I thought that since my camera has the much spoken about “lens micro adjustment” in the custom settings, I would try it out with my own lenses.
The first challenge is the 7D’s lack of instructions in the manual for doing this. It tells you how to input changes to compensate for front and back focusing which will be remembered for each lens; it doesn’t say anything about checking your lenses for this.
This is where the brilliant article on the Northlight Images website comes in. There is a comprehensive summary of the methods available to check your focus accuracy and a downloadable image for your laptop or LCD screen to use as a critical focus tool. You can find it here:
Essentially you set the camera on a tripod and ensure it is level. You show the downloaded image on an LCD screen, at least 50 times the focal length of the lens away. When you view this image using live view on the camera screen, moire interference patterns will maximise and sharpen the interference fringes. Essentially you manually focus till the fringes are at their sharpest and most obvious. You then switch off live-view and half-press the shutter with autofocus switched back on on the lens. If autofocus is nailing focus as well as you do manually, then the focus ring won’t move at all. As far as I can see, this can be done in reverse as well. Autofocus as normal, switch AF off on the lens, and switch to live view. If AF nailed it, then any manual turning of the focus ring with live view on should make the image worse.
Anyway, the upshot of all of this is that all my lenses seem OK. I couldn’t get any reproducible errors on any of my lenses and I went through them all twice. Truthfully, I find that hard to believe; I wonder if anyone else has tried this and found the same as me. Have I just been lucky in my lens purchases, or are lenses simply made to a high tolerance these days? Please tell me if you read this and you have tested your lenses. For the record my tested lenses are:
Canon 17-85 F4 IS
Canon 70-300 IS
Canon 60mm Macro
Canon 50mm f1.4 prime
Sigma 28 mm f 1.8 prime
Sigma 10-20 f4 – f5.6 zoom
Anyway, the photo above shows the test in process. Do check the Northlight Images link, it’s excellent.