balance between clarity and brightness

What your calling clarity is properly called resolution.  It's the ability of the glass to clearly show a reference point object at a known range at a known magnification power.

In other words, with the scope at 6X, 100 yard range, will it show a 1/4" black square on a white background with sharp corners & straight edges, or not?  Or will it only resolve a 3/8" square?  Bringing a target object closer via increased magnification doesn't do much good if the target itself becomes blurry.  Resolution is pretty much the major optical difference between a cheap 6-24X target scope and an expensive one.

Light transmission, or brightness, is a separate function.  The major influences there are tube diameter and coatings, given the same quality lenses.

Foo