looks like a quick solution, a bit of fringing but how does the framing work out? i had a sigma 50-500 and it got me good results at f9 - F11 the fringing might tighten up if you used a higher f ratio - bad tradeoff woud be upping the iso to get shots quicker? can you do a standard shot sequence from 150-600?
http://www.iceinspace.com.au/forum/s...79&postcount=9 a blast from thee past