Speaking of GQ, I posted the following on Lulu earlier today --
The Arca R focus helix is approximately 110mm in diameter, has 172 tick marks (numbered every 5 ticks, from 0 to 34), and moves the lens in or out 2.5mm per revolution -- i.e., 2500/360 = 6.94 microns/degree, or 2500/172 = 14.53 microns/tick mark. Lacking a vernier scale, about the best that could be achieved in terms of repeatability is 1/2 a tick mark, or 7.27 microns. Arca claims 10 micron repeatability for the Rm3di <http://www.rodklukas.com/resources/Arca-Swiss_NewProducts_2010.pdf>, which coincides with the minimum shim increment from Alpa.
The scale that Arca uses appears to be based on the circumference of the focusing mount: 110 x pi = 345.6, which, when truncated to an even integer, yields 172 ticks and a final diameter of 109.50mm.
This afternoon I worked out the theoretical extension table (in Arca units) for the SK 43XL. Plugging in Schneider's published focal length for this lens: 44.6mm, I can almost but not quite duplicate the table that Arca sent with my lens. At distances beyond 10 ft, the differences are 1/2 tick or less. At closer distances, the differences increase, but are still less than one tick up to 6 ft. However, if I use 44.95mm as the focal length, I can precisely duplicate the Arca-Swiss table.
So, in my case, has Arca-Swiss measured the actual focal length of my lens and used that to generate the table? What else do they take into account in generating focus tables?
On a related point, infinity focus is stipulated as an extension of 0mm. What if the sensor is a little farther back? How does one compensate?
Dave.
[As I noted on Lulu, I'm still lacking a mount for my back, so I'm reduced to admiring Arca-Swiss engineering as opposed to using the camera.]