What Is Binocular Collimation and Why It Determines Viewing Comfort
Earlier articles have mentioned the term "collimation" more than once. This piece explains it fully, because it is the invisible ceiling on binocular quality and the real reason two pairs with identical specs can deliver wildly different experiences.
The definition of collimation: a pair of binoculars contains two parallel optical paths, one in each barrel, and those two paths must be strictly parallel and aimed at the same target. That degree of parallelism is what collimation accuracy means. Tolerances in machining and assembly have to be held within a few arcminutes before the eye can fuse the two images into one.
What happens when collimation drifts: with a slight offset, your eyes strain to force two barely misaligned images together, and after ten minutes or so you feel eye strain and dizziness. With a moderate offset, straight lines appear doubled or shifted up and down between the two eyes. With a severe offset, the images simply will not merge and you end up squinting through one eye. Many complaints that a binocular makes you dizzy have nothing to do with the user failing to adapt; the collimation itself was out of spec.
How to check it yourself: aim at a target with a clear edge about 50 meters away, such as a utility pole or a window frame. Once the images merge, close one eye and then the other in turn. The edges seen by the left and right eye should line up precisely in the same position. If they are offset by more than about one tenth of the target's width, you will notice the discomfort.
Why knocks and drops matter: collimation is locked in place by adjustment screws on the internal prism mounts, and a single drop can shift a prism mount. If a binocular that has been dropped shows double images or causes dizziness, do not try to push through it. Sending it back for realignment is the only proper fix. This is also the value of buying from a brand that inspects its output, since every unit passes a collimator before it leaves the factory.
In one sentence: collimation is the soul of a binocular. Buy from a maker that aligns each unit at the factory, protect it from drops and impacts in use, and if you feel dizzy, do not blame your eyes. Check the collimation first.