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12-17-2009, 06:33 AM   #1
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Have I a back focus problem?

I have the k10 add on software that allows you to adjust the focus point.and it's all working fine and I can turn it on and off ok via the laptop any time but can some can some one please explain the charts use.

With my k10 on a tripod set at 45 degrees to a table and the printed on A4 paper the chart absolutely flat, the aperture at its widest and the distance to the paper just off the minimum focus distance, there is a big difference how much of the scale is visible depending on the focal length of the lens due to the minimum focal distance of that lens.

So, with my in camera adjustment set on 0 with a 50mm lens the difference between the focus points seem to start on the chart at the 30mm mark the one furthest away [back focus I presume] is blurred but the front 30mm [ closest] one is not. Is this enough of a problem to warrant adjustment?

The charts instructions I have seen on line all seem to be more worried about up to the 6mm mark being out compared to the other same distance.

Also as I mentioned above, as you test a longer focal length lens the minimum focal distance increases and you see more of the chart, so, which distance with a difference are acceptable and which ones are not?

There seems to have been some very vitriolic comments made on the different charts / deigns/and angles that can be found on a focus chart search on line, but the distance to look at is all I am concerned with.
Alistair

12-17-2009, 08:57 AM   #2
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Is it possible for you to post a photo taken with this setup where you mark the focus point in the picture? And pls add exif data...

- Bert
12-17-2009, 09:58 AM   #3
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If your not seeing problems in real world photos then i wouldn't go into it at all.

Its at best a headache.

At worst you'll actually adjust your camera for test conditions you don't have occurring in your photographs and make the situation worse
12-17-2009, 10:26 AM   #4
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QuoteOriginally posted by adwb Quote
There seems to have been some very vitriolic comments made on the different charts / deigns/and angles that can be found on a focus chart search on line, but the distance to look at is all I am concerned with.
Alistair
All that matters is that you be sufficiently close to guarantee the focus target provided on the chart is the only thing within range of the focus sensor. If you're too far away, the camera might choose to focus on some other part of the chart instead.

I think my experiences with focus chart are probably typical - about month of pulling my hair out trying to figure out what's wrong with my camera, or the camera I bought to replace it, or the camera I got in exchange for that one because it appeared to have problems too, or the three or four cameras I tried in the store before exchanging *that* one, or if maybe the problem had to do with my lenses. Culminating in a gradual realization that the problem was with my testing method all along, and that the camera is *far* more precise and accurate about such things than I was being. Well, at least I learned a lot along the way. And I suppose everyone has to go through something like that at some point.

12-17-2009, 11:06 AM   #5
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the big defficiency with focusing charts is that you are not focusing on a flat surface and you really don't know what the sensor is focusing on.

the best focusing test is to focus on something that is about 10-15% of the frame height, that is absolutely square to your camera, and then have the focus scale lean away from the flat surface. This way you will get an accurate indicator. if the camera is fornt opr back focusing you will see the error but as Marc suggests, the test has so many flaws and the camera is much more accurate than the charts.
12-17-2009, 04:11 PM   #6
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Well, I wouldn't say the test charts themselves have flaws - I don't see the fact that it is not a flat surface as being problematic. they are generally pretty well designed so that *if* the test is performed correctly, thre is an unambiguous focus target. But it's really easy to perform the test incorrectly - not choosing the focus point, not having the test chart square with the camera, being too far away for the focal length to have the target fill the AF sensor, shooting at too shallow and angle and creating the same problem, using tungtsen light, doing the test handheld and moving between the time of focus lock and shutter release, etc. Unfortunately, most of these same problems will plague attempts to perform ad hoc tests without a chart. But the cool thing is, these are the exact things that people do wrong in the real world shooting as well. So learnign to perform a focus test well can make you a better focuser all the time.
12-17-2009, 05:05 PM   #7
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I often had "problems" at the very beginning, using the K10D and one particular lens, a Sigma 100-300 F4.
But then, after too many chart tests of all sorts, I found that the culprit was me, not the equipment. Granted, I made some very small adjustments, but that didn't make much difference.
Now, if I think I have a BF/FF problem, I use whatever is around me, in good light, preferably outside on a sunny day and aim/shoot at different objects, close, mid-range and far away. What I want is what will show on the pics on the computer monitor. As well, inside shots of small objects can be just as good.

