For purposes of shade selection it is better to use manual focus at a specific magnification (1:2) to prevent the blurring of needed details within the oral environment.  Set magnification first and then adjust your distance to properly focus.  Always include the selected shade tabs in the frame, either above or below, on the same focusing plane, parallel to the teeth being photographed and hold the camera parallel to the teeth as well.  This will help capture color a bit more accurately.  Try to keep the retractor out of the photo.