This isn't scientific, especially as I had an IR Cut filter on, but I wanted to compare the relative image sizes of Betelgeuse in its current dimming phase to Rigel and Bellatrix, as the brighter the object, the larger the area it occupies on the camera image.  Betelgeuse usually varies magnitude from 0 to 1.3, Rigel 0.17 to 0.22 and Bellatrix is 1.65. In the image Betelgeuse is in the centre and compared with  a negative image of Rigel (the dark red outer area) in size below and Bellatrix above (with Betelgeuse in negative blue).  This shows that Betelgeuse is currently much fainter than Rigel and only slightly brighter than Bellatrix.

No doubt there is a way to use the raw image data to measure actual brightness and I'd be happy to be shown how.

Peter