Binoculars are two telescopes linked together and viewed separately by each eye. To produce a crisp and perfectly merged image, lenses and prisms inside the binocular barrels should be perfectly aligned or “collimated”. When the two images produced become identical the brain will view the image as one.
Frequent use or rough handling may result in these two getting misaligned. Mis-collimated binoculars will result in poorly defined, overlapping images or cat’s eye-shaped images in each eyepiece. Any minor change in alignment or merging may result in subtle effects that can cause eye strain, headache, and fatigue.
Most manufacturers allow users to align the unit to some extent by themselves. Adjustable tilt setscrews are built on each barrel to adjust the prisms accordingly until images are back in alignment. Usually, a slight tweaking of the setscrews that tilt the prisms nearest the eyepieces should do the job. But, when in doubt, it’s best to get help rather than doing it yourself. Not all repairs can be done on your own; fixing issues concerning the lenses should be done by the manufacturer or a professional.
If you do decide to do minor collimation on your own, here are some guidelines to assist you.
#1 Take the binoculars with you outdoors in daylight. Locate a prominent structure far away from you, about 100 yards or more, a house for example, with straight and horizontal lines.
#2 Place the binoculars on a flat, solid surface pointing the unit horizontally. Depending on where you are, a tripod where the binoculars can be mounted on may also be used for stabilizing the unit while positioning yourself behind the binoculars.
#3 Center in the field of view, focusing on the prominent structure.
#4 Check horizontal collimation.
#5 Check vertical collimation.
#6 Locate the two prism tilt adjustment setscrews on each barrel of the binoculars near the eyepiece. For models with a waterproof rubber covering, use a small (1 mm) flat bladed screwdriver to gently pry open the gap between the edge of the rubber cover and the prism cover, and slowly peel back, until you find the setscrews.
#7 Using the same screwdriver, adjust the prism tilt setscrews.
#8 On the left side barrel, by turning the setscrew clockwise, you will move the image to the right and down and by turning the setscrew counterclockwise you will move the image up and to the left.
#9 On the right side barrel, turning the setscrew clockwise will move the image to the left and down. If you turn the setscrew counterclockwise the image will move to the right and up.
#10 Remember to turn the setscrews back and forth in small increments, frequently evaluating the images as they shift apart from one another or fuse together. Stop turning the setscrews when you see the images merged as viewed through both barrels.
If you think the image is still a bit out of alignment, repeat the steps above until the images merge horizontally without any vertical displacement. Usually, minor vertical misalignment can be easily resolved by adjusting one side.