Lou,
I use LensAlign and FocusTune with my Nikon D810 to fine tune focus for each lens with the camera. I was surprised by the amount of lens-to-lens variance there was and by the magnitude/value of fine tuning necessary for precise focus. Lesson learned.
Your question has prompted me to plan to use the same setup with my 645Z and lenses. As you may know, the LensAlign and FocusTune tools are designed to be used at closer focus distances than Dave recommends for the 645 lenses. I'll use those tools as a starting point and evaluate my results. Stay tuned.
Joe
Hi Lou and Joe,
Thats the both interesting and confusing thing about using lens align/focus tune when used to AF fine tune Pentax 645 lenses (especially legacy lenses). It makes logical sense to AF fine tune at minimum or close distance (lens wide open) with these tools simply because depth of field for a given lens is at its minimum when focus is set for closest distance (and lens is wide open). As I explained in my post above, when a lens is fined tuned this way, any slight variance in focusing accuracy or even focus shift, will be covered at greater distances by the increase of the depth of field (as focusing distance increases). One would then conclude that at infinity where depth of field is greatest, most any focusing error would certainly be covered. Unfortunately that was not the case in a good number of legacy lenses.
There apparently was a wide varience of how each sample of legacy lens focused and exactly focusing what distance focusing was optimized for. Most of these lenses were constructed and put together in the film era and as such, tolerances were somewhat sloppy. I found with some lenses when adjusting at minimum focusing distance, longer subject focusing would be way off. I'm still not sure of the theory behind this but often times when adjusting a lens at close range, said lens might either not reach infinity focus or might even overshoot it, and correct focus could not be achieved.
It was at this point that I moved back the focusing distance as a starting point, where I first fine tuned a lens and then depending on sample, would check and readjust at different distances if necessary. Compromises often had to be made with some lenses, in order to get acceptable accuracy at close, mid distance and infinity.
Keep in mind fine tuning is done with lens shot wide open. In addition to the growing depth of field at longer focusing distances, the depth of field naturally is bigger when using the lens atopped down. Therefore if one uses their legacy lens at say f9, and also at longer distances, and didn't af fine tune their lens, they may never notice that their lens was "off" or possibly not reaching its full potential.
The one exception to all this was the Pentax FA 120 macro. All samples seem to require almost no AF fine tune. Due to the nature of this lens being a macro lens and thus optomized for min close focusing distance to be exceptional in the macro range, Pentax was very careful to adjust these lenses for near perfect accuracy in focusing. One cannot afford a focusing error where lens is used with a depth of field of an inch or so (in macro range).
Conversely, some of the longer telephotos needed a great deal of AF fine tuning. Part of this is due to the varience too in 645D bodies.
As time went on...the current 645z bodies are factory adjusted to much closer tolerances than the 645D as are many of the lenses now being produced today and as such, AF fine tune is not as critical but I believe still necessary to get best performance.
This is much like the story of early Leica M8 bodies and legacy leica lenses made in the film era. Inaccurate focusing was all over the place and both bodies and and lenses had to be sent in to be adjusted. Since the 645D was Pentax's first 645 digital medium focmat body and it was being used with lenses adjusted in the film era, it too was prone to severe misfocusing with certain samples of lenses combined with bodies that weren't carefully adjusted for "null" (neither front nor back focusing).
As others has aptly pointed out, these annomolies/focusing discrepancies still exist with current Nikon bodies and lenses for example after so many generations of digital bodies and lenses designed in the film era.
Dave (D&A)