Agisoft Metashape > Camera Calibration

Calibration and image variant parameters

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dsk81:
Hi,

I have been looking at camera calibration for sub-optimal images lately and have a couple of questions.

Some of my projects in Metashape are a bit quick and dirty building site surveys using only my smartphone, and I don't always remember or have the time to use manual mode and set it up properly. So the photos may be taken using autofocus – this means the cameras internal geometry changes. For my camera, most notably if I look at my screen while focusing from near to infinity, the field of view (focal length) changes significantly. Also, distortion changes visibly from obvious pin-cushion at focus = infinity, down to clearly opposite barrel distortion at focus = 0 (nearest possible).

After realizing this, I have started to experiment using image-variant parameters in camera calibration settings for my images for projects where the default procedure with adaptive corrections don't return correct alignments, typically these problems occur when photos have differing focus and so Metashape can't find a correction that works across all photos.

It seems logical to mark at least f  as an image-variant parameter, since this visibly changes with changing focus distance. But what about distortion, would it be good to mark some of the other parameters as image-variant? I suspect k1, k2, k3, k4 maybe, since these deal with radial distortion, which is what we refer to when talking about barrel and pin-cushion distortion in layman terms. I am not sure about cx, cy, p1, p2, b1, b2, though.

Questions
1: How can I determine what parameters are likely to change with changing focus?
2: Should corrections be applied in a specific order to reduce possibility of diverging parameters? As in, first try matching with common parameters to find the best statistical mean fit, then allow some image variation in subsequent optimizations?
3: And are any of the parameters not normally needed for you average smartphone or point and shoot / consumer DSLR lenses?

I understand there is probably not one definite answer to this, but I have a hard time understanding the different calibration parameters and what they do, and how Metashape applies them if set to auto or adaptive correction.

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