Wednesday, June 16, 2010

Trouble getting one image in binoculars?

Q, from Jennifer:

My sister loaned me her Eagle Optics 8 x 42 Rangers while we were out birding and I found I could focus them to see only one circle through them with no dark shadows. On other binoculars, I usually have lots of trouble getting that to happen.
So I bought a pair. Farther distances, no problem. One circle. But I noticed that when I focus on shorter distances, I see 2 overlapping circles. Is that normal? Is there something else I should be doing for shorter distances?
Also, is there anything optically wrong with holding the binoculars with the focus wheel on the bottom? I find I can reach it with my thumb that way. It's much easier than trying to reach over the top with my fingers.
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A: What you are experiencing with focusing at shorter distances is somewhat normal. It's an effect called parallax. Some people experience a more pronounced parallax effect than others. This is a factor of your IPD (inter pupillary distance) or the distance between your eyes. Here is an article that explains the phenomena in more detail if you are interested.
If you are focusing at shorter distances, the best thing to do is take a few steps backward and the problem should be less noticeable. You can also simply close one eye if it really bothers you. There are not going to be other binoculars out there that will reduce or eliminate this effect.
Regarding how you hold the binoculars, I don't see anything wrong with holding them upside down if that's what you find most comfortable. Thanks, Jennifer.

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