www.ukbutterflies.co.uk

Apologies for spamming (3 posts within as many minutes) in the Photography forum. This one was the original reason I logged in...

Ever since buying my Panasonic 100-400mm (200-800 equivalent) telephoto lens for my Oly MFT cameras, I've used it for almost 100% of my butterfly photography. The only frustration has been occasionally slow focus acquisition times - I just assumed it "was what it was" (for this lens).
However, after reading a post on DP Review that mentioned, in passing, that the lens suffered badly with UV filters, I removed mine, and am so glad that I did. Focus acquisition times have significantly improved (so important for butterfly photography).

Just mentioning, as it MAY be worth trying on your telephoto lens (whatever it may be).

BR, Jon
P.S. The UV filter I was using was an expensive one, btw.

And if you still want to protect the front element of your lens, try a clear protect filter rather than a UV filter. The latter are generally an anachronism from the days of film (which could be over-sensitive to UV compared to the human eye, causing some shots to have a misty or blue tint).