Higher-end DSLR cameras often come with an option to fine tune their (phase detect) auto-focus system. This is sometimes desirable when one discovers that for certain lenses, images present some back or front focus.
I describe here a cheap yet very efficient method of testing your various lenses – camera combinations to obtain sharp focus. If you want to know why I believe it is efficient, read the final remarks at the end of this article.
1. Print a test page containing a pattern of finely spaced dots. For your convenience you may download my own design here. Notice that in the middle of the page, there is a bigger dot that serves as a reference for the location on which to focus.
2. Take another blank page and precisely cut-out a hole that is a bit bigger than the bigger dot of the dotted test page. This page should remain perfectly flat and the borders of the cut-out should be very clean. To achieve this, I found that using a cutter is a good method.
3. Both pages should look like this:
4. Place the dotted test page on a flat surface (floor or short table) and place an object on one side of the page so that it will stay in place.
5. Precisely position the blank page with the hole on top of the test chart so that the bigger dot on the test chart is visible. Things should now look like this.
6. Using a steady tripod, position your camera such that it is aimed at the bigger dot (visible through the hole made in the blank page) at a 45 degree angle. The distance between the camera and the test chart should be close to the minimum focus distance to reduce depth of field. For that purpose, the widest aperture of the lens should also be selected.
7. Using the auto-focus of the camera, focus on the bigger dot. You may take a picture to make sure the bigger dot is roughly in focus.
8. Without moving anything, deactivate the auto-focus of the camera. This step is not necessary if you are using a special button like AF-ON to achieve focus and that the shutter does not activate the auto-focus.
9. Remove the blank page while leaving the test chart in place (this is why securing it with an object is useful) and take your real test picture.
10. If your auto-focus is fine tuned for your particular lens, the picture should look like this:
If it look like this, you have back focusing:
If it look like this, you have front focusing:
In the cases of back and front focusing, you should consult your camera manual in order to adjust the focusing until the big dot is located at the center of the sharp zone of dots.
- Why the blank page with the hole? Answer: 1) most testing methods I have seen so far ask you to focus at some place and look at something in another location like on a ruler. As the depth-of-field should be really shallow for good testing, any slight difference of distance between these two locations will render the measurement inaccurate. 2) By hiding the rest of the test chart when achieving focus on the big dot, you know exactly where the camera is trying to achieve focus because all the phase detection is performed at that exact spot.
- Why a dotted pattern? Answer: as you see in the pictures, a texture like the one provided by the dots makes it very easy to locate the area where the image is in focus. As long as your big dot is located at (or very near) the center of this area, your auto-focus is fined tuned.