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Old Jun 27, 2009, 8:03 AM   #11
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My 50-135 does have a bit of a back focus, it didn't look bad enough to mess with when I shot a focus chart with it, but I notice it occasionally when out shooting in the real world. I really noticed it when I was shooting comparison shots with the 77 Ltd, which is bang-on for focus. I really should adjust it, but never quite get around to doing something about it. It's far better on the K20 than it ever was on the K10, though.

The way I understand it is the focus line needs to be in sharp focus. As far as how much in front of and in back of is in focus is somewhat subject to interpretation. I think I read somewhere that it's a 1/3-2/3 split (not 50-50) but don't remember which is supposed to be where.

Part of the problem when trying to adjust focus points is the fact that you really do need a 45% angle and good light. And to have the camera far enough from the chart to not have the minimum focus distance come into play. I think I'd need to be at work to have all that work out right!
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Old Jun 27, 2009, 10:07 AM   #12
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My 50-135 does have a bit of a back focus, it didn't look bad enough to mess with when I shot a focus chart with it, but I notice it occasionally when out shooting in the real world. I really noticed it when I was shooting comparison shots with the 77 Ltd, which is bang-on for focus. I really should adjust it, but never quite get around to doing something about it. It's far better on the K20 than it ever was on the K10, though.

The way I understand it is the focus line needs to be in sharp focus. As far as how much in front of and in back of is in focus is somewhat subject to interpretation. I think I read somewhere that it's a 1/3-2/3 split (not 50-50) but don't remember which is supposed to be where.

Part of the problem when trying to adjust focus points is the fact that you really do need a 45% angle and good light. And to have the camera far enough from the chart to not have the minimum focus distance come into play. I think I'd need to be at work to have all that work out right!
thanks for the reply
as far as 1/3-2/3 split with the 2/3 at back focus end my lenses wont need adjusting
i spent many hours trying to achieve a 50-50 split and that is where i was seeing the inconstiences

Dave
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Old Jun 27, 2009, 6:10 PM   #13
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hope you can help
i too have this lens and love it but,i never realised i had a back focus problem until i tried this focus test chart. i have not been able to make any head way with this chart as for some reason i get completely different results if i use different zoom settings and also different chart to lens distances. its never consistant.
is it that you only have a problem if the focus line is not clear? do you adjust focus to a balance ? do you adjust at an average of the different zoom settings? do you adjust to an average to different distance settings?
do you adjust only for the 16mm setting? I have
my lens takes great pics but yet the focus chart says i have a very bad back focus problem that would require a +10 setting at 16mm, but only a +5 at 30
even my 50-135 lens takes stellar crisp photos, but yet the focus chart also inticates that that lens has a very bad back focus problem

What to do, what am i doing wrong. is this chart really for prime lenses and not zooms

Dave
Hi Dave, per the instructions, I mount my camera + lens on a tripod and point it at 45 degree angle to the printed focus chart which I tape down to a table top. I then focus on the target print that says "this text should be in focus" (or something like that). I typically test at the wider appertures (i.e f/2.8 to f/4.0)and maximum zoom (50 mm) since DoF is at its shallowest at these settings. As the aperture is reduced and zoom is widened, the chart loses its value since DoF increases showing more of the chart in focus.

My DA*16-50 as tested shows that the focus is exactly where it should be (i.e. no front or back focusing). In all the time that I have used it, I have not noticed any focusing issues which to me validates the test. As a result, I have never had to make any adjustment to the K20D. If there is a back or front focusing problem, you could adjust the settings on the body and repeat the test until it shows the desired result. Once again, I would use max zoom and f/2.8.

If your lens is still under warranty, you may try having Pentax re-callibrate it. Hope this helps answer your questions.
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