wide angle misfocusing problem
Let's review what I've done and tried. At 7 meters range (hot very far after all), I focused on a bush in a pot. I use center focus point linked etc as RioRica stated. The camera's focus area is smaller than the bush. The camera misfocuses at 18mm, reading 4 meters on the lens barrel and visibly soft when viewed on PC screen. Between me and the bush there is open space - nothing else that the focus area could have grabbed on.
I try LV - the image focuses and the lens barrel reads 7 meters. The image is visbily sharper than the previous version.
I try another brand camera, as I said, well known for the accuracy of it's AF system. Same conditions - result - sharp focus on the bush with AF i.e a different result than the Pentax under identical conditions.
Ergo, somewhere between the Tamron lens and the Pentax camera, something doesn't work right at 18mm, at short range.
So I tried a kit lens. Since there are no distance markings on the barrel, I stuck a piece of tape on the barrel and made marks, firstly where the KR focused with AF and then where it focused with LV. Once again, there was a difference. Please note the subject didn't move, neither did the photographer.
So now I have the same problem with two different lenses but with the same camera. Incidentally, I have had the problem in other conditions - the bush was a deliberate choice of test target at a set distance with no other objects between it and the camera. The range is short enough and the bush large enough to ensure that the focus area of the camera is not overlapping the subject.
I want to try one more lens to rule out a lens based issue but as I have carefully tried to rule out all the excuses Marc can think of to exonerate the KR, I'm left with a strong possibility that the KR AF system doesn't cut it under some conditions.
In a sense the problem is now academic as I have shifted to another system and keep the KR due to existing lenses and accessories. I have to remember however if I am using the Tamron lens, to Manual focus at anything up to around 35mm , after that the AF seems OK.
But I have raised this issue as I think it is valid information about Pentax performance and it may alert other users of the Tamron or Pentax 18 - 250mm lens to check the sharpness of the focus at medium short distances at wide angle.
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