>>11411916Focal length, "power" is the difference between focal lengths for zoom lenses. If that was a 10mm-25mm zoom lens then it would have a power of 2.5x. A focal length of 25mm has more, "resolving power," than 10mm, allowing distance objects to appear closer.
If you mean 10mm or 25mm diameter of a telescope. That just changes their light gathering ability, "light grasp/aperture gain." A larger diameter telescope can usually gather more light allowing for dim objects to be resolved more easily.
You would not want a telescope with a 10mm or 25mm focal length, but ones that are 10mm or 25mm diameter won't be all that great either since they will be dim.
Lastly, 10mm and 25mm eyepieces used for telescopes is another thing. Those are normally focal lengths, not diameters. Thus the 25mm eyepiece will "zoom in" closer to objects at 2.5x more than the 10mm eyepiece.
