Water will condense on cool binocular lens surfaces

February 02, 2009|By JEFF RUGG / Creators Syndicate

Q: You recently wrote about binoculars, but I am overwhelmed with all the types available. I received a Nikon Action 7x35 pair of binoculars for Christmas, but found after trying them out when hiking, they fog up. Now, they seem wonderful other than that, but I am a huge nature enthusiast and do lots of hiking, bird watching, wildlife sightings . . . anything in nature, but was very disappointed this past weekend when I could not use them. Would you have any suggestions as to what I could purchase? I want to return these. Now, understand, I don't want to spend hundreds of dollars, but am willing to spend over $100 or so.

A: As you take the cold binoculars (or eyeglasses) from outside cold air into warm, humid conditions, water will condense on the surfaces that are cooler. It happens all the time as car windows fog up with warm, humid air contacting a colder piece of glass. Even a room temperature (75 degrees) binocular lens can be fogged with warm, moist air on the cooler glass as you breathe out the 98.6 degree moist air from your lungs.


Fogging will always happen to binoculars on the outer two glass layers (and the cold metal of the binocular itself). These two pieces of glass can be wiped dry. When the inner pieces of glass and the prisms get fogged, they are impossible to wipe dry; and as they air-dry over time, they can get dry water stains that are impossible to remove. A waterproof binocular is made when it has proper seals and they purge the moist air out and fill it with nitrogen gas. So get a waterproof pair and you will be able to keep the fog out of the inside of the binocular. The outer two pieces will still get fogged.

When you come into a warm, humid environment from a cold one, put the binocular, cameras, GPS and other devices into a sealed plastic bag. The moisture will condense on the outside of the bag, not on the expensive equipment. When it is warm and the water has evaporated, you can open the bag.

Q: I thought with all the high-tech features of binoculars these days that there must be some way to keep the outer pieces of glass from fogging up. Can I coat them with the stuff I use in the bathroom to keep the mirror from fogging?

A: I have never tried that, but it sounds like it might work. Because expensive binocular lenses are already coated with a variety of chemicals that are very effective at making a clearer view, I don't think I would use the anti-fogging chemical on an expensive pair. It would be an interesting test on a cheap pair.

Q: I found your article on binocular lingo very helpful. We have started bird watching, as we have wild lands behind our home. We have bird feeders and an Eastern Bluebird nesting box (we have had one successful nesting event). We have inexpensive binoculars, which have not been very good for viewing. I was pursuing spotting scopes on eBay. I have seen 50, 60, 75 and 90mm scopes with magnification from 15-45, 20-60, 25-75 and 30-100, respectively. Do you have some recommendation as which one would be best?

A: The larger the front lens is on a binocular or a scope, the more light it will bring in during low-light situations, such as early or late in the day and into dark shade. The tradeoff is that the whole rest of the scope has to be bigger. The area of a circle changes by the square root of the diameter, so there is a significant increase in light as you go from 50 to 60 and so on. Get as big of an objective lens as possible when everything else is the same.

Higher magnifications significantly cut the amount of light getting to your eye. The diameter of the dot of light coming out of the eyepiece is found by dividing the objective lens number by the magnification number. This diameter needs to be in the 3-5 range to match the diameter of the pupil of your eye. A 60mm diameter objective lens gives a wonderful 6mm beam of light at 10 power; a 3mm beam of light out of the eyepiece when at 20 power; and only an unusable 1mm beam of light at 60 power.

Except in unusual circumstances, higher magnifications that give smaller than 3mm beams of light are not usable. Even though you can zoom to the higher magnifications, the light will be dim and the beam of light will be so narrow that it will be hard to keep your eye centered on the subject.

High magnifications also have smaller fields of view. It will be hard to find the subject and when it is found it is hard to keep it in view, especially if it is moving. It is fun to zoom into the moon to see it close up, but it never stops moving and at high magnifications it is hard to follow.

Copyright 2009 Creators Syndicate Inc.

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