losing (sharpness) at full zoom

I have been having a problem with getting subjects sharp at the 300mm mark of my tamron100-300, but it only seems to be a problem when the subject is not close ie. 60 feet or further away.Here is an example I shot today at about 80 feet.
This was shot using a monopod. Thanks for any help.Forgot to add this is a 100% crop

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I am experiencing the same. The sun light has been making a difference. Sometimes I think I am set for auto focus when the lens is not. I am learning that when I take more time with each capture the results are better. Manual focus works for crowded scenes.

Actually, for a 100% crop at 300mm with your lens, it really doesn't look that bad. All of the consumer tele zooms in that range are known to be soft at 300mm.

I agree here.
Rob.

Thanks for the replies, so does that mean I should strive(other than obviously getting closer) to maybe only zoom to the mis 200mm's to keep the pics sharper??? thanks for the help!! Trevor

Try using a good, as in very sturdy, tripod for another test at 300mm. I suspect that you will have better results. The longer the focal length one uses, the more problematic even small movement of the lens during the exposure becomes. While you used 1/400 second for the exposure, a bit more rigidity may be the solution.

thanks Skip, I will give that a try. I was drooling over a manfrotto setup yesterday, might have to get on my wifes good side and go get one.Thanks again.