Focus problem when shooting with sigma 70-200

AAC

When I used the sigma 70-200 2.8 zoom to shot a speaker's close up on stage 20 feet away from the camera using no flash, the focus was not right (but not blurred) but the exposure was OK.  Looks like hand shaking but I was using tripod.   
I used tripod, auto focus, center metering, "P" setting.  
However, when I changed to wide angle lens (10-20) with same setting, the picture looked OK.
Should I change to manual fouces and tune to shutter (125 or 250)priority ?

AAC

More information for comments :
1) Tripod : "M" - ball head, model unknown, given by my brother.
2) Sigma 70 - 200 f2.8 on Nikon D50, no flash.
3) Shutter speed : 125, 250 and P setting
4) Shotting indoor seminar speakers close up, speakers normally standing on stage and didn't move very quick
5) Room light : regular classroom setting, no spot light.
6) Centre metering

The problem was, when I used wide angle to capture the whole room, it looked fine. But looked blurry in close-up.
Does it matter with sutofocus or manual focus ?  If I increse the shutter speed, the picture is dark. Tune up ISO ?

Thanks.

[ 本帖最後由 AAC 於 2007-12-30 07:04 編輯 ]

AAC

Well, I tried again yesterday @ hotel function room by setting the shutter speed to 125/2.8 and 90/2.8 with manual focusing and the picture quality improved but not satisfied.   
There was no spot light on podium, just regular function room light. If flash is restricted and no spot light, by using a f2.8 zoom lens, what can I do to make it better?  Tune up high ISO result in noise, lower shutter speed result in blurred picture.

[ 本帖最後由 AAC 於 2007-12-31 13:33 編輯 ]

AAC

原帖由 echo 於 2007-12-31 15:22 發表
你要分清問題係乜, focus problem? 手震, contrast 唔夠? 曝光 under? 因為你講 quality 唔好.

我估最大問題係張相 "鬆" 唔夠 "sharp" ? 我估得岩唔岩?

試下收一級光圈用 f4 啦, 可能有改善. ...

Thanks.
Then I need slow shutter speed to compensate the explosure and again I can't freeze the speakers on stage, right ?