[Elphel-support] Camera Calibration

[Andrey Filippov]

Hi Sinaya,

There is nothing to move inside the camera (if the lens is attached
firmly). I'm not sure - what kind of processing do they make - is it open
source or proprietary program? Do the results depend on the quality of the
target, sharpness of the images and flatness of the target (assuming there
is no wind so the target shape does not change between the shots)?
Can the people who perform calibration provide you with some numbers - what
are acceptable deviations of the target spots from their ideal position?
What is the acceptable non-flatness?
My guess is that the lens is high-quality, low distortion one (and it is
2/3", not 1/2.5" so the distortions inside the smaller sensor area are even
smaller than specified for the lens). Measuring distortion parameters of
the low distortion lens is much harder than measuring parameters of the
lens with the higher distortions. If you had an ideal no-distortion lens
you will not be able to find even the position of the lens center!
So I believe that the distortions of the lens are lower than the quality
(geometry, flatness) of the target you use (the focusing quality also
varies a lot) and that leads to erratic results. We have our target printed
on much more solid material and we still have to calibrate the target
itself (it is 3.022m x2.667m), because with the f=4.5mm lens we can detect
~0.1mm shifts of the pattern, and the printer and material stretching can
not provide such precision.
So I would recommend you to print the target on a really wide paper (at
least) and make sure it is flat - do not use ink jet printer that made your
target non-flat with large waves. Than you may be able to measure
distortion, but I'm not 100% sure that this method will work without target
calibration.

Andrey

On Mon, Jul 9, 2012 at 5:51 AM, Sinaya Dayan <sdayan at njrati.org> wrote:

> Hi Andrey,****
>
> Here is one of the image sets we took along with the target measurements
> and its calibration report.****
>
> The analyst approved that the images are good for calibration. We have set
> the focus to infinity (@48.5 mm) the entire time and the target size was 3
> x 2 meters. We have not changed anything in between images part from the
> camera’s position. ****
>
> “Hardware problems” according to the analyst could be a small internal
> movement that could jeopardize the image and result in an in consistent
> error. ****
>
> Please have a look and let me know what you think.****
>
> Thanks,****
>
> Sinaya****
>
>
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