How does real life camera sensor size affect tracking on Blender?

Question

I don't quite understand deeply what the relation of the real life camera sensor in relation the tracking with the virtual camera size. I've always worked with direct aspect ratio (i.e: 1920 x 1080). In the case of Arri Alexa sensor size to full 35 mm. And that's about it. That's all I used to PFtrack to do his bidding. Later I'd refine track. Put some markers, export to .FBX and voilá: I got my Matchmove.

But I ask this question because when I do the same with my phone tracking is offset. On a Droid Turbo, the resolution of 10MP (4320x2432) can record at 1920x1080 24 fps. The focal lenght is 4.5 mm. Viewing angle 66.5ºx40.3º. Sensor size 5.9x3.3 mm. Crop factor: 5.9x

With those numbers, I don't know how to setup PF track or any other tracker since: If I set the sensor size: I get an automatic size frame which I cannot modify (it gets ghosted) If I set the Image size: I get an automatic crop factor which I cannot modify (to set the sensor size).

I guess this should be pretty straight forward in PF track or any other Tracker (Boujou, Blender, Shiloette, etc..) but I don't know where to put what information provided above.

Could someone help me with this please?

Answer 1

If you use Automatic Refinement for the focal length during the solve process in Blender then it shouldn't really matter, since Blender will calculate the (more or less) correct focal length for the given sensor size. For the final image it doesn't really matter anyway, as long as aspect ratio and the camera solution itself are fine.

But if you want to be sure you have the correct solution by comparing it with real world data, like your actual camera sensor size and focal length, you can fill in those values in the Camera and Lens panels of the clip editor and see if you still get a proper solution. If the automatic refinement calculated something that is way off from the real world data, then probably something is wrong.
Of course, as you described, you have to account for the crop factor. An iPhone will hardly have a true 35mm or so focal length, but rather something like 4.2mm. But if you take into account that it also is not a full frame camera, then it's really just about the correlation between focal length and sensor size that matters in the end.

See this video, where I try to describe it: https://www.youtube.com/watch?v=yjLTg37hFUM&index=9&t=0s&list=PLtuvwW4VAp5tu2RdbRHThM6FVFfvFur1g