I just bought my first Canon DSLR (a 5D MkIII) and the newest Sigma 120-300/2.8 with the USB dock (although so far the Sigma focus seems to be doing the job out of the box). I want to get a 1.4x TC for occasions when I don't take my other system with me.
So far the few reviews I've seen of the lens have been done using a Canon TC , and I have read very nice things about the latest Canon TCs. Logically a Sigma TC should work better but I am wondering if they actually do? Anyone have any information, experience or even theories about this?
I'm not reallly concerned about the cost difference in this case and of course I need to check compatibility. It would be convenient to use only a Canon TC if I decide to shift solely to Canon for wildlife.
And thank you to Admins for keeping my account active while canceling the recurring charge when I didn't use the site for a long while. That actually wasn't necessary, but I appreciate all the same. (Assuming it wasn't an error of course )
I have the 120-300/2.8 OS (non-Sport) and treated it to the Canon 2X III last year. I was disappointed with the IQ and recently bought Sigma's own 2X TC, which does deliver slightly better results, but still needs stopping down at least 1/3 if not a full stop. I have not put the Sigma TC to the test for moving subjects so I can't advise about AF performance. However, I have read that with the Canon 2X III TC the AF is slowed by the camera on purpose for greater accuracy. With my Canon lenses the Canon TC beats the Sigma TC hands down, so it's not a question of one TC being outright better than the other - more one of best match of components.
I don't know how my experience translates to the 1.4X world, but my experience with the 120-300 and 2X TCs seems commonplace.
Yes, that matching of components is what I am most interested in. Unfortunately, living in Thailand trying both out and then returning the one I don't want isn't really an option.
I presume Sigma converters are designed to be campatiple with their lenses. I imagine also that different folks will perceive slightly different results when testing. I use the Sigma 500f4.5 EX and the Sigma 300f2.8 EX and now discard the Sigma 2X converter in favour of the new Canon converter. I have found that the fringing is far less when using the new Canon 2X converter on both my lenses. I have found no measurable difference in AF speed when testing the converters on my 300f2.8 lens. I would suggest that you test each of the converters in as punishing conditions as possible as well favourable conditions and then make your decision. I know that may be easier said than done.
There is a massive thread on another site (not sure if you can name them on this forum) of over 250 pages on just this lens (in it's various incarnations of course). It is universally accepted there that the Canon/Nikon TCs do not do as well as Sigma's. It was following that advice I bought the Sigma x2 and it is a very good TC that matches the 120-300 very well. I have not been disappointed, which has been the reaction of those using Canon/Nikon TCs. I did try filing off the offending lug and tried the Nikon TC14EII on my 120-300 .. but still prefer the AF of the Sigma x2.
Thank you Adrian. Perhaps I will try asking around to see whether someone will allow me to return one. You're right, of course and you could have added that I should really familiarise myself with the new gear first - including finding the optimal adjustments (if any) for me; otherwise I won't really know what the results of my try-out mean.
Oh my, Kevin! 250 pages? I feel like Rumplestiltskin, and thank you for replying at all - you must be thoroughly sick of the topic after all that! I saw something on dpreview, but it ended up to me more about the lens, which isn't relevant to me for now, and it didn't run 250 pages. You could sent me a private message with a link, but I'll add terms like "offending lug" to my search and I am sure I'll find it. Big thanks.
Birds are only a small element of my photography interest and I don't shoot them often. However, if it is of any use, I went to shoot motorbikes last week and brought back a few photos. The following pair were shot within the same second using my 5D3 and 120-300 OS non-Sport + Sigma 1.4X TC at 420mm, 1/1600, f/4.5, 200 ISO and are cropped to about half the original frame or less.
They should each be 1920x1080 pixels, but the preview shrank them to fit the screen so I hope they don't get resized when I post them for real because that will degrade IQ.
Oh well, I guess they got shrunk. Here are links to the originals...
Thank you, Tim.
You need to resize them for posting here. I've seen folks both crop the parts of interest or just post a link for this kind of stuff, where you are trying to show details. But I can see there are no flies on them at this res. I rarely shoot either birds or sports bikes, but sharp is sharp and fast is fast so this is very nice to see given it is essentially the same gear I have.
Is your experience that AF iis only slightly affected with the Sigma 1.4x?
AF seemed speedy- no noticeable difference in speed, but without back to back tests in controlled and repeatable conditions I'm not going to put money on it. Quite simply, fast enough is fast enough and it is fast enough, based not just on these samples but countless others.
Sigma or Canon TC for Sigma lens
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I rarely shoot either birds or sports bikes, but sharp is sharp and fast is fast so this is very nice to see given it is essentially the same gear I have. 
