08-05-2011, 06:29 PM
[quote name='Beholder3' timestamp='1312538823' post='10404']
Hi,
is here any owner of a Pentax-Mount Sigma 24mm / F1.8 EX DG lens?
I recently bought one and it seems to share an error with another specimen from another forum user.
Here is the story:
When trying to use autofocus to take a picture of anything in a far distance (e.g. 500m plus, horizon landscape etc.) the autofocus sets focus to a wrong point, resulting in unsharp pictures. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/sad.gif' class='bbc_emoticon' alt='
' />
#
It is absolutely possible to get a sharp picture of that distance mith manual focus.
The really freaky thing though is, that this focus problem only seems to kick in from F1.8 to F2.8. With my lens as soon as I switch to F3.5 or higher autofocus works great and sharpness jumps up.
This issue is not existent in general with the camera or other lenses. It has also nothing to do with general softness at larger apertures.
It is 100% repeatable.
If you own this lens, can you check if your lens also behaves like described above?
It is not noticeable easily in many shots since that lens might not be primarily used for combinations of far distance shots and wide open apertures.
[/quote]
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).
Hi,
is here any owner of a Pentax-Mount Sigma 24mm / F1.8 EX DG lens?
I recently bought one and it seems to share an error with another specimen from another forum user.
Here is the story:
When trying to use autofocus to take a picture of anything in a far distance (e.g. 500m plus, horizon landscape etc.) the autofocus sets focus to a wrong point, resulting in unsharp pictures. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/sad.gif' class='bbc_emoticon' alt='
' /> #
It is absolutely possible to get a sharp picture of that distance mith manual focus.
The really freaky thing though is, that this focus problem only seems to kick in from F1.8 to F2.8. With my lens as soon as I switch to F3.5 or higher autofocus works great and sharpness jumps up.
This issue is not existent in general with the camera or other lenses. It has also nothing to do with general softness at larger apertures.
It is 100% repeatable.
If you own this lens, can you check if your lens also behaves like described above?
It is not noticeable easily in many shots since that lens might not be primarily used for combinations of far distance shots and wide open apertures.
[/quote]
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).