Perfectly aligned lenses and prisms in binocular barrels produce a crisp, merged image that is said to be "collimated." Rough handling can knock binoculars out of collimation, leading to fuzzy or overlapping images and possibly cat's eye-shaped images in each eyepiece. This can cause eyestrain, headaches and a degraded user experience. To correct miscollimation, manufacturers build adjustable prism tilt setscrews into many of their models. If prism tilt is excessive, however, or lenses are misaligned, have a professional collimate your binoculars.
Go outdoors in daylight and focus your binoculars on any large object with distinct horizontal and vertical edges, at a distance of about 100 yards.
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Relax your eyes. Alternately open and close them, about every second. You should see the image move slightly to the left when opening the left eye and to the right when opening the right eye. This slight movement is normal and desirable.
Adjust the prism tilt screws to restore collimation if you detect horizontal movement.
In the daytime and outdoors, look at a distinct horizontal line, such as a roof line, gutter or fence rail.
Slowly move the binoculars about 4 inches away from your eyes as you keep looking at the images in the eyepieces.
Check that the horizontal feature you focused on remains an unbroken straight line -- in other words, no part of it is displaced vertically.
Adjust the prism tilt screws if you detect any vertical displacement.
Prism Tilt Screw Adjustment
Locate the two prism tilt setscrews just ahead of the eyepiece on each barrel. For rubber-armored models, pry open the rubber covering with your screwdriver to expose them. Be careful not to overstretch the rubber; you want it to snap back to its original position when done.
Remove the protective glue from the setscrew's top and insert the screwdriver in the slot.
Turn the right barrel's setscrew clockwise in very small increments to move the image in the right eyepiece to the left and down, counterclockwise for up and right.
Turn the left barrel's setscrew in the same manner, clockwise or counterclockwise, to adjust the image's horizontal and vertical alignment in the left eyepiece.
Adjust one or both setscrews as necessary, frequently checking progress using the horizontal and vertical collimation tests, until the two images merge horizontally and there is no vertical displacement.
The ultimate collimation test is to view a bright object, such as the planet Jupiter, in the night sky. It should be well merged, with no double images. Properly collimated binoculars will also have nice, round exit pupils. If the exit pupils are oval, even with the images perfectly merged, collimation is not complete and your binoculars will need professional attention. Always register your binoculars' warranty.
Binoculars with porro prisms have the two setscrews. Binoculars with roof prisms may have two additional setscrews, for a total of four, that need to be adjusted in sync with each other.
Do not attempt to remove binoculars' objective lenses or other interior lenses to fix collimation problems. Lenses need to be removed, properly aligned and reinstalled by skilled technicians using specialized equipment.