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...I've got a sigma 30mm f1.4 that is fine when I'm shooting headshots (quite close) with the small ma adjustment but then focuses behind the subject for full body shots. If I were to correct this then I'd get a slightly off result for headshots. If the amount the lens is 'off' by varies along with the distance from the subject, what do I do to sort it? Is there anything?
Since depth of field decreases with decreasing subject distance, it seems to me that accurate focus for headshots would be the most important. Purhaps doing your adjustment at a test distance that's somewhat further than your normal headshot distance, but closer than your normal full-body distance would work by splitting the difference.
What distance were you using before? I usually try for 50X the focal length, so for the 30mm I'd use 1500mm = 60 inches = 5 feet as a starting point.