To set my Micro Focus Adjustment (5D MKII & Canon lenses) target distances, I have read a few conflicting suggestions:
1. 50x the lens focal length
2. average shooting distance when using that lens
Is there a correct or absolute distance?
Does it change when using a lens close or far away from the subject? I would have thought that if it is fractionall shifting the sensor or focus plane then that should be good for all distances with that particular lens and body
Hi Frank,
Obviously you want the AF to focus correctly on the distances that matter most, for you.
The 50-100x focal length suggestion, which will require a large target and shooting distance for longer telelens designs, is a traditional distance often used in the film era. What really matters is that the fine detail on the target has a higher resolution than the highest resolution component in the imaging chain.
At 50x focal length, the magnification factor of the image projection on the sensor plane is almost 1:50. That means that if one prints a target oneself, say at a 300 or 360 ppi resolution, the finest detail will be about 0.006 lp/mm, and combined with the 1/50th magnification that will produce optical information with a resolution up to 9000 lp/mm. I can assure you that that exceeds the capabilities of virtually any lens in air, and certainly exceeds the (non-antialiasing filtered) sensor resolution by a large margin. So 50x FL is a very conservative and very safe criterion.
I would AF microadjust at the relevant focus distances for your common shooting distances, provided they are not much closer than say 25x focal length if you want to ensure some validity at occasionally larger focus distances as well. At close focusing distances your test target needs to have higher resolution, and that may become difficult when closer than 25x focal distance. But even then, there is nothing that kills resolution as fast as slight defocus.
Cheers,
Bart
