I'm working from memory here so I might not be 100% accurate here but it gets you started. Your average scope comes with a zoom lense, let's say a 15-45 power. There is a lot of physics, geometry, voodoo, etc that goes into magnification instruments and it is impossible to optimize the image produced, field of view, eye relief, etc with a zoom piece. The highest and lowest powers are optimized the best they can but the rest are a compromise. Compare the specs of say a fixed 30x lense to that of a zoom lense, the field of view and the eye relief will be far better because the lense geometry is optimised for the fixed power. There are some very good comparisons between fixed and zoom scopes on the birding forums that will do it more justice than I can. I would love to have a zeiss, kowa, or Swarovski with a 30x fixed lense for general glassing and a zoom lense as a backup for specific situations.