Questions About Optics

Question and answer: Optics for birding.

Q. My husband bought a pair of 7x35 binoculars on line, they weren’t as powerful as we wanted so we sent them back and got a 7x50. Other than they are bigger, I don’t see much difference. My husband thinks they are better but I think he doesn’t want to admit we (meaning he) screwed up. Are 7x50 more powerful than 7x35?

A. Most of my couples counseling involves an optics purchase gone bad. This is one case where size truly doesn’t matter, although the 7x50’s are half again bigger than the 7x35, they are the exact same power, 7. That is they make objects appear 7 times closer. The 35 and 50 refer to the size of the objective lens (the big one) in millimeters. In low light conditions the 50’s may be brighter, but in the day time you won’t see any difference.

If you hold your binoculars at arms length and look through the eyepiece, you’ll see a circle of light referred to as the “exit pupil”, on the 7x35’s it will be 5 millimeters and on the 7x50’s it will be 7.14 millimeters. You can measure it with a ruler or divide the objective size by the power. In the day time the pupil in your eye is open around 3 mm, so the extra light on each binocular isn’t getting into your eye anyway. When the sun goes down, your pupil will dilate more and that extra light can get in. But...once we reach a certain age, (50, give or take) our pupil doesn’t open past 5 mm anyway. So if you’re over 50 you probably can’t justify the purchase of the heavier, more expensive, binoculars.

Where this principle is more important is comparing compact (say 7x21) to a full size 7x35. In day light, or the binocular store, your pupil will be around 3 mm, the same as the compacts exit pupil. Step out at dusk or dawn to do some birding and the 7x35 will greatly out perform the compact pair, as no matter your age, your eyes will let that extra light in.

To make matters worse, the field of view is usually less on a 7x50 than a 7x35. This is more important for birding so you can more easily locate and track a bird moving around quickly at close range. When used for their intended purpose, astronomy or boating, the narrow field of view isn’t as important since you are looking at object farther away and relatively stationary. (This is where hubby says, “I bought them to star gaze….yeah, that’s the ticket.)

Something to think about when buying binoculars on line is the return policy. Can I return them for a full refund? If so, narrow your selection down to 2 pair order both and return the one you don’t want. There’s nothing like having both pair in your hands to compare at the same time. That’s why you should always take your old pair with you when shopping. You want to make sure what you’re getting is better than the old pair, I’ve seen people with great binoculars who didn’t know what they were worth (a gift or inheritance). In some cases they just needed exterior cleaning and adjustment. In some cases they had to be sent away for interior cleaning and realignment, but I’ve had $500 binoculars come back in “like new” condition for a $35 repair and saved the person $200 on a lower quality binocular.

Q. How do I adjust the dial on the eyepiece of my binocular?

A. This is hard to put in writing but I’ll try, it’s extremely important to have your binoculars adjusted for the differences between your eyes. Often after getting the adjustment right, you’ll go from hating your binoculars to loving them. The adjustment you’re referring to, I hope, is called the diopter. You may have used a binocular microscope at some time in your life, and the adjustment on a pair of binoculars is the very same.

In most cases it’s on the right eyepiece, the numbers start at 0 in the middle and go +1,+2 on one direction and -1,-2 in the other. Some models have the diopter adjustment on a dial above the center focus. Make sure it is adjusting the right eye by going through the whole range while looking through only the right eye. It should go in and out of focus.

Step 1: Put the lens cover on the right eye.

Step 2: Focus, using center focus only, on a stationary object 25-40 feet away.

Step 3: Put the lens cover on the left eye.

Step 4: Moving the diopter and without touching the center focus, bring the same object into crisp focus.

Step 5: Remember or better still mark the setting on the eyepiece.

Step 6: Repeat every year or so. As often as you get your eyes checked.

You won’t have to touch the diopter again as that eye will focus with the center focus, it’s only to compensate for the difference between your eyes. I often see people fiddling with the diopter constantly while bird watching. They adjust it every time they look at a different bird. This is a needless, headache inducing exercise.

Note to eyeglass/contact lens wearers. In most cases your prescription corrects for the difference between your eyes. In these cases your diopter setting is likely to be “0”. I quite enjoy the reaction when someone first looks through a binocular meant for eyeglass wearers, with the eye cups dialed down and the diopter set to zero, the difference, even with a lower end binocular is incredible. It usually starts with a string of swear words (english and/or french) then, “Come here honey you have to look through these.” But, they won’t pass them over, so I get another pair for “Honey” and one for myself and we settle in for some back yard birding.