Sigma lenses we do have ARE DEFINITELY SHARPER at the same aperture in the corners vs the same focal length Canon's. It used to be that Sigma's prices were so much cheaper than Canon's that if you were careful and TESTED each Sigma lens in-store, you could get a GREAT set of lenses at a GREAT PRICE versus the equivalent Canon. NOW, Sigma is raising prices and IMPROVING its quality control, so the differences are becoming less pronounced as time passes. I still say Sigma is sharper in the corners BUT Canon DEFINITELY has much better looking Bokeh! So again, for in-studio, wedding portraiture and head-and-shoulders shots, use the Canon's!
For my PERSONAL CHOICE at 50mm, 85mm and 135 mm, I pick Sigma for all three focal lengths over the Canon's. At 200, 400 and 600+ mm I would DEFINITELY pick Canon because of Canon's superior build quality and better internal focus mechanisms on such long focal lengths.
HOWEVER, I should also note that on a Bang-for-Buck basis, the BEST sports zoom out there is the Sigma 150-600mm f/5-6.3 DG OS HSM Sports Lens for Canon EF. This is an AMAZING ZOOM LENS! I can be at the far end of a football (i,e. Soccer) field and STILL get a fast-action play shot on the opposite Goalkeeper under those usually awful stadium lights! DO NOT get the cheaper "Contemporary" version of this lens -- rather DO GET the SPORTS VERSION ($1800 U.S. for December 2018) -- It is so incredibly GREAT for big field sports and long-range wildlife shots!
See weblink:
https://www.bhphotovideo.com/c/product/1082152-REG/sigma_150_600mm_f_5_6_3_dg_os.html
Another set of lenses if you can afford the $5000 price tag is the Zeiss Otus series which are the SHARPEST LENSES corner-to-corner EVER!
It's a MANUAL ONLY lens but it's sharpness is undisputed.
For the BIG THREE focal lengths of 50mm, 85mm and 135mm which most people will get at the prosumer level, the Sigma is STILL CHEAPER than the equivalent Canons by around 15% to 25% or even more depending upon your country. Just remember to TEST each Sigma you buy personally at the store!