Hey Doc

(Test pattern)

All modern cameras are capable to perform auto focus. Regardless of which method the auto focus is based on, there will be some discrepancy. Commonly refers to as front-focus or back-focus. Front-focus means the auto-focus plane is in front (closer to the camera) of the intended point, Back-focus means its in the back. This discrepancy is within the lens used. Few professional digital cameras have the ability to manually make the adjustment for each lens used on that camera. I have learned a way to test for the lens. Here is how.
1. Open this test pattern on your monitor, make size adjustment that it fills the entire screen.
2. Place the monitor 45 degrees angle in the direction where the camera is pointing.

Camera ---------- \ (monitor)

3. Mount the camera on a tripod. Place the tripod at the location that the test pattern is filled the entire viewfinder. Set the camera to shutter priority.
4. Set the auto focus sensor at the center of the picture, where the "|" bar is located.
5. Select widest aperture on your camera (example f2.8) and activate the auto-focus by pressing halfway down the shutter button. Move the camera back or forth slightly then focus again and take the picture.
6. Follow the procedure for each setting if you are testing a zoom lens.
7. Upload the pictures to your computer and use the image viewing program to see the results.



Here is the result of my Sigma 70-200mm zoom lens at 100mm setting. Clearly the auto focus plane is between the center bar and the number 1 (it is front focus of one degree). If I want, I can make this adjustment in my Nikon camera so the auto focus will be compensated for this lens. Make sure all settings are tested and averaged. Since the offset is not that great for this lens, I will not make any adjustment because, remember, don't fix it if it ain't broken.