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poor focusing in a plane

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Offline rkomar

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poor focusing in a plane
« on: 12 / July / 2014, 14:53:53 »
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This isn't a problem with CHDK, I'm just asking this here because many of the members are so knowledgeable about cameras.

I have a Canon A4000 camera aligned perpendicular to a piece of glass (set up using a mirror against the glass to make sure the camera is pointing straight at it).  The distance between the camera and glass is about 25cm.  I press the page of a book against the back of the glass and take a picture.  The problem is that the whole text is never entirely in focus.  Getting the focus right in one part results in poorer focus in another part (in diagonally opposite corners).  It is as if the alignment is bad between the lens and the sensor.  I used CHDK to take pictures at various focus distances, and could see the focus shift from one corner of the image to another as the distance went from 190mm to 250mm.  I'm surprised at just how large the focus setting difference is between the extremes.  I've uploaded the images to http://www3.telus.net/rkomar/focus/ for anyone who wants to take a look (the images are taken with focus settings of 190, 200, 210, 220, 230, 240, and 250mm).

My question is, is this particularly poor for even a point and shoot camera, or can I expect the same from other similar cameras?  Do I need to go to an expensive model for better results?
« Last Edit: 12 / July / 2014, 14:56:11 by rkomar »
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Offline reyalp

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Re: poor focusing in a plane
« Reply #1 on: 12 / July / 2014, 17:45:28 »
Quote from: rkomar on 12 / July / 2014, 14:53:53
My question is, is this particularly poor for even a point and shoot camera, or can I expect the same from other similar cameras?  Do I need to go to an expensive model for better results?
I think the only way to get a useful answer to that would be to test a large number of cameras. If one of the people working on large multi-camera rigs could compare compare, that could be interesting data.

If your rig allows, you might try varying the distance and using different zoom settings.
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Offline rkomar

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Re: poor focusing in a plane
« Reply #2 on: 12 / July / 2014, 18:29:44 »
I used this and another A4000 to photograph album covers from about 90cm distance, and noticed the same problem then.  So, I think it persists at other focus and zoom settings.  When shooting the album covers, I just figured I had something wrong in the setup, but for the book pages, I've been very careful to ensure the camera is aligned perpendicularly.  I'm just surprised that the focus problem is as bad as it is.

The problem stands out so much, that I figured others would recognize it if it also happens with some of their cameras.  I'm not expecting any solutions to the problem, but rather comments on whether it's a common problem.  I'm debating whether I should just buy different cameras or not.  The current images can be binarized into something useable, but it would be better to have good focus over the whole page, of course.
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Offline srsa_4c

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Re: poor focusing in a plane
« Reply #3 on: 12 / July / 2014, 18:54:20 »
Just an idea: do different IS settings have any influence on this phenomenon?
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Offline reyalp

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Re: poor focusing in a plane
« Reply #4 on: 12 / July / 2014, 19:21:13 »
Quote from: rkomar on 12 / July / 2014, 18:29:44
I used this and another A4000 to photograph album covers from about 90cm distance, and noticed the same problem then.
Both your a4000 show the same diagonal difference in focus?
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Offline rkomar

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Re: poor focusing in a plane
« Reply #5 on: 12 / July / 2014, 19:35:22 »
Quote from: reyalp on 12 / July / 2014, 19:21:13
Quote from: rkomar on 12 / July / 2014, 18:29:44
I used this and another A4000 to photograph album covers from about 90cm distance, and noticed the same problem then.
Both your a4000 show the same diagonal difference in focus?

I've only been using one of them so far when shooting the book pages.  I remember focus problems with both when shooting albums, but that was about a year ago, so my memory may be off.  I'll test the second camera in the book rig, and report here later.
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Offline rkomar

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Re: poor focusing in a plane
« Reply #6 on: 12 / July / 2014, 19:40:10 »
Quote from: srsa_4c on 12 / July / 2014, 18:54:20
Just an idea: do different IS settings have any influence on this phenomenon?

My script sets the exposure time, which I based on the ISO 100 setting I use.  So, instead of using the script, I used chdkptp to set the zoom and ISO setting, and then just did a "shoot half" and "shoot" for each ISO mode I tried.  As such, the focus is automatically set by the camera each time.  It's harder to tell that way, but I'd say that the focus problem is still there when I set the ISO mode to one of 1, 2, 3 or 4.

I wouldn't have thought that the ISO setting would affect the focus.  What gave you this idea?
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Offline waterwingz

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Re: poor focusing in a plane
« Reply #7 on: 12 / July / 2014, 19:42:10 »
Quote from: rkomar on 12 / July / 2014, 19:40:10
I wouldn't have thought that the ISO setting would affect the focus.  What gave you this idea?
srsa_4c is talking about image stabilization (IS) settings. 

When you put a camera on a tripod, many sites recommend you turn IS off as the camera gets confused if the image is really stable and starts trying to move the image stabilizers anyway.

Example : http://digital-photography-school.com/image-stabilization-on-tripods/
« Last Edit: 12 / July / 2014, 19:44:07 by waterwingz »
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Offline philmoz

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Re: poor focusing in a plane
« Reply #8 on: 12 / July / 2014, 19:44:27 »
Quote from: rkomar on 12 / July / 2014, 19:40:10
Quote from: srsa_4c on 12 / July / 2014, 18:54:20
Just an idea: do different IS settings have any influence on this phenomenon?

My script sets the exposure time, which I based on the ISO 100 setting I use.  So, instead of using the script, I used chdkptp to set the zoom and ISO setting, and then just did a "shoot half" and "shoot" for each ISO mode I tried.  As such, the focus is automatically set by the camera each time.  It's harder to tell that way, but I'd say that the focus problem is still there when I set the ISO mode to one of 1, 2, 3 or 4.

I wouldn't have thought that the ISO setting would affect the focus.  What gave you this idea?

I think srsa_4c meant IS (Image Stabilisation), not ISO.

Also are you shooting at the wide or tele end of the zoom? At the wide end the camera may be doing more distortion correction on the image data to account for lens issues. This would be most noticeable in the corners.

Phil.
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Offline rkomar

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Re: poor focusing in a plane
« Reply #9 on: 12 / July / 2014, 20:10:33 »
Sorry about the ISO mixup.  IS makes more sense.  I have IS turned off, since the camera is mounted in a rig.  With the camera about 25 cm from the glass, I use a zoom setting of 37 to get the paperback filling most of the height of the image.  As long as I keep the distance below 30cm, I find I can zoom to the paperback size and still focus.  If I pull back more than that, I'm not able to focus at all when zoomed to the paperback size.

The JPGs I provided are also got from the RAW DNG images I saved.  I do the barrel distortion correction myself, and produced the JPGs from that.  (I normally produce PNG files, but to keep the size down on the web, I converted to JPG).  When I look at the DNG images, I see the focus problem there, as well.
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