The Answer = 8.2 feet per 100mm of Focal Length
25 times the Focal Length of the lens under test (25xFL), which resolves to 8.2 feet per 100mm of Focal Length. It is not critical that the distance be precise, although the closer the distance the more critical it is. In general +/- 10% is fine.
- 100mm = 100/100 x 8.2 = 1 x 8.2 = 8.2 = 8 feet is OK
- 200mm = 200/100 x 8.2 = 2 x 8.2 = 16.4 = 16 feet is OK
- 50mm = 50/100 x 8.2 = .5 x 8.2 = 4.1 = 4 feet is OK
- 25mm = 25/100 x 8.2 = .25 x 8.2 = 2.05 = 2 feet OK
To simplify even further, think about it this way - take the focal length and move the decimal point 2 place to the left:
- 100 becomes 1
- 200 becomes 2
- 50 becomes .5 or 1/2
- 25 becomes .25 or 1/4
Then shorten 8.2 to 8 and multiply:
- 100mm = 1 x 8 = 8
- 200mm = 2 x 8 = 16
- 50mm = .5 x 8 or 1/2 x 8 = 4
- 25mm = .25 x 8 or 1/4 x 8 = 2
So 500mm is 5 x 8 = 40 feet and 70mm is .7 x 8 = 5.6 or 5 1/2 feet
Note: When shooting a crop frame camera (for example Canon 7D series or Nikon 7000 series) you should use the exact focal length of the lens without regard to the crop sensor. When using a zoom lens, use the focal length that the zoom lens is set to.
Since the AF system is not a linear system, it is not possible for the adjustment to be *perfect* at all shooting distances. So it is important to test and adjust the camera/lens combination at a distance that will work well at most all shooting distances.
If the test/align distance is too short, the AF system could be compromised at longer shooting distances. But when the test/align distance is equal or greater than the recommended 25xFL distance it is likely that the AF accuracy will be improved at all shooting distances, both closer and further than the test/align distance.
Since the AF adjustment can only be *perfectly* accurate at one distance, if you shoot at a specific distance or small range of distances (like head and shoulder portraits), you should always test/adjust at that specific distance even if it is less than the 25xFL recommendation. But be careful when shooting at longer distances.
We suggest you measure from the center column of one tripod to the other. The reason for this, is that when you retest a specific camera/lens combination, you can repeat the same distance and not have to remember how you measured the distance.