I thought I had a problem with focusing my Sigma 150-600mm sports lens which can’t make up its mind on whether to back focus or front focus when fitted to a Canon 7d Mkii, so decided enough was enough and purchased a Canon 500mm f4 L but this is doing the same, so I set up a focus chart to do a micro adjustment on it, placed it as far away as I could from the camera which is 15 mtrs at 45 degrees to camera, good light, level and facing directly at the lens at the same height, set camera on a sturdy tripod switched off the os set aperture to wide open with the lowest iso and took 6 test shots but these came back as follows first shot heavily back focused second the same third a not so much back focusing about half way fourth front focusing slightly fifth same sixth shot a little back focusing , after adjustment it got closer to the focus line but still inconsistent, so connected the lenses to my 5D mkiii which and done same test and adjustments with better results but still inconsistent and then my old faithful 50D which results were more stable but still not perfect, now the acid test, with my 5Dmkiii ever so slightly back focusing on four out of six shots I went out in the field switched on the os and took some shots hand held got back and checked the shots and they all front focused. Before I condemn my Canon gear to the local canal and take up golf is there anything I need to do differently.
Put your focus chart parallel to the camera sensor, by putting it at 45 degrees you are just confusing the AF. It is alright if you have something like a ruler at 45 degrees to show the focus error but the main point of focus should be flat and directly facing the camera.
Hi Ray, thanks for the reply, the chart has a ruler each side of the main focus line like this on A3 paper, see insert, I've tried the target one also which is flat and has a 45 degree ruler at one side also but it's not consistant and when using in the field with OS switched on it focuses on a different plain, I will do a flat chart at 90 degrees directly facing the camera as you suggest it sounds more logical, I'll keep you posted.
