I have three hot-shoe viewfinders because my Sigma compact cameras lack an optical viewfinder. But sometimes bright light prevents the use of the live-view LCD - hence the hot-shoe items. I have:

A Sigma VF-21 equiv. 50mm
An Olympus VF-1 equiv. 28mm (I think)
An unknown brand with two sizes of frames - the larger approx equiv 50mm.

With the Sigma and the Olympus, comparing the finder view with the unaided eye, the finder is about 50% 'magnification'.

With the unknown brand, the same comparison, gives the finder as just under 100%.

Can anyone cast some light on the big difference between the unknown brand and the other two?

BTW, the lenses are much larger in the unknown brand.