Wed 30-Jan-13 12:05 AM | edited Wed 30-Jan-13 12:08 AM by HBB
I suspect that those suggesting you shoot something (a 12 inch ruler for example) at a 45 degree angle mean the following:
1) Place camera/lens configuration on tripod as above.
2) Place 12 inch ruler, or yard stick, at a 45 degree angle relative to the sensor plane of the camera, and parallel with the lens axis, at a subject to lens distance that fills the image with the ruler/yardstick.
3) Open the lens up to maximum aperture.
4) Focus on the midpoint of the ruler (6 inches) or yardstick (18 inches) and capture an image.
5) Look at the image at "actual pixels" on screen and see if the sharpest part of the image is at the 6 inch or 18 inch point. If the midpoint of the ruler/yardstick is the sharpest point of the image, this probably rules out a focus issue. If the sharpest point of the image is ahead of the midpoint, you may have a near focus issue: the lens is actually focusing closer than indicated in the view finder. If the sharpest point of the image is behind the midpoint, you may have a far focus issue: the lens is actually focusing beyond the point indicated in the view finder. Either of these may possibly be be fixed by a fine focus adjustment in the camera, unless there is a problem with the lens that is beyond the range of fine focus adjustments.
Hope this helps explain things a bit.
Neil is our resident guru in these matters ... you are in good hands.
HBB in Phoenix, Arizona Nikonian Team Member
Photography is a journey with no conceivable destination.