Alignment and Durability
Binoculars actually consist of two separate optical instruments, an individual telescope for each eye. So it is essential that they stay in alignment. When binoculars are functioning properly, both sides focus on the same field of view, but a sharp jolt can easily throw them out of alignment so that the two fields no longer overlap. If the binoculars are severely misaligned, you will see a double image when both your eyes are open, and the subject will look blurred. Binoculars that are only slightly out of alignment may actually be a worse problem, because your eyes strain to bring the two images together, leading to eye fatigue and a headache.
Checking alignment
To check the alignment of your binoculars, try this simple test:
In alignment
Look at the roof of a house with them, then, continuing to look through
the eyepieces, move the binoculars about eight inches away from your
eyes. If the binoculars are in alignment, the horizontal line of the
roof should be at the same level in both fields.
Out of alignment
If the roofline appears offset the binoculars are out of alignment.
Realigning binoculars is no simple task; it should only be done by an experienced technician.
Durability
You get what you pay for with binoculars, as with any other consumer
item. Inexpensive binoculars are more likely to go out of alignment
than higher-priced models, because their prisms and lenses may be glued
in place rather than securely held by metal brackets. Temperature
changes or slight jars can easily throw inexpensive, poorly-made
binoculars out of alignment. Birding can be very tough on optical
equipment; it makes more sense to invest in rugged, well-made
binoculars initially than to repeatedly replace or repair less durable
ones.
