About 2 months ago JJ @ OrangeWeddingFilms.com posted a video stating he was going to put together a test between the Canon 24-70mm F/2.8, Sigma 24-70mm F/2.8, and the Tamron 28-75mm F/2.8 in Video use. Looks like he's posted a few first samples just yesterday and in some cases you do see difference in Contrast, color, and sharpness. I personally like the Canon 24-70mm F/2.8, but if you're happy with the results, you can save a bit of money on the other non Canon brands. Here's the break down on lenses, prices, and what you're comparing.
First up Canon 24-70mm F/2.8 Wide Telephoto priced at approx $1349.00
click image to find at bhphotovideo.com
Second Lens is the Sigma 24-70mm F/2.8 Macro priced at approx $569.00
click image to find at bhphotovideo.com
Third Lens in the test is the Tamron Zoom Wide Angle-Telephoto AF 28-75mm f/2.8 priced at $499.00
click image to find at bhphotovideo.com
To follow up on his first review against these lenses, i've owned them all at one point. I still have the Canon 24-70mm F/2.8. Sigma makes some really great lenses, I still rock my Sigma 20mm all the time. I started with the Sigma 24-70mm F/2.8 before my Canon L, using it for photography. Compared to the Canon it was slower on auto focus, more noise when focusing, and problems with communication when using Infrared metering from my Speedlite. This is when I upgraded to the Canon 24-70mm F/2.8 L. The Sigma 24-70mm F/2.8 of course is half the price of the Canon 24-70mm, so if you're looking for a strictly Video lens with this range, all these quirks might not be a problem for you. If you're a hybrid shooter of both Photo and Video, you'll get better performance and image quality from the Canon.
I'm not going to get deep into the Tamron lens, I love Tamron lenses like my 18-270mm as my travel lens choice, but if you're going to be dealing with the weather, this is where the Canon 24-70mm L lens proves it's worth. Many of the cheaper lenses provide decent image quality at an affordable price, but lack the weather sealing, quiet motor, and fast autofocus. I had problems with an earlier lens (I won't mention any brands) shooting in light fog which crept into my lens and pretty much ruined it with moisture from the inside. Again, if you're just doing strictly video and not worried about auto focus noise & weather, you'll find great deals in the Sigma and Tamron brands, and also Tokina which isn't part of this test because they don't have a lens available in this range.
Update: Just confirmed with JJ that his test was using the older Sigma 24-70mm F/2.8. There is another more silent and higher end 24-70mm F/2.8 HSM priced at about $899.00 from Sigma that has not been tested. This is said to be a close rival to the Canon 24-70mm F/2.8.
click image to find at bhphotovideo.com
