Reading your comments Neuro I am starting to wonder I might have done it all wrong! First, when you say ~25x the focal lenght, do you mean say 25 x 50 mm = 125 cm or ~4ft?
Also, when you say "shoot several shots at each AMFA setting - I do 8 total (2 starting at infinity, then 2 without refocusing, then 2 starting at the minimum focus distance, then 2 without refocusing)", so do you mean you do all your test using manual focus? And when doing these 8 shots above, do you always stay at the same distance from the ruler?
Yes, 25x the focal length would be 25 x 50mm = 1250mm = ~4 feet.
All the shots are the same distance from the ruler, and you need to use AF for AFMA. What I mean is that for the first shot, I turn the manual focus ring on the lens until it's set at infinity, then activate AF and take the shot. Then I turn the MF ring to infinity again, AF, shoot. Then just shoot twice without manually changing the focus before shooting. Then I turn the MF ring to the minimum focus distance, AF and shoot, repeat, then two more shots without moving the MF ring. Hope that clarifies.
I got the SpyderLenscal myself and got so discouraged trying to micro adjust my 50mm 1.2L that I gave up. ...as you can see I am trying everything I can to increase the % of in focus shots with my 50 1.2 (from microadjustment to the ST-E2 speedlite transmitter
Two words: focus shift.
The 50mm f/1.2L is something of a special case. That lens suffers from a particularly bad focus shift (many lenses have some focus shift, especially, fast primes, but usually not enough to notice). Focus shift is when the focal plane of the lens changes when you change the aperture of the lens. All lenses focus with the aperture wide open, then stop down the aperture to your chosen setting as the shot is taken. In the case of the 50mm f/1.2L, if you select an aperture narrower than f/1.2, down to about f/4, focus shift means that the lens will actually focus on a point that's behind your chosen focal plane. At f/4 or a little narrower, the DoF is deep enough to mask the effect, because by then your chosen focal plane is within the DoF. At very close subject distances, the DoF is shallower, and the effect of focus shift is exaggerated (you may need f/5.6 or even f/8 to get a deep enough DoF to mask the shift).
So, what can you do? Here are some options:
- Stop down to f/4 or narrower. But, I'm going to assume that shooting at f/8 is not the ideal solution...
- Shoot at f/1.2. There's no shift if you don't stop down.
- Use Live View. Granted, that's not a good option with typical 50/1.2 subjects.
- Manual focus with the DoF Preview button pressed. You'd almost certainly need the Eg-S focusing screen instead of the stock screen.
- More complex AFMA. You could perform an AFMA at f/2, and you'd get different results than wide open - that adjustment would compensate for the focus shift at f/2, but not be applicable at f/1.6 or f/2.8, for example. So, you'd need to have a list of AFMA values, and change the setting to match the aperture you want to shoot at. Might work if you can pick an aperture for a shoot and stay there. (Side note here: the 1D X can store two AFMA settings for a zoom lens, one for the wide end and one for the long end; in theory, Canon could allow multiple, aperture-dependent AFMA settings for the 50L, which would certainly help with this issue.)
- Intentionally front focus. Use an AF point over a feature that's a little bit in front of what you really want to focus on.
- Tweak on the fly. The 50L has full-time manual focus, so you can use AF get you close, then turn the MF ring slightly to bring the focal plane forward a little. You'd likely want to be using back-button AF for that, and it would take a fair bit of practice to get it working reliably.
The 50L can deliver amazing shots, but due to the focus shift issue it takes some work and practice to get the most from the lens.