Binoculars Question


March 17, 2007, 04:00 PM
I've never owned binoculars but I am looking to purchase some. I understand the magnification but I am wondering what difference lens size makes. For example... comparing 2 binoculars, let's say 10x42 vs. 10x25. Let's also assume that field of view at a given distance is relatively the same. Is there an advantage to having a larger lens? I know larger lens usually equates to larger/heavier but is there a light gathering advantage to having a larger lens? These would probably most often be used for daytime sporting events, possibly star gazing at night. My apologies if use is slightly off topic but I get such good info/advice here I figured I'd try. Thanks, Roger.

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March 17, 2007, 04:33 PM
Quote: "is there a light gathering advantage to having a larger lens? "

The larger diameter front lens gathers more light rays which not only makes the binoculars brighter but also improves resolution. Of course resolution is also dependent on the quality of glass, grind quality, etc. I don't recommend any binoculars with front lens diameters less than 32mm or larger than 42mm. IMO the 25mm lens are not as bright or sharp and the 52mm lens are too heavy. This is personal choice and your choice may differ. I don't think any normal binoculars ( 8X - 10X) are going to be that useful for star gazing although you can get a better look at the full moon. Last suggestion: Buy the best quality glass you can afford. Leica, Swarovski, and Zeiss are all considered top notch. Review their web sites for more detailed info on binoculars, spotting scopes, etc.

Good shooting and be safe.

March 17, 2007, 05:36 PM
Thanks LHB1,
I'm looking for an inexpensive pair because they will be used infrequently. Probably not looking to spend more than $150. What characteristics of the binoculars create increased fov?

March 17, 2007, 06:04 PM
while i normally don't scrimp on optics at all, i did buy my wife a pair of those gunshow special Korean ncstar binoculars for bird-watching. we've had them maybe 4 years now and no problems. of course, birdwatching from the kitchen windows doesn't usually cause the nicks and dings and dragging through the mud sort of wear that hunting does. so YMMV

March 17, 2007, 06:16 PM
Field of view is primarily determined by design of the eyepiece lens system. Picking a binocular in $150 or less category is pretty much hit or miss IMO. Only advice I can give is to NOT buy any binocular in a plastic blister pack but ONLY buy binoculars that you can physically handle and look through. You might contact some of the bigger internet sellers of binoculars and see what they say. They may give good advice or they may try to sell you whatever they make the most profit on or have overstocked.

Good luck and good viewing.

March 19, 2007, 11:31 PM
Hi Roger - Hope I don't overload you, but here goes:

The word is “binocular,” not a “pair of binoculars,” which means two binoculars. The prefix “bi,” from the Latin bini, means double or two. Add it to “ocular,” from the Latin oculus, meaning eye, and you’ve described a two-eyed instrument. A binocular.

A common binocular glass is BK 7, the “B” meaning borosilicate. “K” stands for Kron or Crown, one of two main types of optical glass. Another popular Crown glass is BaK 4, a barium-silicon compound. BaK 4 prisms are denser, with better light transmission and clarity than BK 7. While BaK 4 is usually more costly than BK 7, you won’t see much difference through binoculars. They’re both prism glasses. You can tell prism type by looking into the objective end of a porro-prism binocular. If the exit pupil appears round, it’s BaK 4. If it looks square, it’s BK 7.

Porro prism binoculars allow better depth perception. Good roof prism glass uses phase-correction-coated prisms, called “p-coated,” which corrects the phase shift that occurs when light leaves uncoated roof prisms. Without p-coating, even the very finest roof prism binoculars don’t match the optical sharpness of the finest porro prism models.

The size of the field depends not only on eye relief, but on the size of your ocular (not objective) lens and the magnification. When you look through a glass, your eye becomes the apex of a cone of light whose base is the perimeter of the ocular lens. From the side, that cone is seen as a two-dimensional triangle. If the angle at the apex (your eye) is 30 degrees, (a 30-degree angle subtends 150 feet at 100 yards) and your glass is an 8x, then dividing 150 by 8 yields a field of 18.75 feet.

