Some good info I got from a member on Nikonians and thought I'd share it with you. Especially informative for anybody getting a super telephoto lens.
I got the new 150-500mm and was a bit discouraged, but feel a bit better now. I have been shooting between 100-300 yrds and have to do heavy cropping which gave me a soft image. But when no cropping is necessary it looks pretty good.
Now I got this response from Dave "Telecorder"
Good for you! You're learning the limitations with your understanding.
Many people move up to a long lens with unrealistic expectations; They soon find (or not) that user technique and subject distance are two critical issues that work against them if they're not fully understood. Add in OS/VR and the circumstances/expectations are compounded.
In most all cases at the long end of a lens' reach, new users moving up belatedly find that their technique needs dramatic improvement that will only come with extensive use/experience. I firmly believe a lot of new user reporting issues with, and 'blaming' a new long lens as being poor, have more to do with their techniques than the glass.
Likewise, subject distance seems to be dramatically misunderstood. Its been my experience that an image needing to have more than ~30-40% of the original field of view cropped to compose an image is where the image quality begins to suffer. There are just not enough remaining pixels covering the subject for the desired detail.
Take a 2-3' tall Bald Eagle. The vertical FOV of a 3008w x 2000h image at 200' is only ~6' or 72" for my Bigma at 500-mm. To fill the vertical FOV, one would need to crop out 1/2 - 2/3 of the FOV.
Thus, my 'rule of thumb' is to employ as good of user technique, 1/focal length shutter speeds or faster and recognize that I'll get few good-great images if my subject distance vs subject size will require more than 30-40% FOV cropping. Any more than that will usually result in merely an ID snapshot.
A good site to get an appreciation of these issues is by Mothman13 at-- http://www.texasmothman.com/photogra...ng/birding.asp
While there are exceptions to most everything, I believe recognizing and understanding these critical aspects and addressing them will go a long way to enjoying imaging with a long lens.
If anybody wants to add to this please do so.