Viewing Angle- Straight-Through vs. 45° vs. 90°
We’re often asked about the best choice for binocular viewing angle. This depends partly on budget, and partly on how you intend to use the binocular (terrestrially, astronomy, or both). We say “budget” because all binoculars have a straight-through view, except for relatively-expensive binocular telescopes, which have a viewing angle of either 45° or 90°. Straight-through view binoculars are great for all terrestrial viewing, but they can be less than optimum for astronomy simply because viewing becomes uncomfortable above +45° (the range of viewing comfort for earthlings is +45° to -45°). But amateur astronomers may accept viewing outside of the comfort zone because of the high cost of binocular telescopes, and also because of the ease and simplicity of using hand-holdable binoculars that require no tripod.
Some amateur astronomers are deeply divided over the best viewing angle for binocular telescopes- the choices being 45° and 90°. The argument from those that prefer 90° is that 90° viewing is the most comfortable for viewing to zenith (directly overhead). This is true, as with a 90° binocular, you’re looking straight ahead to view directly overhead. But how many objects in the night sky are directly overhead? The vast majority are not of course, and those that are overhead will soon be at a lower angle in the sky due to earth’s rotation. Yet viewing directly overhead with a 45° binocular is not uncomfortable- it just means that you’re looking upward at a +45° angle to do so. The key to viewing comfort when looking up at a 45° angle is binocular eyepiece height- as long as the binocular mount has enough height range, viewing to zenith is within the comfort zone of +/-45°. On the other hand, 90° binoculars become uncomfortable when viewing to less than +45°, because you’re exceeding the -45° comfort range. Less than 45° means half of the sky is outside of the comfort zone! Viewing to the horizon is extremely uncomfortable with a 90° binocular, because you’re looking straight down (-90°), making them unsuitable for any type of terrestrial viewing. Only 45° binoculars offer comfortable viewing of the entire sky, as well as to the horizon. This is why Oberwerk manufacturers only 45° binocular telescopes.
Oberwerk Brand Ambassador Fiske Miles demonstrates viewing to zenith with the Oberwerk BT-82XL-SD binocular telescope. He does appear to be within the viewing comfort zone, wouldn’t you agree?