Birding is almost a curse, you end up spending thousands of dollars on lenses. I was looking at some shots I took last winter with my 2.8 300mm tamron 107b, the earliest version. Many times I was at 2.8, 1/30 iso 320. Higher iso the sharpness falls off dramatically. Tough to get shots without movement either of the subject or camera, even on a monopod.
Shooting long on water is tough. The 55-300 controls CA and flare quite well. You won't find that with older lenses and iq over water will be hard to get. The Sigma 500 zooms seem to be ok,
Maybe I'll list what I've tried:
55-300 pentax. Too slow for most of the shooting I do here. We are in a valley and it is plain dark for 6 months. In good light a pretty fair lens. Focus hunting sucks, makes me want to throw it somewhere.
Tamron 23a 60-300 adaptall. Cheap, can be had for about $60. Nice and sharp and a lens that gives you a smile when using it. Couldn't get a decent shot on water though.
m42 300 f4 Pentax. Abberations were something fierce. Sharp but out of focus colors just wierd. A bit better on water. Sold it.
500mm Tamron mirror. F8 is too slow. Not too bad for sharpness, oddly tough to handle. Sold it.
Pentax 300 A* f4. I really try hard to like this lens but can't. When I get a good shot it is amazing, but CA and lack of sharpness makes me leave it at home. Lack of tripod mount limits it to light where handholding is ok.
Tamron 107b 300mm 2.8. Oddly effective in low light but as light increases you have to stop down severely to control CA. Works well with a 140 tc. They say the 60b and 360b are better optically. Useless for water; can't control the stray light that bounces off the waves.
Tamron 400 f4 adaptall. The best of the bunch. Love the length and gives clear shots stopped down. Not good on water, need to stop down to f11 to avoid stray light and CA.
Sigma 170-500. Not as sharp as I like, but very nice to handle. Again a lens I try really hard to like but quality is just not there. Focus is quick.
I dream of a Sigma 500 f4.5 and probably will buy one next year. I will get myself an autofocus 300mm f4 either sigma or pentax in the next little while. Most of the year a 300 2.8 is almost a necessity, so am considering a sigma 300 2.8 and a tc. Not sure if it would be satisfactory though.
Bah. A gambling or cocaine habit may be cheaper than this hobby