You can easily aim straight-through binoculars without a finder as long as the magnification is below about x20 when there is a real field of view of 2.5 degrees or more. Aiming becomes more difficult at higher magnifications and can be extremely difficult with angled eyepieces unless you are exceptionally familiar with the binocular view of the region of sky you are observing. Some form of finder is therefore extremely helpful in the latter cases. In order to avoid fogged optics, you should ensure that you mount the finder so that, when you use it, you are not breathing on the eyepieces of the binoculars. There are four distinct options: simple mechanical sight, reflex finders, finder scopes, and lasers.
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