[Elphel-support] Camera Calibration

[Sinaya Dayan]

Hi Andrey,

Thank you for the detailed response. I will forward your suggestions to the calibration company to see what they think of the solutions you bring up and hopefully we will be able to finalize this step.  I will let you know what was decided as soon as I hear back from them.

I appreciate all your help.
Best Regards,
Sinaya
From: elphel at gmail.com [mailto:elphel at gmail.com] On Behalf Of Andrey Filippov
Sent: Tuesday, July 17, 2012 1:10 PM
To: Sinaya Dayan; support-list at support.elphel.com
Subject: Re: [Elphel-support] Camera Calibration

Hello Sinaya,

Let me summarize what I see about the problems you encountered trying to calibrate Elphel camera with the 2/3" low-distortion sensor and what you can do about it.
1) The reason of your failure to calibrate is that the lens+sensor combination has much lower distortions than your method of calibration is capable of. Do you realize that if you had an ideal lens (pin-hole camera model) than you will not be able even to find where the lens axis crosses the sensor surface? The results you showed indicated that on some of them the program placed the lens center completely outside of the sensor area. Can you imagine the camera where the lens (or sensor) is so loose that it moves by several millimeters sideways :-)?

 There are at least 3 factors why these lens+sensor behave so nicely in the camera and have so little distortion:
a - high quality, low-distortion lens you selected;
b - long focal distance for the small sensor (it is approximately equivalent of f=200mm for 35mm format)
c - using just the center area of the lens format (only about 1/2 of the radius)

2) What can you do about it:
a) (What I would recommend) Use the focal length found with your test target, and edit the rest of parameters in the .CAL file manually: put the lens center at the sensor center, zero all the distortions. Such .CAL file will be accurate to the pixel size and you can use it with your software.
b) Use high quality target printed on a single piece of good material (it may cost you some $500-$1000 - the price we pay for our targets to the print shop)
c) (not a solution, but you can just use it to verify my words above). Put a small format (1/2.5" or at least 1/2") short focal length (i.e. 4-8mm) lens in your camera, make the test photos with your self-printed multi-sheet target and send them to the calibration company you uses before. I'm pretty sure all the "hardware problems" will go away and they will be able to provide you with the .CAL file for large-distortion lens.

Andrey
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