Defective AutoFocus compensation on Ixus 115 HS - Feature Requests - CHDK Forum
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Defective AutoFocus compensation on Ixus 115 HS

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Offline iuianbr

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Defective AutoFocus compensation on Ixus 115 HS
« on: 15 / August / 2013, 07:21:53 »
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Hello!
I've get a camera (Ixus 115 HS), with bad focus. All pictures/movies seems to be slightly out of focus, despite the fact that camera seems to focus without apparent problems. It is out of warranty, and repairing (just tuning, more precisely!) might be as expensive as the camera itself. I've heard to be a common defect for this camera (also add the 220 and 310), with many people complaining about it! Can we get a script or something to set a correction factor, like -/+ something, or multiply with a value almost unitary (like 1,003 or 0,997) to compensate for the misaligned AutoFocus? I'm a newbie, I don't know exactly how is done the Autofocus in camera, maybe he has a correction table itself, from manufacturer, or is just a mathematical function, with adjusted factors, etc. Is there a chance for us?
8o)
PS. Can the AF be adjusted directly, by turning some knob inside the lens/camera? Is there a tutorial?
8o)
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Offline mastodon

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Re: Defective AutoFocus compensation on Ixus 115 HS
« Reply #1 on: 15 / August / 2013, 13:13:28 »
I have an SX200 with the same problem.
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(A 410), SX 200, (SX 280), SX 700
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Offline outslider

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Re: Defective AutoFocus compensation on Ixus 115 HS
« Reply #2 on: 15 / August / 2013, 13:56:59 »
The problem is not so trivial and first you have to know, what is going on.

So, the questions are:

1) Does the camera always do wrong focus?

If yes:
2) Is the error always the same? I mean camera focuses always behind the object or always before?

If yes:
3) Can you find (manually) any way to determine what is the error? So whether it is always for example 10cm behind focus? Or camera focus is always 0.9 of correct one?

If yes:
If you determine the exact relation between correct focus and this from the camera you can write a script, which would do following:

- half_press shutter
- get the focus which was set by camera
- calculate good focus
- set the focus, probably with aflock and just shoot.

If answer for some of above question is no then you are out of luck. CHDK have no focusing features on its own. In theory you could change focus stepwise and try to do a lot of math on the viewfinder data to catch the point, where the focus is the best, but it would be not so simple...
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if (2*b || !2*b) {
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Compile error: poor Yorick
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Offline mastodon

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Re: Defective AutoFocus compensation on Ixus 115 HS
« Reply #3 on: 16 / August / 2013, 08:13:29 »
I use CHDK to see/check the calculated focus values. It is not always bad, just sometimes. That times it focuses always before the subject. So just I script over the top of the bad algorithm, is not enough.

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(A 410), SX 200, (SX 280), SX 700
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Offline outslider

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Re: Defective AutoFocus compensation on Ixus 115 HS
« Reply #4 on: 16 / August / 2013, 10:01:34 »
So I gues you'd need to write your own focusing algorithm based on the data from live view... And that's not so easy. And I believe that if you'd really want to do this it should be coded in CHDK source code rather than in script.

Maybe some code frome edge detection could be helpfull. I guess, that the best focused image should have the highest amount of detected edges. But that's still thing that is hard to implement better, than well-payed guys from The Cannon done...

Moreover, I have noticed, that untill you half-press shutter button camera tends to use higher aperture than the set one (seen on sx130), which obviously changes DOF. So the algorithm would need to press the half_shoot button at start.
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if (2*b || !2*b) {
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Compile error: poor Yorick
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