How to decide the smallest eyepiece a scope can use?

Based on the Mmax value as calculated above, you can decide the minimum eyepiece focal length that you may wish to use for a scope. The equation is:
f = F / Mmax

Where: f is the minimum eyepiece focal length (mm); F is the focal length of the scope; Mmax is the maximum usable magnification of the scope.

For the Nipon 26-78x78 scope, the minimum eyepiece which can be used is: f=780/156=5 mm. This would provide about 156x power which is nice for viewing Mars or resolving the rings of Saturn. To resolve Saturn's rings, you may only need about 30x power, but to see them more clearly, higher magnifications are required.