[bend the light]

Hi all,

I bought a lens in a charity shop. Knew it was cheap and cheerful, but thought it would do me ok as a long lens for birding when I'm in Wales during the holidays (going to visit the red Kite centre, for one).

The lens is a Cosina 100-400 f4.5-6.7. Push and pull to zoom. But, I have a problem...

I can focus, autofocus works, and the zoom works well, and I can take a shot when on manual focus. But when I swithc to autofocus, it does the focussing, but then doesn't take the photo. The shutter won't go! The focus confirmation light (green dot on bottom right of viewfinder) flashes slowly when the shutter is depressed, but no shutter actuation.

anyone had any issues like this? Is it Cosina? I don't get the Err99 that you sometimes get with non-genuine lenses on the 400d, so it's not that...