Objective Dia.

PVS-7D SFK with P+ Tube

The objective is the end the light enters. The larger this diameter, the more light there is entering the system resulting in greater potential detail and contrast. Spotting scopes with large objective diameters that are also lightweight and practical for carrying into the field are becoming very popular. Full descriptive text along with how this will effect your view can be found in the Basics II article in the All About Optics section.