View Full Version : Tips for doing AF fine tuning with software

03-16-2013, 04:06 PM
If you have decided to make the plunge and use software to help you fine tune the autofocus of one or more of your lenses with a camera body, there are a few tips that I can provide that may help you get better results from that work. Let me emphasize the word work. Whether you use FoCal or FocusTune, doing autofocus fine tuning is drudgery. I would rather go to the dentist than do another series of tuning tests, but then I would not be sure I was getting the sharpest images my camera and lens could capture would I?

I will cover the tips in what I think is the order of importance. You will find some of these same tips in the documentation that comes with FoCal or FocusTune. I just want to emphasize some of them.

Light on Target

It is really important that you have sufficient and stable lighting on the target you are using for the tuning. If the target illumination is too low, the camera will have trouble doing the focus operations. Low light on the target will also result in lower shutter speeds which in turn will result in greater adverse affects of vibration on the focus results.

The ideal light source is bright sunlight on the target, but be sure you calibrate on a day that does not have a cloud cover that changes during the testing. Consistent lighting on the target will result in better apples-to-apples comparisons that the software makes at each autofocus adjustment point.

I found that I got the best testing results when I ran my tests in my garage using a construction work light as my target illumination source as shown in the attached image titled Jordan Garage Test Setup. Not only did I get good even lighting on the target, I eliminated the concern for wind causing my camera/lens to move during the testing…a stability concern we will review next.

Test results have shown that the white balance of the camera has little to no affect on autofocus fine tuning results. However, it may make you feel better when you see a pure white in between the black on your target, then change the WB to get the pure white.

Stability of Camera/Lens and Target

Having a rock solid mounting platform for the camera/lens and the target will give the adjustment software the best opportunity to concentrate on the sharpness of the test images at each autofocus fine tuning adjustment point. The longer the lens, the more critical it is to have platform stability. Use your heaviest tripod and most stable head for the calibration tests.

One sure fire way to defeat all of your plans for camera/lens stabilization during testing is to be in too big of a hurry if you have to handle the camera during testing. After you make an autofocus fine tune adjustment manually, pause for at least three seconds before you take the next shot or allow the software to take the shot. A solid tripod/head should damp any vibration imparted by your hands during that pause.

If you plan to do your autofocus tuning outside, be sure to pick a day with little or no wind. Having a rock solid tripod/head will not be enough to prevent wind-induced movement that will adversely affect testing, especially for long lens that have more surface exposed to the wind. To prevent wind from moving your camera/lens you can either do all of your testing in a room or at least have the camera/lens in a room (e.g. garage) and place the target outside at the distance recommended by the software vendor. If you place the target outside, the stability of the target is as important as the stability of the camera/lens, so nail it down while keeping it parallel to the back plane of the camera.

FoCal offers an image stabilization option that you can turn on to be sure the software is capturing the sharpest image possible. You can turn that option on if you like, but it is better to eliminate any possible camera/lens movement than to try and correct movement when it happens.

When you are doing the autofocus fine tuning of a long telephoto such as a 400mm or a 200-400mm at the long end of the zoom range, you will see more variability of the calibration test results (wider spread of data points) than you will see with shorter lenses. When you install a teleconverter on a lens, you will also see an increase the variability of test results. Two of the attached figures illustrate this issue. The image titled Low Variability of Test Results shows the fine tuning achieved with a 200-400mm lens. The image titled High Variability of Test Results shows the fine tuning that failed with the same 200-400mm lens after a 1.4X teleconverter was installed.

To reduce the variability of the test results and improve the tuning when you see results like the High Variability of Test Results image, you may have to turn on Vibration Reduction (VR) or Image Stabilization (IS) on the lens. Reducing the variability of results with VR or IS will enable the software to determine the final autofocus fine tuning faster and with less camera shutter clicks. Under 40 clicks to achieve AF fine tuning is good; over 60 clicks is pushing the limits of the tuning capabilities of the software system.

If turning on VR or IS does not give you good results, you can also increase ISO in order to increase shutter speeds. The increase in shutter speeds will give you sharper images and better results for the same reason higher shutter speeds work better for fast moving subjects. You freeze the movement. However, be sure you do not exceed the maximum ISO specified for your software; increasing ISO adds electronic noise to images and reduces image quality that can impact test results if the maximum is exceeded. FoCal warns you about increasing ISO well before you hit the maximum recommended.

Printing of Target

Follow the software vendor’s instructions carefully when printing the target that goes with their software. If the target is too reflective or has too much of a matte finish, the autofocus fine tuning results can be adversely affected. The software has to “see” the images a certain way to evaluate the sharpness of each image. The target printing specifications ensure the software gets what it expects to see.

Also be sure you print the version of the target that matches the version of the software. The software vendors use specific patterns in the targets to compare between images when selecting which image is sharper than another. If you are using the wrong version of the target, the software may not be able to properly perform the comparison.

Alignment of Target

If you use FoCal to do your AF fine tuning, be sure to use the feature that tests the alignment of the target. Until you get the green check mark on the target image on the computer, do not start the testing. If you use FocusTune, you will not get direct feedback from the software as to target alignment. That is one advantage of having the LensAlign kit that helps you do the alignment required by FocusTune.