indoor photo taking: grainy blacks & sections of photo out of focus

bluedragon

Hi everyone, I have a D3100 with the regular Nikon 18-55mm lens and I use a tripod.
I take photos of clothing which are on a mannequin, indoors. The lighting isn't at all professional but I have had some really sharp photo's of the clothing. When I take the photo's I stand about 12' away from the mannequin.
Almost always the black pants come out grainy, the photos are sometimes darker than the previous days photos (when nothing has changed) and some parts of the photos are out of focus. My question is, is there something I can do differently? Maybe a particular setting? My current settings on the dial are Guide Mode/Shoot/Bring More Into Focus/Aperture F18/ISO 100. The ADL is off.

Any suggestions would be really appreciated.

Riddelske

Black pants will come out grainy because the camera always refers to 18% grey.

bluedragon

Problem solved by a very helpful person. Thank you.