[Beholder003]

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.

[Brightcolours]

[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).

[Beholder003]

[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.

[Brightcolours]

[quote name='Beholder3' timestamp='1312621994' post='10421']

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.

[/quote]

Then you are looking at a focus shift problem... as the AF gets done with the lens wide open. And those are usually by design, not a sample problem.



Is it in practical use a real problem, though? I can imagine that usually you have the need for bigger apertures with closer by subjects, and with far away subjects at 24mm you stop down past f3.5 anyway? So a problem that is easy to live with, just about understanding the intricacies of this particular lens?