Real angle of view (RAOV) is used to determine field of view (FOV) and apparent angle of view (AAOV). To determine a binocular’s FOV at 1000 yards, multiply RAOV by 52.4 feet. AAOV is determined by multiplying the RAOV by the magnification. A binocular with an FOV over 400 feet or an AAOV over 60 degrees is considered a wide angle.

Your eyes can resolve about 60 seconds of arc, or one moa. Magnification gives your eyes more resolving power, but as magnification exceeds resolving power of the glass, you get a larger image that is no clearer. Rayleigh’s constant, 114.3, divided by the objective lens diameter in millimeters, yields the maximum resolution in seconds of angle. To get the maximum power at which the eye can use this resolution, we divide 60 seconds of arc (the naked eye resolution) by the maximum resolution to get the magnification. 50 mm = 26.2x, 32 mm = 16.8x.

For good definition (the optical quality most of us call sharpness), the diameter of the objective lens should measure, in millimeters, at least four times the magnification. For the finest definition, an objective at least five to six times the magnification helps enormously.

March 20, 2007, 09:03 AM
Go to a store that carries a substantial line and spend an hour looking at the different models. There is a difference. You aren't just spending extra money for a brand name

Check out the Nikon Monach line. About 2x your maximum, but probably 2x as good (at least) as the cheaper $50-$75 binculars. My choice is the 8x42 ones.

March 20, 2007, 09:34 AM
Two other factors to consider are:

1. Eye Relief
If you wear glasses, a shallow or short eye relief (i.e. 5mm) will result in your inability to see the entire image because your eye is being held away from the eyepiece by your glasses.

I wear glasses and it takes 20mm worth of eye relief to allow me to see the entire image without having to take them off.

Even if you don't wear prescription glasses, at some point in time, you will likely be wearing sunglasses, so this issue will arise.

2. Water Resistant
I take mine out on the water in a Kayak, and out in the woods under foggy and damp conditions, so I need a binocular that is considered "water resistant" down to 1 meter of depth.

Once moisture gets inside the binocular, it fogs up the works, big time. Ask me how I know.

FWIW, they are a Pentax 10x50, PCF WP II. Weight about 2 pounds.

March 20, 2007, 09:49 PM
Thanks for the advice. Crittergitter, I appreciate the clarification of the word binocular. You lost me on about 90% of the rest but the relationship between magnification and lens diameter and the resulting effect on sharpness was quite understandable and something I would have never known to consider. Muchas Gracias.

Charles S
March 20, 2007, 10:10 PM

Thank you the information you provided on binoculars, I learned a lot about glass and the process. I had always read that dollar for dollar porro prism binoculars were a better value, your explanation the properties of each type explains that.

April 3, 2007, 10:33 PM
You're most welcome. Honesty demands that I reveal most of my post is information I collected over the years from optics articles and books and saved for reference, and is not truly "mine."

April 4, 2007, 07:53 AM
I've had a Nikon Travelite IV 9x25 ($100 or so now and 9 ounces IIRC) since
'93 and it's just a little too hard to hold it steady enough to watch the Redskins cheerleaders without bracing an elbow on a knee. :) I think a 7x or 8x would be better. The 8x40 or 8x42 seems to be the overall favorite for birdwatchers, too. The 9x25 has just a little too narrow FOV for football from the top row of the bottom level.

I would like to have some 8x40's for $300 or $400, but the Travelites pretty much fit in most pockets and have survived years of duck hunting and fishing without being dunked.

Another thing to consider when debating cheap vs. pricey - good glass is necessary for extended viewing. If all you want to do is glance at something to see if it has antlers cheap binocs might do. Cheap binocs give me eyestrain and a headache after five minutes usually. Good ones are good for hours at a time.


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