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Author Topic: focal length px changes during pre-calibration  (Read 8093 times)

hanscastrop1@gmail.com

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focal length px changes during pre-calibration
« on: September 06, 2024, 04:20:21 PM »
Hi
F (focal length inpx) parameter is calculated using focal length mm devided by pixel size mm. In the initial tab it shows the right F(px) as 7634.75008
after using 9  checkerboard images for pre-calibration it is changed to 8971.2534.
My question is: why?
and: can I fix the initial figure prior to the actual calibration, or is that not the way to go?
Thanks

Alexey Pasumansky

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Re: focal length px changes during pre-calibration
« Reply #1 on: September 24, 2024, 06:32:54 PM »
Hello hanscastrop,

If you want to keep some of the interior orientation parameters unchanged, you need to fix it via Tools Menu -> Camera Calibration dialog before running checkerboard-based calibration procedure. In such case values for all the fixed parameters will be preserved according to the values in the Initial tab of Camera Calibration dialog.
Best regards,
Alexey Pasumansky,
Agisoft LLC

hanscastrop1@gmail.com

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Re: focal length px changes during pre-calibration
« Reply #2 on: October 04, 2024, 08:11:41 PM »
Thanks Alexey,
the point is: I don't know if I want to keep some parameters unchanged. I would think the self calculated figures for f  would be oke and maybe fixated. But doing that the object looks way distorted and the figure calculated by metashape is way diifferent en delivers much better result. I don't know what causes this extreme difference.
Hans.

Alexey Pasumansky

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Re: focal length px changes during pre-calibration
« Reply #3 on: October 07, 2024, 04:28:13 PM »
Hello Hans,

If possible, please share sample dataset, calibration images and the project where you observe considerable difference between pre-calibrated values and those that are estimated by Metashape.

Please send the download link to support@agisoft.com.
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Alexey Pasumansky,
Agisoft LLC

hanscastrop1@gmail.com

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Re: focal length px changes during pre-calibration
« Reply #4 on: October 10, 2024, 12:08:52 PM »
Hi Alexey,
I will send files if you still think it is nescessary a this message;

The considerable distance is not between  pre-calibrated values and estimated, but between initial tab and estimated/pre-calibrated.
I add illustrations where you can see the initial (calculated) values are f 7634  and the estimated values are f 8969. My question was why they are so different?

I was wondering if I should fix the initial (f 7634) value, but I see clearly now that value should not be used.(noticing this value was confusing for me.) Also distortion plot shows a lot of red (chessboard calibration); is that a problem?
Remains the question: why so big a difference.

Thanks.

Hans