Tuesday, July 6, 2010

Micro AF Adjustment

The one gimmick that came out with the newest Canon DSLR models (50D, 7D, 5D II, 1D III, 1Ds III, and 1D IV) that really made me want to upgrade is the Micro AF Adjustment (MAA) ability. Basically what it does is allow you to calibrate the lens to achieve perfect focus without having to send the body and lens to Canon Service Center. Sometimes the perceived "softness" of a lens can be attributed to an uncalibrated lens causing back-focusing or front-focusing. This feature allows you to calibrate any one type of lens to the body.

In order to aid MAA, there is a product called LensAlign Pro Focus Calibration System with Enumerator (US$179.95) which makes it easy and fast. This is what the product looks like:

My friend Methodical at POTN gave a nice recipe on how to use this apparatus with MAA.

Method for all test shots:

- Av mode
- Widest aperture
- ISO 400-1600
- Cable release
- In camera 2 second timer
- LensAlign Ruler

First off I’d suggest, as suggested by Michael Tapes, that you use a tripod to mount the LensAlign as it takes the task much simpler to align the target. The kit comes with the standard ruler for the shorter focal length lens and the longer ruler (4’) for the longer lens.

Canon 85 1.8 and 50D: This is the initial shot before going into editor. You can see what it looks like before any analytical work is done.

This time I did not use Paul Westphal’s suggestion of 50x the lens focal length. I used the 25x the lens focal length and various ISO settings, which cut my working distances in half (see above). This allowed me to work out of my garage (camera and lens in garage, LensAlign in driveway). The working distance (camera setup) from my garage to the end of the apron is about 84’ – perfect for my working distance.

Same shot as above except I used photoshop emboss filter to highlight the focal point on the ruler. You can clearly see that the lens is back focusing.

I set up the tripod with the LensAlign and back sighted the bulls eye to the lens. This is an easy step – just look through the bulls eye hole back to the lens and center. At this point, you are about 95% on target. I then go back to the camera and lens in the garage (out of the sun) and put the camera in live view mode, zoom in to 10x to check the alignment with the bulls eye and make adjustment at the camera end if necessary. Once aligned, I shut off live view and adjust camera and lens, via the view finder, so that the focus point is centered on the bulls eye (you will need to enable focus point so that it is visible in the viewfinder). I then lock things down, check to make sure all the settings are correct, defocus the lens and take 3 base shots, defocusing each time and waiting for things to settle down, especially with the long focal length lens. I download and open those photos in PSE7 and take them to the Editor. In Editor, I emboss each photo (find edge filter is useful too), and do an auto sharpen if necessary, and use various zooms (i.e. 50, 100% etc.) to analyze where the focus falls on the ruler (front, back or spot on). If, for example the lens front focused, I go back and take 2 test shots each using various MAA dialed in, say, in this example +3, 5, 7, 9 etc.). I download and open in PSE7, take them to Editor and perform the same steps as above. Once I determine which adjustment centers the focus on the ruler, I dial it in the camera, save all files for future use, and created a spreadsheet with the before and after LensAlign MAA – this is personal so that I can compare the MAA between methods. After testing a few lenses, I kind of got a good feel for what MAA to use to make additional test shots to get things closer. Btw, I tested a couple lenses with the MAA from my old method using the LensAlign and some were way off.

Here's one of the test shots with a - 4 micro af adjusment, which put focus to the center - on "0" (i.e. corrected the back focusing issue)

Again, I faced 2 uncontrollable variables, light and wind, but this time consistent light was not really a major concern as it was when using my old method to perform the MAA because the ruler will show the results no matter what as long as you have some light on it. So my major enemy was wind. A couple times it knocked the large ruler out of place so the remedy was to set it in place and use some tape (painters) to keep it in place (place under the ruler of course). It would be good if you have a large indoor place to do the test for the longer lens. My method for the wind enemy was patience. I just waited till the wind stopped.