Canon Datascript Focus Calibration

As I said yesterday, this noon I visited Canon Datascript authorized service center to calibrate my EF 28mm f/1.8 USM. Well, not that I found anything disturbing about its performance, but exactly Saturday May 14th 2011, the day when I bought the rookie 50mm and testing its focus accuracy with SpyderLensCal (The shop offered me to use theirs), I'm curious about my 28mm focus accuracy, so I setted the shoot mode to AV and used f /2.5 to get the shallow DOF so that the focus point can be easily defined, I used 0 EV, ISO setted to [Auto] and this is what I got.

SpyderLENSCAL

I increases the saturation a bit on the zoomed picture to exaggerate the spherochromatism - Its a color fringing at unfocused area common in fast lens, it will appear green behind, and magenta before the in-focus area, it's different from lateral color fringing - and you can see that the number that has no color fringing is number 1 (while it suppose to be at number 0) which means it focused slightly behind the intended point. I made 4 shots and all gave me the same result. I don't need to look at it from my laptop LCD, it can be easily spotted even from the camera body's LCD.

So I showed this result to Canon's technician, he brought the camera up their lab and done some testing. Finish testing my lens, he explain to me that the lens has no problem. He then enlightened me with some interesting fact about focus calibration which I'll share on a list:

• Lens calibration should be done at f/2.8 and f/5.6 (but some source said that canon recommend to use the largest aperture possible, kinda conflicting).
• The distance of the lens and the calibrator chart should be no less then 50 times the focal length. ex: my lens is 28mm, so the distance should be at least 1.4 meters. for a 50mm lens, the distance should be at least 2.5 meters - no luck if you are an American with silly inches, yards, and miles eh..hehe
• The Chart must have equal brightness at the center and all four point direction. it has to be measured by a lightmeter, the value should be 8,5 and 10 for lens and body focus calibration respectively.

So looking by the fact that my calibration result was done in a store with dim florescent light bulbs and taken at almost closest focus distance, my result is definitely invalid. Here is an example of how Canon do it:

Canon Focus Calibration Chart

I have no idea why they use two kinds of lighting, but canon stated that they do it based on standardised procedure from Japan. And the fact that quite many consumers test the focus accuracy the way I did is why many people get a different result from what Canon get.

But the problem is, even viewing this result made by the technician I found that my lens does suffer some back-focus, the number 1 is sharper than number 0, so I think I'll fiddle a little with my 50D's microfocus adjustment. I just found some interesting link and article that might worth a try. Note that the technician mentioned that microfocus adjustment only works if your target is less that 3 meters. If you try to focus to infinity, it will ignore the adjustment. kinda strange, I'll check into it some other time.

Anyway, about my gallery, I haven't had any chance to hi-jack mu sister's Mac to process my images. My Acer's LCD sucks, so I refrain on doing any digital post-processing with it.