Is it normal for the Nikon D7000 to randomly back-focus when using the viewfinder (phase-detection AF system)?
The two lenses I am using are a Sigma EX 17-50mm f/2.8 DX and a Sigma EX 70-200mm f/2.8 FX.
It is nearly 100% guaranteed to do it in low-light, which by "low light" I am referring to your everyday indoor lighting of 3-4 bulbs in a ceiling fan fixture and maybe a lamp in the room.
The back-focusing is most pronounced when I am at 17mm wide open. Pictures are unusable under these circumstances.
I used the 17mm end of the Sigma wide open at 2.8 in daylight today and took a picture of a wall that I had positioned the camera parallel to, and it was extremely out of focus. However, I switched over to LiveView and it was perfectly fine. Both shots were taken on a tripod with the exposure delay function on.
Whenever I use LiveView in the same setting, my picture is as sharp as one would reasonably expect. This is not an issue of "softness". You can clearly see something in the background being tack sharp while the face that is centered in the camera and takes up at least 50% of the image is completely soft.
I've sent the camera in to Nikon Service Repair once because of this problem and this is how it comes back, the same way.
Is this something I should live with or is it cause enough to return the item?