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Possible lens calibration on 870IS?

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Offline ColT

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Possible lens calibration on 870IS?
« on: 14 / September / 2014, 08:48:21 »
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Hi,

I'm repairing a Canon Ixus 870IS. I have replaced the lens compartment, and i experienced that when zoomed in, picture is out of focus. The camera is reporting that focus is OK. In wide angle zoom, the image is sharp, focus is OK.
Stabilization works fine.

I don't know, is there any way to calibrate the focus on this camera, with CHDK or without it?  Or should i adjust the sensor plate screws?

Any help would be appreciated :) Thanks :)
« Last Edit: 14 / September / 2014, 08:52:00 by ColT »
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Offline ahull

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Re: Possible lens calibration on 870IS?
« Reply #1 on: 14 / September / 2014, 10:09:17 »
I suspect you would need to do some pretty deep inspection of the firmware to figure out how to adjust the calibration.

There is very little available on line to assist, but this might give you some pointers..

http://blog.amarkulo.com/downloads/2010-01-08-canon-powershot-and-ixus-service-manual.pdf

... its not for the camera you mention, but is one of the few manuals I have ever been able to find.

Canon appears to use bespoke software for calibration, so you would have to emulate this.

This would be a very difficult task, and very time consuming I would expect. You can adjust the position of the sensor with the screws, and this might be a simpler, if less exact approach.... unless anybody else has any ideas.

« Last Edit: 14 / September / 2014, 10:35:20 by ahull »
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Offline waterwingz

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Re: Possible lens calibration on 870IS?
« Reply #2 on: 14 / September / 2014, 10:31:44 »
Quote from: ColT on 14 / September / 2014, 08:48:21
I'm repairing a Canon Ixus 870IS. I have replaced the lens compartment, and i experienced that when zoomed in, picture is out of focus. The camera is reporting that focus is OK. In wide angle zoom, the image is sharp, focus is OK.
This raises an interesting question that might be related to the manual focus troubles we have been seeing with some specific cameras.

I'm starting to think that cameras do not focus using any information calibrated for distance.  Instead, they use a feedback loop to adjust the lens mechanism until they see a sharp image as measured by average rate of change of pixel to pixel intensity variation in the focus zone (I think).  They then use the position of the lens focus mechanism and the zoom setting to calculate (or lookup) the subject distance for EXIF info purposes.   

Cameras with MF capability reverses the above mentioned process when in MF mode, using the requested subject distance setting and current zoom position to calculate the needed Mecha setting.

Under the above assumptions, if something is misaligned in the lens mechanism, it may still be possible to focus but the reported subject distance will not be correct. Furthermore,  if CHDK tries to set the subject distance on one of these cameras,  the calculated mecha position needed will be wrong and the camera will not be focused where it was requested (in fact test have shown where it not focusing anywhere at all).

So for CoIT's issue,  it may not be a calibration problem at all.  Instead,  the focus mechanism is somehow misaligned and not able to achieve focus lock at long subject distances.  If so, then the mechanism must be fixed to solve this problem - any recalibration is not going to help.

All of the above is just my current working theory.  I've been using a test script comparing mecha position to reported SD to play with validating this.   


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Offline ColT

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Re: Possible lens calibration on 870IS?
« Reply #3 on: 14 / September / 2014, 10:39:19 »
Hi!

Thanks for the ideas :)

Yes, it is strange that the focus rectangle goes green, and the image is blurry. The contrast AF shouldn't dot this. May be the 870IS's focus algorithm is different, and somehow it can be misaligned :S I have tried to adjust the sensor, but it's not complete now ( id need a proper screwdriver). All that is achieved is that as i could adjust the screws, i could see that image is a little sharper or blurry.

I thought that i replace the lens black plate from the damaged lens, but i cannot properly disassemble (flex cable damaged :S )

The lens i newly put in this 870IS, is a brand new lens from China. So i wonder how it can be misaligned, but i can be sure that it is.

I check the service manual, maybe there is a software method to adjust the focus :)

Thanks!
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Offline srsa_4c

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Re: Possible lens calibration on 870IS?
« Reply #4 on: 14 / September / 2014, 10:42:12 »
Quote from: waterwingz on 14 / September / 2014, 10:31:44
Instead, they use a feedback loop to adjust the lens mechanism until they see a sharp image as measured by average rate of change of pixel to pixel intensity variation in the focus zone (I think).
A camera with damaged focusing ability can happily report AF success even if can't see anything sharp during the AF process (my own experience).
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Offline ahull

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Re: Possible lens calibration on 870IS?
« Reply #5 on: 14 / September / 2014, 10:43:28 »
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Offline ColT

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Re: Possible lens calibration on 870IS?
« Reply #6 on: 14 / September / 2014, 10:50:23 »
Yes, it will be good to have this for Ixus...
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Offline waterwingz

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Re: Possible lens calibration on 870IS?
« Reply #7 on: 14 / September / 2014, 10:54:15 »
Quote from: srsa_4c on 14 / September / 2014, 10:42:12
A camera with damaged focusing ability can happily report AF success even if can't see anything sharp during the AF process (my own experience).
Well, it must be coming to that conclusion based on something.   

Older range finder cameras let you manually align two images from different optics paths to determine subject distance.   And some Polaroid models had an ultrasonic range finder.  Modern cameras obviously have no such options so that just leaves a decision about what it thinks it sees in the sensor data.  Hence my theory about "sharpness".   And of course sharpness is only relative - it might find what it thinks is the sharpest position and that might be totally blurry to the human eye.  But it's sharper than any other position to the camera.

Again, more theories only.
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Offline ahull

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Re: Possible lens calibration on 870IS?
« Reply #8 on: 14 / September / 2014, 13:53:51 »
There are naturally rather a lot of strings in the firmware relating to "Focus"...

PRIMARY.BIN.SD600.strings.txt: 2a9da8 FOCUSADJDATA
PRIMARY.BIN.SD600.strings.txt: 2a9db8 FOCUSCAMADJDATA
PRIMARY.BIN.SD600.strings.txt: 2a9ef0 FOCUSLENSPARAMETER

... these for example... but what exactly they relate to, is open to debate.
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