Pixiq

In the past I didn’t really think about the accuracy of autofocus. After all, we pay a lot for our high tech camera and lenses and only in rare cases should there be a problem. After seeing the menu item “AF fine tune” in my Nikon D3 I began to wonder what google could reveal about this mostly ignored feature.

After some research I ended up at Jeffery Friedl’s Blog and specifically his Autofocus Test Chart. It’s an excellent post on autofocus and has a well crafted test chart you can download and print for free, so that’s what I did.

What a shocker! The AF for my 24-70mm f/2.8 lens was not correct. Here are the results of my test and some pointers for setting it up. 

1. Use a good tripod, set the camera up with the lens attached and leave it there for the entire test. I tested the Nikon D3 with a 24-70mm f/2.8.
2. Place the test chart on a table and make sure the paper is not curled. Tape it.
3. The camera must be at a low angle 45deg or less to the paper. It is critical to get the camera in line with the chart. You can hang a plumb line from the tripod center then extend a straight edge from the center tick marks in the chart to the plumb line.
4. Shoot at high shutter speeds, wide aperture’s and several zoom lengths. I tested the 70mm and 50mm focal lengths, both at f2.8 and around 1/1600sec.
5. Use this depth-of-field calculator and make sure the focal length and f-stop you use has around the same depth-of-field in front of and behind the focus point.

Take several images at a couple of focal lengths, for each image allow the lens to re-lock focus, ie. start with a blurry bar. I found it's better to download the images (RAW) to the computer and view them at 200% rather than view them in the camera's LED.

The D3 will save the adjustment for the specific lens you are fine tuning, this is probably true for any DSLR with a AF fine tuning adjustment. This allows you to adjust several lenses, save the correction and the camera simply defaults to that correction for that lens. Now I’m off to test my 70-200mm.