08-06-2011, 09:13 AM
[quote name='Brightcolours' timestamp='1312568958' post='10411']
if you are sure the focus is actually better at f3.5 (and not just a difference in DOF and contrast and such), then you are seeing a focus shift when the lens gets closed down. Apparently during focussing the lens is aware of the focus shift, and corrects the focus.,.. but that means that the f1.8-f2.8 range also gets "corrected" and ends up with incorrect focus. It would help if you would try to figure out in more detail what the focus behavior actually is (does it always have the same missfocus, is it front or back focussed, is at f3.5 the focus really correct and such).
[/quote]
I am absolutely sure the autofocus hits better at f3.5+. In this apertura range I can't manage to get a better result with manual focussing and it's not a DOF issue. f1.8-2.8 I can easily get better results with MF.
It is always the same misfocus (with a little variance in amount).
It is backfocus (frontfocus at unlimited range would require an huge error and massive rewinding of the focus to focus on less than 15m (as the target is 220m away). That rewinding is not happening.
if you are sure the focus is actually better at f3.5 (and not just a difference in DOF and contrast and such), then you are seeing a focus shift when the lens gets closed down. Apparently during focussing the lens is aware of the focus shift, and corrects the focus.,.. but that means that the f1.8-f2.8 range also gets "corrected" and ends up with incorrect focus. It would help if you would try to figure out in more detail what the focus behavior actually is (does it always have the same missfocus, is it front or back focussed, is at f3.5 the focus really correct and such).
[/quote]
I am absolutely sure the autofocus hits better at f3.5+. In this apertura range I can't manage to get a better result with manual focussing and it's not a DOF issue. f1.8-2.8 I can easily get better results with MF.
It is always the same misfocus (with a little variance in amount).
It is backfocus (frontfocus at unlimited range would require an huge error and massive rewinding of the focus to focus on less than 15m (as the target is 220m away). That rewinding is not happening.