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10-16-2011, 06:37 AM   #1
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odd front focus problem with kit lens and Tamron 18 - 250mm lens

Hi, I'd like to check if anyone else has found the following problem. Using the DAL kit lens on the K-R, I find that it front focuses when set at 18mm but develops correct focus as I zoom in. For example, focusing on a bush abouit 7 meters away, the camera focuses the lens to 2 meters. When the lens gets to around 35mm, the focus is correct.

Close up shots at 18mm focus OK enough i.e. slightly soft but are still better if I do it manually or with Live view.

Focusing on a subject at 30m or further, at 18mm, the lens does not turn to infinity but stops short. If i zoom in, it changes the focus distance.

I tried it with Liveview and then it focuses correctly at 18mm. I have exactly the same problem with the Tamron 18 - 250mm lens. The problems therefore appears to be camera related.

The consequence of this is that my 18mm shots tend to be blurred from OOF, unless I remember to zoom in, lock focus, zoom out and take the picture.

Anybody else had this problem?

Thanks and happy shooting.

10-16-2011, 01:57 PM   #2
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If your camera is focusing at 2 meters when you have explicitly told it to focus at 7 by carefully selecting a focus point that is centered on the bush, with nothing else within range of the focus sensor - then that's no front focus, it's a broken camera. But since you say you see this at 18mm and then it goes away as you zoom in, I'd say there is almost zero chance you are actually doing what I said - that is, you aren't choosing your focus point in such a way that nothing but the bush is within range. That' pretty common - at wide angles, the bush shows up so small in the viewfinder that something else is very likely in range of the focus sensor and is 'stealing" it's attention.

Feel free to post specific images, but I'd say this is 99% likely what you are seeing - simply a failure to communicate effectively enough to the camera where you wanted to focus.
10-19-2011, 12:00 PM   #3
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reply to Marc

Thanks Marc for taking the trouble to reply - I follow what you say about focus point issues.

It is hard in the viewfinder to see a difference in focus when the lens reads either 2 m or 7 meters at 18mm. It is only when enlarging the image on screen that the misfocus becomes apparent with increased blurring of the image.

Funnily enough, i have another camera of a competing brand, well known for the accuracy of its focusing system and with it's own 18 - 55mm kit lens, it locks focus accurately i.e. at 7 m distance, it reads correctly vs. focusing at 2 m distance.

The problem is something between the Pentax K-R and the lenses I am using but in the absence of any other 18mm+ zoom lens, I can't clearly establish where the fault is lying.
10-19-2011, 11:08 PM   #4
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I see no reason to believe it isn't exactly as I said - just because you point he camera in the general direction of a given pbject doesn't mean the camera can read you mind and know that is where you want to focus. You need to carefully position the selected focus point on a place where here is nothing else within range. This isn't unique to Pentax, btw; it's pretty much just coincidence if you happened to jsut. Otice this now.

10-26-2011, 10:20 PM   #5
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Sameproblem

I'm having the same issue as jlaubza. In my case I was doing forward focus test using a scale at 45 degree, a center target, single focus point at the center, on tripod, at a distance of 4 feet. Like jlaubza, I tested the DAL 18-55 and the Tammy 18-250. For both lenses, they did fine if zoomed in higher than 50mm. But both would focus one to two feet, yes feet, in front of the target when using the lenses at 18mm. I confirmed this with Live View, which hit it dead on. I still need to test this at greater distances. I also am quite puzzled.
10-26-2011, 10:23 PM   #6
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Forgot to mention, I am also using these on a Kr.
10-26-2011, 11:36 PM   #7
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odd front focusing problem

Dear Docswanson, you have no idea how reassuring it is to know that this problem isn't unique - at least I'm not alone!

I very much suspect that this is a common problem with the KR and the cheaper wide angle lenses or at least a wide angle at f3.5 and above - my suspicion is that the camera does not get enough light to refine the focus on a wide angle subject. As you say, with LV, the focus is accurate so the camera is capable in one focusing mode but not with VF focusing.

And since the other brand camera I use does get the focusing right with a similar kit lens, I believe that the Pentax focusing module is the culprit.

If one restricts oneself to 4x 6 prints, you will never notice the misfocus on wide angle shots but if you crop and enlarge, the problem becomes troublesome.

Regards.
10-27-2011, 03:02 PM   #8
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Post the tests. I will practically guarantee the problem is nothing of the sort, and will turn out to be a simple matter of the camera choosing to focus somewhere other than you intended at wide angle settings, simply because more will be in range of the focus sensor. It's a basic problem inherent to all AF systems, nothing unique to any camera or any lens.

10-27-2011, 06:35 PM   #9
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Marc is quite right. In a target-rich environment, like the subject field of a wide-angle lens, what target the AF system decides to focus on may not be what YOU are interested in. Select a focus point, or set AF to center focus, and you can outsmart the camera. You also might want to link the focus and metering points, to sync the focus and exposure.
10-28-2011, 02:11 PM   #10
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Even if you select the focus point, with a wide angle lens, you also must take into consideration that more than one object is likely to be within range of the focus sensor. That if, you might aim select a focus point and aim it at one tree trunk, but with a wide angle lens, that tree is going to render small enough in the frame that the trunk won't fill the entire range of the focus sensor sensor, and the camera may well focus on something behind the tree.
10-29-2011, 12:56 AM   #11
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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.
10-29-2011, 05:54 PM   #12
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Was this test repeatable, or a couple of fluke shots? If repeatable, then I think it would help to post the results. And BTW, how did you ascertain the size of the focus sensor in order to determine that the bush was completely filled its range?
10-30-2011, 09:42 AM   #13
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reply toMarc S

The answer is yes to both your questions. The camera front focuses at wide angle settings with the Tamron 18 - 250mm and the kit lens.
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