06-07-2010, 01:21 PM
[quote name='Klaus' date='07 June 2010 - 12:35 PM' timestamp='1275910547' post='304']
A focus shift at max. aperture isn't really possible by definition. In this case you've "just" a back-/front-focus problem.
That said it can be calibrated this way on purpose assuming that it hides the worst at smaller apertures.
[/quote]
Yes: I calibrated my Sigma to focus accurately at maximum aperture (which given that phase detect AF is effectively "seeing" f2.8 means deviating away from enutral) but the price, obviously, was worse performance stopped down.
But I also find that if I calibrate it to be good at about 2 metres, it's out of whack very badly indeed at infinity -- and vice versa.
So I now calibrate it for 1.5 metres at f 1.8 which is the useful environmental portrait range, where AF is useful with the thin DOF, and if I use it for general work I use manual focus.
A focus shift at max. aperture isn't really possible by definition. In this case you've "just" a back-/front-focus problem.
That said it can be calibrated this way on purpose assuming that it hides the worst at smaller apertures.
[/quote]
Yes: I calibrated my Sigma to focus accurately at maximum aperture (which given that phase detect AF is effectively "seeing" f2.8 means deviating away from enutral) but the price, obviously, was worse performance stopped down.
But I also find that if I calibrate it to be good at about 2 metres, it's out of whack very badly indeed at infinity -- and vice versa.
So I now calibrate it for 1.5 metres at f 1.8 which is the useful environmental portrait range, where AF is useful with the thin DOF, and if I use it for general work I use manual focus.