When I got the lens I did some informal testing, and the lens did well. AF was spot on, no micro adjustment was needed. Sharpness exceeded the 70-200 f/2.8 IS, and overall build and handling was good. The real test of the lens would be in the field with tough conditions. Over the weekend I shot two triathlons, both in harsh lighting conditions, often shooting backlit or in the shade, but sometimes in the direct sun. Most of my shooting was of cyclists, often moving near 30mph directly at the camera. Thousands and thousands of shots were taken, and consistency was the key. The weekend was a good torture test to see if the lens could keep up. Shots from Saturday are in the post above, and shots from Sunday are at the end of this post.
On Saturday I started shooting bikers coming up a hill away from the transition. The background was very busy, but gave the photos some context. The lens performed well, and I got some good shots, but soon had to move to another location. My next spot was shooting the bikers as they rode past on a public road. The area I chose was shaded and had a line of trees in the background. Unfortunately, being a public road, there were also many times when cars were in the background. The 300mm focal length and f/2.8 aperture did a good job of blurring the background and bokeh looked good to me. In a shaded/backlit situation the lens performed good overall, but did have some AF misses, and occasionally my viewfinder AF confirmation light would blink, and the lens would not AF at all! This happened several times, and the only way I could get it to AF again, was to switch the lens from AF to MF and back to AF. I am not sure what exactly was causing this issue.
After shooting the bikes, I moved to a section of the run. The course layout put this directly in the sun, with the runners being lit from the side. Difficult shooting conditions, but the lens did a good job. Clean backgrounds were hard to find (residential area), so the thin DOF came in very handy. AF didn't have much trouble with the runners, but I did have several more instances where the lens would not AF at all, until the AF/MF was toggled back and forth. At the end of the day I went through the shots, and it seemed that there were slightly more OOF than I was used to with the 70-200 or 300 f/4. The shots that were in focus were tack sharp, but my keeper rate appeared to be a bit down. Some photo's from Saturday can be seen in the previous post.
On to Sunday, and I was at another triathlon. The weather was beautiful again, with slightly cooler temps and more of a breeze. Still not a cloud in the sky and harsh lighting. Today I was shooting bikes only, and was assigned to a specific, uphill portion of the race. A bit different shot than normal, that would hopefully capture some emotion. The spot ended up being partially shaded, so I set my camera up to shoot the riders only in the shade. This resulted in even exposures on the subjects, with blown out backgrounds. I felt it was the best compromise given the shooting location.
Shooting the riders coming up the hill was a bit slower, and the AF did a great job. Very good keeper rates throughout the day on Sunday, and once again, the shots were very sharp with good background blurring. The hill didn't end up being all that steep, so most riders weren't pushing quite as hard as expected, but the shots still look a bit different than the tucked/cruising ones I shoot a lot of. After all the riders went by, I moved to near the end of the bike section and caught the rest of the riders as they went by past a line of trees. Again it was a shaded area and had a busy (leafy) background. Even with the riders traveling at higher speeds, the AF was spot on and the shots were sharp. Photos from Sunday's shooting can be found at the end of this post.
All of the shots were done in Manual mode. ISO was manipulated to keep a moderately fast shutter speed (generally 1/640-1/1600, sometimes even higher). Shooting in the shade allowed me to set the exposure and not have to adjust it for long periods at a time. The Sigma 120-300 was used 99% of the time at 300mm, and always at f/2.8. I did not use an extender for any of these shots. With triathlons, shooting position is usually pretty much up to the photographer, so I find that primes usually work fine. Zooms can be handy if a large group comes by and you miss people in your normal shooting area, but this usually is not a problem.
I focus using the back button on my 7D. This weekend was my first time using a monopod, due to the heavier lens. I think these two things may have led to some of my AF misses on Saturday. The monopod seemed to put my hand in slighty different positions as I panned along with the racers. I think this might have caused my thumb to slip off the AF ON button from time to time. I paid close attention to this on Sunday and had very good focusing results. I still do not have an explaination for the AF issues on Saturday (where the lens would not AF at all). This did not happen a single time on Sunday, so it may have had something to do with the specific lighting conditions, or possibly even dirty electrical contacts, I really don't know. For sports that require me to move around more, I do not plan on using a monopod. I think this lens is light enough to hand hold for a full day, but I will find out for sure very soon!
Overall, I am very happy with the lens. It takes a little time to get used to the added size and weight (and proper monopod use if applicable), but the results are very good. The sharpness is somewhere between the 70-200 f/2.8 and 300 f/4 that I previously used. The AF speed is also in that range (quite good). It might not be quite as quick as the 300 f/4 or 300 f/2.8, but I found that it does keep up with most any situation (except for my unexplained issues). The price is right, and the zoom is very handy, even if you do not use it frequently. Especially in situations where you do the majority of your work at 300mm, this could allow you to avoid carrying a 70-200 on a second body. For me, I think it is the right lens. I hope it continues to perform, and holds up long term. Someday I may still consider going to the Canon 300 f/2.8 IS, but for now the cost is too great. If this lens fits your shooting requirements, and you budget, I would highly recommend checking it out!