The magnification of a viewfinder is a confusing quantity because DSLRs are still rated by an obsolete standard for 35-mm film cameras. For example, the nominal magnification of the Canon XTi viewfinder is x 0.7. This means that the camera and viewfinder would work like a x 0.7 telescope if you put a 50-mm lens on it - which is not the standard lens for a DSLR.
That, in turn, is equivalent to viewing the focusing screen through a x 3.5 magnifier (because a 50-mm lens alone would be a x 5 magnifier).
Add the Angle Finder C, giving x 2.5, and you have a total magnification of 2.5 x 3.5 = 8.75. That's more power than you'd normally use viewing a 35-mm slide through a loupe. It should be - and, in my experience, it is - enough to judge the focus accurately.
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