Many SLR cameras now come with a way to adjust the auto focus calibration for lenses. Most cameras will save the adjustment for each lens and when you reattach the lens it will automatically apply the adjustment. Those camera’s that just allow a global adjustment will require the photographer to make the adjustment for each lens every time they attach it. Annoying but better than nothing.
As I explained in my post “The problem with sheep” my telephoto lens was focusing behind my subjects. To fix this I had to setup a quick little test environment and run some tests.
The first step is to print a focus chart. Next setup your camera on a tripod at a 45 degree angle to the focus chart. If you are like me and do not have any of your high school geometry sets around you can fold a square piece of paper diagonally so it forms a triangle which will have two 45 degree angles and a 90 degree angle. You can use this with your camera on your tripod to see if the lens is at 45 degrees.
Set your lens to the longest focal length, max aperture and the closest focus distance. In my case this was 320mm, f5.6 and about 4 feet away from my focus chart. This should give you the narrowest depth of field. Now move your lens so it is out of focus and use the camera’s auto focus to focus on the line that says “this text should be perfectly in focus”. I focused right on the “s” in focus.
After each test I unfocused the lens, dialed in a focus adjustment (see your camera’s manual on how to do this), refocused using auto focus and took a picture. Below are 100% crops of the different test shots with different focus adjustments.
As you can see the -20 focus adjustment is the closest to the line being perfectly in focus.
Please follow the link to see the specific instructions I used for focus adjustments on the Pentax K10d.
Now I just have to find the sheep to validate my results in the field!