You already know the focal lengths is 20mm, but you might want to compare/check calibration results.

In camera matrix the focal lengths `fx,fy`

are expressed in pixel units. To convert focals in World units `Fx, Fy`

you need sensor size in same units using similar triangle

`Fx = fx * W /w`

or `Fy = fy * H /h`

where:

`W`

: is the sensor width expressed in world units, let's say mm `w`

: is the image width expressed in pixel `fx`

: is the focal length expressed in pixel units (as is in the camera matrix )

Your Sony α6000 is 6000x4000pix and sensor size is 23.5 x 15.6mm hence:

```
Fx = fx * W /w = 5192pix * 23.5mm / 6000pix = 20.33mm
Fy = fy * H /h = 5192pix * 15.6mm / 4000pix = 20.25mm
```

At the end your calibration looks good