Pretty much any lens can be forced to have CA issues if you try hard enough. That's why there is a CA repair tool in the raw converter.
I love the internet, someone takes a virtually unphotographably backlit picture and then spews out that the lens has a CA problem.
Neither the 31 or 77 have CA problems, but they can be forced to show CA if you take the right picture.
Anyway, I have both the 70 and the 77. Both are nice lenses. The 77 is a bit faster, a bit longer and you can put filters onto it. The 70 is a bit slower and doesn't have any way of mounting a filter built into it.
The 70 AFs a bit better on the DLSRs I've used.
Overall, I prefer the 77 to the 70.
Unless you are really needing the extra stop and a bit of speed, save your money on the 31 and use your 16-50. While the 31 is as good a lens as you can buy from anyone, anywhere, by f/5.6 or so, you aren't really going to see much between it and your zoom once you start making prints.
I don't post pictures here often, this was shot with the 77.
Last edited by Wheatfield; 10-15-2009 at 07:33 AM.