I just got home the the National Matches last night so I could not come back here and update until just now. I made a mistake when I said the focal length of the 2.5-25X42 was 100mm, it is actually closer to 120mm. That's what happens when you try to calculate from drawings rather than measure directly. Anyway, at 120mm, the F-number is closer to f/2.8. The reason I like to know the focal length and the F-number is that I can calculate the depth of field from the objective lens group and from there figure out some specific focal distances and understand the DOF of the scope at various distances.
@Beaver10, I'm a little surprised you had difficulties focusing at various magnifications. I'm not even going to try to diagnose this as you no longer have the scope and diagnosis on a forum is a waste of time at best and perilous at worst. That March scope is highly prized by hunters for its incredible zoom range and small build. I'm glad the bigger NXS 3.5-15X50 is working put for you.
@Ringman, as you may remember from your photography days, the smaller the F-number, the larger the lens, the more critical the focus becomes, and the shallower the DOF. The glass quality being equal, the larger lenses will provide better resolution and more light than the smaller lenses at the same focal length.
