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05-23-2011, 10:58 AM   #1
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K7 focusing issues.

I hope someone can help me with this as I am at my wits end.

I have a K7 and seem to be having serious focusing issues. back and front. As an example using my Sigma 24-60 f2.8, indoors I have the microadjustment set at -4 @35mm, Using it outdoors yesterday I and needed to push it to +10 and even that wasn't enough to get things completely in focus.

I seem to be having this issue with all of my lenses. Some more, some less. All the zooms have focus shift when I change focal lengths from front focus as the short end to back focus at the long end. I know this and can adjust in the studio, but now that summer is here I need to be able to work both outside and inside, and move from one location to another.

I never had this issue with my GX10, but no matter what I do the K7 just doesn't want to play nice.

Any thoughts on the matter would be greatly appreciated.

05-23-2011, 11:26 AM   #2
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Without knowing more about how you tested this, I would say the most natural conclusion is that your testing methods are insufficiently well-controlled. Can you post some of your test chart shots?
05-23-2011, 12:31 PM   #3
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I am pretty rigorous in my testing. Focus chart 5 feet @ 45 degrees to the sensor. Camera on tripod with everything leveled and aligned with lasers. I defocus the lens then focus the camera and trigger the camera via remote. I do this 3 times then check the fine focus on the computer before making any adjustments, then do it again and again till it's "perfect"

Here is the issue yesterday, this issue has shown its face before, but here is the most recent example. The first is shot at 60mm@f9 The point of focus is her eye, but it is the hair at the back of her head and flowers behind her head that are in focus. The second again the focus is on her face, but the post about a foot and a half behind the models head is in better focus (but not perfect). Shot at 37mm@f3.2
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05-23-2011, 01:59 PM   #4
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Assuming those are crops and not the whole shots, those look like are classic examples of why you need to test using well-controlled test subjects (like properly designed charts). Neither of the shots above shows FF or BF - they show the simple fact that the focus "point" is not a point at all, but a largish area around the red square in the viewfinder. You may have put the red square right over the eye in #1, but the hair was also in range of the focus sensor, and the camera apparently chose to focus there instead (hard to tell from the given crop). Similarly, you may have aimed the red square at the models face, but the background posts would have been in range of the focus sensor as well.

05-24-2011, 12:49 AM   #5
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How about showing us the test chart shots?
05-24-2011, 04:43 AM   #6
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It isn't unusual for zooms to have some focus shift throughout their zoom range.
You may have to find a happy medium and use depth of field to cover the focus error.
I don't know if this is a worse problem with Sigma than other lenses, but it wouldn't surprise me, The only zoom I have is the 60-250 Pentax, and it doesn't do what yours is doing.
05-25-2011, 08:08 PM   #7
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Thanks for the replies.

I went and tested the lenses in question on a K5 at the store, just freehand, no tripod with the same result. It's the same with all my Sigma lenses. The Pentax ones do it also, just not nearly so bad. Again my old GX10 has far less of an issue with this. There is very little shift using that body. I would assume it would be the same with a K10d as they are the same camera internally.

With the attached pic, you can see the shift with my 70-200mm Sigma. The green dots show the points of sharpest focus. Quite a variable. This is at 5 feet so you can extrapolate how bad it can get at say 20 feet or more. The 70-200 is a more graphic example of how much shift there is.

A link to the full rez copy so you can pixel peep.
http://farm6.static.flickr.com/5062/5760076711_0fe5616fa9_o.jpg

Because the lenses seem to shift much more on the newer bodies I am figuring they most likely need to be re-chipped. On the GX10, when I zoom in or out, and press the focus button, they creep just a little making a small correction, whereas on the K7 and K5 they do not.
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05-26-2011, 04:49 AM   #8
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as marc indicated it might be useful to see the full frame of both samples, to evaluate what the camera might have focused on

05-26-2011, 10:12 AM   #9
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Did you actually shoot these things horizontally? That's not how the chart was designed to be used. And doing it handheld could easily explain a shift of a couple of millimeters. Plus, do to curvature of field, it would be normal for the chart to show some amount of FF looking at the sides of the frames. Can you post one of the images, uncropped, unrotated? It's too har to tell what we're seeing with these.

But in an case, note that even if these were done correctly, your results show discrepancies of at most a few millimeters. That's a normal sort of range for focus to be off and indeed, it can depend on whether the lens started off focused in front of behind the target. But your initial examples were very clearly something else entirely - focus being off by several inches or even feet. No camera or lens is going to be so far out of spec that it could misfocus by that much - it's very clearly a case of the camera simply choosing the focus elsewhere within range of the AF sensor.
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