I just did a brief check of my 16-50. Focusing through a somewhat
dirty window at extremely distant objects, it sometimes focused right
in the center of the inifinity mark, sometimes a bit closer. Focusing
in a room on objects between 2 meters feet away and 5 meters distant,
it focused appropriately between the 2 meter and infinity marks.
As I'm sure you know, the menu focus adjustment corrects for a
throughout-the-range mismatch. If your lens focuses accurately at 2
meters, you won't be able to change the infinity focus. However, I
suppose it could be front focusing throughout the range. Good luck.
BTW, when shooting at infinity and 16mm, I almost always focus
manually. I figure to do otherwise is asking too much of an autofocus
system. But you of course want your lens to be accurately calibrated.
Paul
On Feb 11, 2009, at 1:15 PM, jtainter wrote:
See Paul's query below.
With my first DA* 16-50, at 16 mm the lens would not autofocus past
about the 2 m. mark, even on very distant objects. (I use only the
central af point.) At first I didn't worry about it, figuring that
DOF would cover the difference. But when I focused manually to the
infinity mark, the images were much sharper. I sent this lens, the
DA* 50-135, and the K20D to Pentax, and the camera came back set for
a universal -3 AF adjustment. That seemed to be a good compromise,
and at 16 mm the lens would focus to the near part of the infinity
mark. The lens thereafter performed pretty well, but on this last
trip to Italy I noticed the problem of edge weakness above 40 mm.
Hence it went to Pentax, which sent me a new sample of the lens.
Now I have another sample of the lens showing the same af problem.
Tonight if I'm not too tired I'll test it with the standard focus
adjustment chart. It's possible that I can correct the problem with
focus adjustment. I believe that is what Pentax did when I sent the
first one in.
I usually keep a skylight filter on each lens, but at the moment
there is no filter on this lens. So I know that is not throwing off
the af.
Joe
-----
How does one check infinity focus at 16mm? It would seem that the DOF
is so extensive at that focal length, even at f2.8, that it would be
nearly impossible to determine accurate focus. I don't think I've ever
attempted infinity focus at 16mm on my lens. I'll have to try it and
see what happens.
Paul
On Feb 11, 2009, at 11:59 AM, jtainter wrote:
My K20D came back yesterday. It appears that to correct the sensor
problem Pentax just replaced all or most of the electronics. When I
turned it on, it started up with the initial menu, asking for
language, location, time. Then I had to go into the menus and set
all my preferences again. Now I get to start testing 18 or 19 lenses
for autofocus adjustment.
The new DA* 16-50 F2.8 has a problem like the last one had. It won't
autofocus to infinity at 16 mm. So even in newer copies of this lens
there are problems. I will try to correct this with the autofocus
adjustment, but if that doesn't work it will have to go back to
Pentax--again. I'm getting tired of this.
Joe
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