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03-22-2008, 04:45 PM   #1
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What do you make of this?

I've had the K10 for almost an year and it's been pretty good. I've used it with the kit 18-55 lens, tamron 28-75, the 100mm macro and pentax 70-200. Sometimes I noticed that in some images the object was out of focus but blamed it on my mistakes.
Today I got the 16-46/4 lens, brand new from amazon, and did some test shots of my son. They seemed kinda soft on the camera preview, so I downloaded them and sure enough - they were out of focus!
This was only visible at 16mm, at 45 it was OK.
Long story short - after downloading the focus chart I did some tests and now I'm confused.
With K10 the 16-45 is spot on at 45mm and waaaay off at 16mm - heavy back focus.
With K10 tamron 28-75 is spot on at 75mm and so-so at 28mm - slight back focus also.
I have DL too, so I did a test with that one too. It focuses correctly at any range.
Is this something common with K10 - back focusing at wide angles?
Should I send back the lens and hope that the next one will be OK or should I send the camera to Pentax for calibration? How long does it take to get it back - I have events that I have to cover next month.
I was really looking at D300 and now this problem is adding more weight in that direction.
Here are some pictures:
All shot at f4.
K10-pentax45mm


k10-pentax16mm (crop)


K10- tamron75mm


k10-tamron28mm


DL-pentax45mm


DL-pentax16mm


03-22-2008, 05:33 PM   #2
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I have a ton of questions,

was the camera on a tripod, at 45 degrees to the test chart? This is important because any slight vertical shake could move the focus point, you need to have the focusing point directly at the line.

It looks like you are not squae on the target, this also needs to be corrected.

Additionally, are you aware that the depth of field is devideed 1/3 before and 2/3 behind the focus point?

this last issue, is part of optics, but makes many people think they have back focusing problems when they do not.
03-22-2008, 06:27 PM   #3
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It was not done with a tripod, but I was leaning on a chair and it was prety close to 45 degree, and I did 6 or 7 test shots and all they look the same . I even focused at 45mm and then zoomed out to 16mm with the AF off - it was ok then. But if you let the AF on it will readjust the focus after zooming out and it will be off target.
Also, these are ilustrations of the real life problem - it will not focus on the object I want.
It doesn't matter if this picture was taken at 45 degree or square at the bed - the focus is way off the center, were I was aiming.
03-22-2008, 06:42 PM   #4
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QuoteOriginally posted by minavasht Quote
...
I was really looking at D300 and now this problem is adding more weight in that direction.
...
I believe most digital SLRs suffer this BF/FF issue, more or less. That is why Nikon D300 and Pentax k20D (maybe more) have added a feature to store lens info.

Before changing system, you could try K20D in a shop to see if it works better.

03-22-2008, 06:49 PM   #5
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QuoteOriginally posted by ddhytz Quote

Before changing system, you could try K20D in a shop to see if it works better.
There is not a single pentax dealer in Orlando, can you believe that?
All my pentax gear was purchased online.
This is one of the reasons I'm looking at the dark side - they are everywhere and I can go and test them at the store.
I was really surprised that this lens was so bad on my camera.
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