What do I need to know about spotting scope zoom eyepieces?

Zoom eyepieces are a great convenience, since you don’t have to change eyepieces to closely examine a distant view or pull back for a wide angle scene. However, there are some tips you should know about to get the most out of your eyepiece.

When you focus the eyepiece on a building or a bird, then zoom in, you’ll lose image sharpness and need to refocus. This is because the scope’s zoom eyepiece is not parfocal, so it won’t be in focus at all magnifications. The same is true when using your zoom eyepiece for photography with your scope – you’ll have to refocus if you zoom in or out.

Terrestrial viewing is best done with a magnification of 15x to 60x. If you exceed 60x magnification, a view can be blurry because the of earth’s atmosphere. Heat waves and high-altitude winds move air around and cause differing temperatures of air to mix.  This makes the air act like a weak lens that interferes with the light from a distant object, especially on a hot day. This "bad seeing" can drastically distort your image. Early mornings are best for high power viewing with your zoom-eyepiece spotter because the air temperature is more constant.


Updated 12/18/13