Nikon D90 - focus mode

[nanux]

Hallo,

I am taking photos with Nikon D90 + Sigma 17-70/Nikkor 50. I have the camera set on dynamic one point focus with some of semi-automatic mode (A/S). The problem is, when I have selected one focus point and half-press the shutter button the exposure meter is set somehow and when I recompose the frame, the picture is overexposed (most of the time) or underexposed. Should I use automatic focus? What type of combination of exposure setting and focus mode you are using? I know that everyone shot in different way, but any advice in this topic will be helpful for me

Thanks

[Aegea]

You may have AE-Lock set. Take a look at pp. 88, 179 and 200 in the manual.