Here is Rod's response for you all to read:
Stanley,
Her you go:
A Focus factor is a number Arca-Swiss users use to precisely set infinity on our R series cameras. It takes into consideration how deep the pixels are in the sensor. Being organic, Sensors vary, unit to unit. All tech cameras, being as precise as they are should be tested for sharpest results and best color. Other manufacturers us a different procedure to set focus factor.
On an Arca-Swiss RM3di, you go out and find a target at 1 mile, or 1500-1600 meters. You must use a target that is at the 1 mile or 1500-1600 meter distance, with great detail such as cell phone tower, building, or non moving target with good fine detail.
After leveling the camera, you should set the focus wheel at 0, then 1, 2, 3, 4, 5, and 6, making exposures at each focus position. Be sure to get a good exposure.
The aperture should be approximately the most commonly used aperture by the user. I use F11 as I shoot Landscape with backs typically 60mp or less.
Users with 80mp backs should shoot approximately F8 or F8.5.
This ensures that maximum depth of FOCUS, is achieved, to counter the focus shift, of especially the red wavelengths, in all lenses.
Make the exposures and examine your images at 100% on your computer.
Select the absolute best infinity focus and note which image it was in the order of the shots.
Say that the image # 1 was the best. That is your focus factor, 1.
When you focus the R series camera after that, add 1 to whatever number is indicated on the focus card, which is supplied with each lens, for a given distance.
You might also take note after discovering that, in our example #1 was best,
if #2 or #3 still had acceptable infinity. This is useful for a image environment where there are few if any objects at this infinity, yet you want a personal hyper-focal distance. My factor is #2, but in downtown Manhattan, for example with my 43mm, I typically use #3.5 or #4, as you typically can't see 1 mile very well, so I can concentrate on composing and exposing.
You can program the focus factor into an emodule so it is already taken care of. This makes the DOF readings extremely accurate if using this accessory.
Hope this helps.
Rod
Rod Klukas
US Representative
Arca-Swiss International
PO box 28450
Tempe, AZ 85285
(480) 755-3364