How to compute focal length when images lack EXIF data?

[zharvey-welltok]

In a previous question I was using openMVG_main_SfMInit_ImageListing without the -f argument. I was also using images that did not have focal length info in their EXIF data, and so the resulting sfm_data.json had (essentially) UINTMAX set for each view's id_intrinsic value.

In that question I was advised by @pmoulon to re-run openMVG_main_SfMInit_ImageListing with a focal length of -f 1.2*1920 = -f 2304.

I'm wondering what the two magic numbers (1.2 and 1920) came from, and how to derive them on other image sets in the future. Any ideas? Thanks in advance!

[polto]

@zharvey-welltok , 1920 was supposed to be the width of your sensor in pixels and 1.2µ sensor's pixel size.
You should be able to find both parameters from sensor documentation.
Regards.

[zharvey-welltok]

Thanks @polto but how did @pmoulon know what my sensor was?!? My point being that he somehow found out what my sensor was (my smart phone) and was able to look up the width & pixel size. I'm wondering how he did that since I will be processing images from cameras and sources whose make/model info I will not have access to.

So there has to be something in the EXIF data, or some other way to detect sensor make/model info.

[pmoulon]

Here the explanation about the values:

• 1920 comes from max (width,height) for your dataset,
• 1.2 is a scaling_ratio coming from the FoV of your camera
  • For example if we simulate a 45 degrees of viewing angle
    • the focal_ratio will be => 0.5/tan(0.5 * FoV) => 0.5/tan(0.5 * 45/180*3.14) => 1.2

Then according those you can approximate the focal length as focal_ratio * max(w,h) => 1.2 * 1920

This formula provides a good guess for SfM if your camera have a "classical fov" near than 45.

[zharvey-welltok]

Thanks @pmoulon, this is all starting to make sense! When you say:

"1920 comes from max (width,height) for your dataset"

...do you mean that my sfm_data.json contained width/height information, and that's where you obtained the 1920 from?

[pmoulon]

You can either look to the max size from the JSON or just open the image properties and look to the image size.

[zharvey-welltok]

As always, thanks @pmoulon 👍

[pmoulon]

You're welcome, I'm happy to help and explain things.
Perhaps those information must be somehow expressed into the online sphinx documentation ...

[caomw]

good