[Elphel-support] Camera Calibration

[Andrey Filippov]

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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