Stewart
I use the Canon 300mm f4 image stabilised lens with
a 1.4 x teleconverter for most of my birding /herps shots. It is one of
the cheapest L series lenses and the 420mm I get with the converter is
reasonably useful. I switch to digiscoping if this doesn't give me enough
magnification as I can't afford the Canon big lenses. The downside is that framing the shot is not as easy as with a
zoom.
If I had the option (ie money) I would probably go
for the 100-400mm zoom f4.5/5.6 IS or the 400mm f4 DO IS and use the 1.4
converter to get the extra reach. One thing to check up on is whether you
still retain autofocus if you use a 1.4x converter on the 100-400 lens.
With Canon cameras, once the effective aperture goes over 5.6 the autofocus
stops working (unless you own an EOS 3 body in which case it will go to
f8). So with the 100-400 plus 1.4 converter you will get a 140-560mm
f6.3/7.8 zoom lens but will probably have to use manual focus. If you get
the very expensive 400mmDO IS and add the converter you will get a 560mm f5.6
prime lens and retain autofocus.
Regards
Matthew
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