25X or 50X focal length to microadjust?

Where did these two distances come from? Are you supposed to microadjust using the longer distance if you plan to shoot landscapes, and the shorter if you are doing portraits or still lifes? I don't understand how those distances came about.

Canon recommends 50x. I'd always thought it was to ensure that there's enough depth of field to make it easier to get a consistent MFA value.

Personally, I'll use anything from 20x to 50x and never found it mattered.

The Canon article on this topic http://www.learn.usa.c​anon.com …oadjustment_art​icle.shtml says:

"The adjustments applied using this control are based on the depth-of-field you'd have at a lens's maximum aperture. They are not based on the lens's focal length! When setting the Microadjustment, you'll see a scale on the camera's LCD monitor with up to + or - twenty steps. Each step is a very fine increment, equal to 1/8th of the depth-of-field you'd have with the current lens wide-open. And that 1/8th of the depth of field is only moving forward (toward the camera) or back (toward the background) from the sharpest plane of focus. The main thing to remember here is that these are very fine increments. "


" • Shoot test pictures using the lens(es) you normally use, and at the distances you typically use them. In other words, if you're seeing a consistent focus shift when you shoot group pictures at weddings, don't take test shots of a ruler on your desk with a macro lens."

OTOH, Canon states in this article http://www.learn.usa.c​anon.com …ate_EOS_AF_Quic​kGuide.pdf

"Use Live View to manually focus on a stationary, flat, high-contrast object that is at the center of the viewfinder and parallel to the plane of focus. The camera-to-subject distance should be no less than 50 times the focal length of the lens. For a 50mm lens this would be at least 2.5 meters, or approximately 8.2 feet."

So Canon itself does not seem consistent in its recommendations! After all, 50X is not the same as "at the distances you typically use them". But to Canon's defense, they do state " no less than 50 times the focal length"

Perhaps it is related to AF system accuracy. It has been stated http://www.the-digital-picture.com …-Autofocus-Explained.aspx that

"For EOS AF points, there are two levels of precision - 'normal' which is within one depth of focus for the attached lens at max aperture, and 'high precision' which is within 1/3 of the depth of focus for the attached lens at max aperture."

I decided to create a spreadsheet looking at some of these distances, in an effort to reverse engineer 'Canon logic' (isn't that another oxymoron that we have discovered?! )