All three cameras feature lens microadjustment, but what exactly does this do?
As was highlighted in Jon Tarrant's recent lens group test (l/VDC March 2009), lenses can sometimes quite literally fail to hit the spot, in that their focusing is slightly out of line with the camera on which they are mounted. Both the lens or the camera may be at fault, though all three of these cameras allow calibration of a particular lens, should you discover any inconsistencies.
To determine whether you need to adjust this, you should use a suitable target (specific targets for this are available), positioning it parallel to the focal plane and ensuring it has been evenly lit.
The systems in all three cameras work similarly. Canon's AF Microadjustment feature allows you to either apply correction for up to 20 individual lenses (whose profiles are then stored in-camera), or to make a global adjustment for all mounted lenses. Nikon's AF Fine Tune function operates in much the same way, as does Sony's AF Microadjustment, though the a900 goes on to support an additional 10 optics.
The physical construction of a lens is not affected by this process, and so any changes you make will not be visible should you use the lens on another body. Any settings you apply may be erased from the camera and lenses may be re-calibrated if necessary, though you should only attempt to carry out this procedure if you're confident in what you're doing.
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