focal length px changes during pre-calibration

[hanscastrop1@gmail.com]

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]

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.

[hanscastrop1@gmail.com]

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.