Advice Please

[lozninja]

I am D300 Owner with a nikon f2.8 70-200 mm vr lens Which is beutfull piece of kit .
Now i purchased a Sigma 120-400mm f4.5 os hsm Lens for longer reaching photo's such as buzzards .
When i use the lens on relativly close moving subject or wildlife i get some lovelly shots .
When i am trying to phptograph buzzards in trees or in low light i have real problems getting sharp shots , even on a tripod . I know about faster shutter speeds and iso and over and under exposure, but i think the point i am trying to make is that am i possibly to far away from the subject .
I have never really done this sought of shooting before and i think i may be trying to get too much out of my lens . Is there some sought of rough idea of if the subjects too far away ? When i have taken the shot if i look on my preview screen the birds are like pinpricks and i have to zoom in 4 or 5 times before you see them .
Also at this distance i have real problem keeping thins sharp . I do use a tripod manfroto one but its not one of the expensive ones and with that nikon and 400mm sigma on it things are not that solid . Would you still have the vr on or off in this situation ???.

Any Ideas or assistance People ????

Many Thanks

[Duke Leto]

Hi post a couple of examples and allow the excellent BF members to assist, I would say as a guess if the bird is that small on the screen then that's going to be a problem.

[jourdaj]

Have you tried calibrating the focus of your lens in the D300? I found my Sigma 400mm f/5.6 to be off by several millimeters before using the calibration tool in the camera. Now it is spot-on, and makes a world of a difference in capturing the distance shots. Also, don't be surprised if images are soft when the lens is wide open. Stop down your aperture 1 full stop after calibrating the lens.

Best!