Results matter most and I would suggest that you send us some sample shots of small objects in tight groups but placed at different distances ... an example would be the way bowling pins are arranged: they are all close together but at different distances form the bowler and at different angles. You aim and shoot on the pin you choose for focusing and you should have a nice sample of what your AF is doing.
Of course, bowling pins are hard to place on a table-top! But, you know what I am saying: small objects grouped but at different angle of view and at different distances from the camera.
That worked for me.

JP
12-17-2009, 07:43 PM   #8
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QuoteOriginally posted by jpzk Quote
Of course, bowling pins are hard to place on a table-top! But, you know what I am saying: small objects grouped but at different angle of view and at different distances from the camera.
That worked for me.

JP
I have used the "small objects" test quite a bit the last few weeks with my K10D and my 40mm ltd. As far as I can tell, when wide open, at a range of four feet it focuses with perfect reliability at a point two inches behind my intended subject. The error is less noticeable with the lens stopped down(of course), but, I think it still effects image quality-my film (K-1000 and SP-500) cameras still produce images that seem to pretty handily outstrip my K10 as far as sharpness goes.
I got my K10 last June, so it should still be under warranty, but I see so many horror stories of user-hostile customer service from Pentax that I'm afraid to call them and have it sent in. At this point I'm weighing whether or not to just do the software hack, albeit it I have version 1.00 of the firmware and I don't think any of the debug programs run on 1.0
I guess dealing with issues has become part of the experience of having a DSLR, regardless of manufacturer, but its kind of depressing all the same.

12-17-2009, 09:53 PM   #9
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I think you have a measurebation problem.
12-18-2009, 05:24 AM   #10
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ok for what its worth I have all the batteries 20 mm apart. the focus was on the centre one and then the lens not touched but the selection lever then put on manual and the rest added
one at f22 one at f1.9 at minimum focus distance
the last one at f1.9 at distance of 50mm apart the centre one 900 mm from the lens
the don't look the same to me front to back especially the long distance one ?
Alistair

Last edited by adwb; 03-31-2012 at 07:01 AM.
12-18-2009, 07:45 AM   #11
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Looks like your focus is spot on with the first two pictures, right on the centre battery. The zoomed out picture looks like the battery to the right of the target has the sharpest focus-did you move the camera or change a setting on the lens?
12-18-2009, 08:28 AM   #12
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QuoteOriginally posted by Blackadder Quote
I have used the "small objects" test quite a bit the last few weeks with my K10D and my 40mm ltd. As far as I can tell, when wide open, at a range of four feet it focuses with perfect reliability at a point two inches behind my intended subject. The error is less noticeable with the lens stopped down(of course), but, I think it still effects image quality-my film (K-1000 and SP-500) cameras still produce images that seem to pretty handily outstrip my K10 as far as sharpness goes.
In that case, given that you have tried just about everything, you might have to send it to Pentax.
As far as service is concerned, I had no problem in the past when I sent my K20D for a small tweak. They corresponded timely with me and I had the camera back within 3 weeks. That was Canadian ... Pentax Canada in Toronto.
I hope you can get the same service from where you are.
All the best!

JP
12-18-2009, 08:39 AM   #13
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Alistair, I concur with Blackadder (quote below):

QuoteOriginally posted by Blackadder Quote
Looks like your focus is spot on with the first two pictures, right on the centre battery. The zoomed out picture looks like the battery to the right of the target has the sharpest focus-did you move the camera or change a setting on the lens?
Given that the first two pictures show the centre battery focused properly at both f22 and f1.9, at the minimal focusing distance for that lens; and that the last one, at 90cm from the lens, you have proper focus albeit the last one being slightly off to the right.
At such a distance, considering it is a maximum aperture as you mention (wide open), I would assume that there might have been some shifting to the right. The objects are small and the distance used might give you an impression of incorrect focus, but I still tend to agree with Blackadder that there is an actual shift to the right of the equipment on that last pic.

JP
12-19-2009, 04:47 AM   #14
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Thanks for the input every one, I think its ok as well, but with the paper chart as per the chart instructions there is a greater area in front in focus than behind but enough is enough, I'm, leaving the focus set to zeo for the time being.
I was interested in the short range distances from a macro point of view to get the best spead of d.o.f in either side of the centre focus point.
Alistair
12-19-2009, 02:47 PM   #15
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You're better not relying on AF in macro photography anyhow - if you need precise placement of the focus field, you pretty much want to do that manually.
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