Hello from France!
Coming late to the discussion, it might be useful to mention various relationship existing between the commercial magnification of a loupe, its focal length, and corresponding dioptres.
In principle, the focal length in mm is given by : 250/magnification and conversely, the magnification is given by 250/(focal lenght in mm).
Hence a 50 mm lens, be it designed as a standard lens for the 35mm format of be it an ultra-wide angle covering the 5x4 format, behaves like a 250/50 = 5x loupe.
50 mm makes 1000/50 = 20 dioptres! We are very, very far from reading loupes sold as eyeglasses!
The problem is that we would like at the same time to see fine details, hence the need for, say, a 8x loupe (or a
jeweller's / watchmaker's loupe); and see the whole image, hence the need for a reading loupe with very long focal lenghts: a 4 dioptre reading loupe has a focal length of 1000/4 = 250 mm, therefore it is a 1X loupe ; no need to say that no loupe vendor could argue about the usefulness of a 1x loupe ;-)
I'm a bit of a collector of loupes, eventually my preferred loupe for looking at the ground glass of LF camera is the (now discontinued) 4x Rodenstock, the direct competitor of the 4x Schneider, covering a circle of about 50 mm in diameter.
But I like very much the Russian 4x loupe "Horizon panoramic" covering the 6x6 format ; the one I have was bought in Russia, so it has a sentimental value for me
There are actually two models of the Russian "Horizon" 4x loupe:
The regular model covers 50x50 mm
The "panoramic" model has a different, more complex, optical design and covers 60x60 mm.
Difficult to say more, but based on my experience, you should not spend too much time when focusing and scrutinizing, eventually your eyes get strained and you are at risk of missing proper focusing ... So it is important to find a loupe or a magnifying instrument allowing to work quiclky with the best comfort for the eye.
One important point for those like me suffering from astigmatism and therefore who have to keep their ophtalmic glasses when looking through a loupe: all loupes are not created equal regarding the actual image field that you can see.
I have found that both the Russian Horizon 4x Panoramic and the Rodenstock 4X to be very comfortable when using spectacles (is this still the British term in use in 2019 ?), or ophtalmic glasses.