I keep reading about "Set your lens to 35 or 50 times the focal length for focus calibration" (Using whatever method of chart, gauge, software, etc.).  Seems easy enough.

I decided to use the Sigma USB dock to tune the 35mm ART f/1.4 lens to zero since the FoCal suggested a -7 AF Tune setting to correct for backfocus after the mirror alignment I made and subsequent test with it.  Figured I may as well make it as good as I can.

Bum part was shot distances of over 30 feet seemed very blurry after FoCal's final number.

The Sigma dock's manual is very sketchy in the dock's operation/calibration.  The 35mm lens is tuned in four distance ranges or zones:  12", 16", 28", and Infinity (Which is harder to do than I expected, and my long tape measure is in "Inches" so that's that!).  Using a suggested 50x the focal length, I ended up testing the Infinity at 69" which resulted in nothing at any distance being sharp.  That's bad and puzzling too, as FoCal said it was "tuned perfect."

Looking at the hyperfocal tables, seems the 35mm at f/1.4 that 95 feet is the minimum distance out to infinity to set that final distance range in the Sigma software.  Good luck targeting 95', and determining what is sharp ahead or behind that 95' target later, plus setting that distance test up in your small yard.

I ended up using 20' on the patio as the 4th distance zone in the Sigma software for their Infinity and it sort'a works a lot better than the 50x the focal length of 64".  So the distance range for testing was 12", 16", 28" and then out to 300" for zones 1-4 respectively in the Sigma software.  Sort of bizarre, but at least the infinity shots were much better now than using the 69" for it prior where nothing was sharp far out - and I needed to go back into the camera's Fine Tuning to correct it still (Which now screwed up the close range too!) even after I got the green light from FoCal.

So if one is using 35-50x the focal length in testing, it might be way off for distance zone the way the makers make their distance range setups in their lenses.  That infinity Zone IV (It's 63" to Infinity in the Sigma software that writes to the lens.) is sort of tricky and probably the zone most of us shoot within.  Most all the test charts are working within the first 3 distance zones and not the furthest one we work in, maybe 6-12 feet with models, so sharpness shots could be questionable out there.  I'm curious where Nikon puts their Zone V and VI distance at too as they did it different than Sigma's four.

Cudabacs used to be the authorized Lecia/ Hasselblad repair center in San Francisco. They had a shop optical collimator setup with about a 6-8" diameter lens, a lighted target at around 6 feet and a stop rest to place the camera. that basically made sure that infinity was, for all practical adjustments, actually at infinity. I just take the camera outside and point it at distant trees or power poles.

With the cravat that I don't own a zoom lens, they are tested at the closest focus and infinity. Assuming that the focus screen to lens flange distance and the sensor to lens flange distance are the same, all should be well. Tossing in a autofocus sensor distance and zoom lens into the mix is mighty complicated.

Seems I got Rich at FoCal's attention regarding the suggested targeting distances when using all these lens tuning targets coupled with the lens tuning firmware, i.e. Sigma and Nikon lens tuning docks.  https://www.facebook.com/ReikanFoCal/

It's a mess when one tests at a set distance (e.g. 25-50 times the focal length), and then the internal lens firmware has some "cross-over distance point" where you can move a couple of feet off your calibration target's distance and the firmware decides you are in a new and totally different distance zone and throws the entire focus off leaving you scratching your head as to what happened.  Sigma has 4 zones (Nikon has 6) for primes, and with their zooms there are 16 zones to tune (ugh!).

It'll be interesting to see what becomes of it all.

Aside, I've been reading about the people on dpreview who like to use the rear AF button on the body and claim it's more accurate.  I suspect it's like pressing the shutter button twice: First press spins the lens into a tolerance zone (hysteresis), and the second press of the AF button on back (or shutter button again) tightens up that tolerance band and it doesn't spin so wildly as with the first press.  A Nikon authorized tech told me the hysteresis tolerance is a little loose else it will hunt and be slow to focus so it is a compromise for speed (i.e. A slow-to-focus lens may be more accurate.).  The FoCal software seems to demonstrate that as it goes through the tests where the QoF (Quality of Focus) improves on the curve on subsequent shots based on the same AF tuning number.  First shot seems pot-luck at times.

Just an FYI for those that are interested in this sort of techy stuff...