I love my DA* 300mm. It is my favorite lens, and when I had the opportunity to illustrate a book about Danish wildlife last year, I used it almost exclusively.
Still, with the flight distances of birds in my densely populated and intensively hunted homeland, this is a short lens for wildlife.
I have the Pentax 1.7 AF converter, which I have used on a few occasions, f.ex for stationary animals like owls. It doesn't provide true AF though, only "assisted autofocus", which is very imprecise and often locks on the wrong focal distance, which is why it is better for stationary animals. I only use this combo when the conditions are right, and between the light lost to the converter and the light quality in cloudy Denmark (I think Seattle is wonderfully sunny, seriously), they rarely are.
I sold my Sigma 150-500mm APO, because it was not entirely sharp on the long lengths except stopped down to at least f/7, and then I might as well use my converter + 300mm. Using the Sigma at <400mm seemed silly, when I have the DA* 300mm
Most likely, I will have to stick with what I have - if I save up for the K3, the AF situation might improve - but I would still like to be able to dream! I am just not sure what to dream of...
The new DA 560 mm is horribly expensive, but that aside it didn't exactly get rave reviews. With my converter, I have 510 mm as it is, but poor low light performance and AF - and in a much more wieldable package. Would the 560 mm be a huge upgrade in terms of IQ? Is IS a bit faster, but bigger.
There is the Sigma 400mm F5.6 APO, which is only available second hand, but I HAVE seen a few around the marketplace - and they are almost within budget. It gets very good verdicts on sharpness, and it isn't huge. The reach is another 100mm compared to be "bare" DA* lens, and a bit faster than the DA*+converter, and full AF. Would that feel like a significant upgrade?
The Sigma 150-500mm APO wasn't for me, and if I went for a zoom lens again, it would need top performance at the longest focal distance, because that is where it would get used.
What I would really love is the opinion of someone who has and loves the DA* 300mm, but bought something else to get longer reach, and who is really happy with the results.
Thanks for any replies!