During the development of FoCal, a large number of cameras have been tested with a huge variety of lenses.
Generally, a professional grade modern manufacturer own-brand lens will give the best AF performance. Cheaper, consumer grade lenses are often less accurate in their positioning of the lens elements for focussing and can result in poorer performance. Third party lenses – even good quality ones – can be more likely to suffer from similar problems to the consumer grade lenses with more inaccuracies in focusing than manufacturer own brand professional level lenses.
FoCal takes a series of shots at various AF microadjustment values and looks for the best one. Whilst this sounds simple, it has taken hundreds of hours of development and testing to create FoCal so that the results are reliable and consistent.
All of the AF performance issues described previously are random and will mess up any shots you take, so FoCal has been developed to identify random errors and remove their effects while predicting and validating the best AF microadjustment value in as few shutter actuations as possible.
Using sophisticated image analysis which has been developed in to work with the FoCal target, the software can detect tiny changes in the AF result. The analysis can also check for and adjust against small changes in the test environment to get the best set of data possible. A finely tuned mathematical lens model developed from the results of profiling a large number of lenses can quickly determine the best microadjustment value.
So, whilst it all looks quite simple, when you click the button on a Fully Automatic Calibration operation, you are taking advantage of an incredibly sophisticated and finely tuned system.
One thing to note about FoCal results: When you change the microadjustment value on the camera for a particular lens, the focus position determined by the AF system is shifted a tiny amount. This amount depends on the lens and the camera and is decided by the camera manufacturer for each type of lens.
It is important to understand that all of the things that can create inaccuracies in the AF system (see Autofocus behaviour) can lead to shot-to-shot inaccuracies that are greater than a single AF microadjustment step!
That last statement is so important – here it is again:
The ERROR in focus position caused by the whole AF system can be BIGGER than an AF microadjustment step. This means that there isn’t one perfect AF microadjustment value for this setup.
The following two links are articles published by Roger Cicala of LensRentals, and give a good view of camera and lens focus variations from an independent third party with a lot of experience in this industry:
http://www.lensrentals.com/blog/2011/10/notes-on-lens-and-camera-variation
http://www.dpreview.com/articles/7333489584/variation-facts-and-fallacies
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