Recently I upgraded from a 7D to a 5D mark III. With the new body I also purchased a 70-200 f2.8L IS II. This combo has worked quite well for me through a couple events so far. But, as I am preparing for a (family) wedding this weekend, I noticed a weird trend. Attached you will find two pictures, no time was spent on them, they are out of camera, and they were just to show the exposure difference. Both pictures were taken at 1/100, f2.8, FC:0, iso 400. The first was taken with the 70-200. The second, with my Sigma 85 f1.4. As can be seen, there is a large difference in the exposure from shot to shot. I should also note that the speedlite used was a Yongnuo YN-568 (batteries were fully charged in both instances)
This is the first zoom that I have spent much time using, especially with a speedlite. Before I relied predominantly on my 50/1.4, 85/1.4, and 100 Macro.
Can anyone lend any guidance as to why I am having such variance in exposure? I though it may be due to the length of the lens, my first attempt was to take the lens hood off. This seemed to have no significant effect.
Perhaps it is something easy that I am missing. Forgive me if this is the case. I am still relatively new with photography. If its just a trait of using the long zoom with a speedlite then that is something I can adjust to. By increasing the FC on my speelight to ~+3 I can get a similar exposure to the prime lens.