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Increasing Field of View in Binoculars.

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#1 Art Bianconi

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Posted 12 October 2007 - 05:59 AM

Is there a simple, effective way of reducing the magnification and increasing the field of view of a binocular?

If, as an example, I were to change the exit lens on a 10 by 40 thus reducing the magnification to 5X, could I expect to see a proportionate increase in the field of view?


Thanks

Art

#2 EdZ

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Posted 12 October 2007 - 06:10 AM

Only if both original and new eyepieces have the same diameter field stop diameter, so the likely answer is NO.

This is what happens:

typically the f# of a binocular is about f/4 to f/4.4. Some smaller 8x and 7x binoculars are about f/3.8. So, let's take an f/4.2 10x42. That means the objective is F=176mm and the eyepiece if F=17.6mm. Assuming 1.25" eyepieces, then to change that to a 5x binocular you would need an eyepiece with F=35. First, that's a huge honkin eyepiece and second the field stop won't grow by double, because it would be limited by inside barrel diameter. So you could go from a 60° 10x42 to at best a 44° 5x42. Tfov would grow from 6° to only 9°.

Almost all examples work out the same way, no matter what size you modify.

Some other examples:
f/3.8 8x32 72°/9.0° needs a 30mm eyepiece to make it 4x32. One of the best of the bunch here, Max Afov would be 51°. It would be a f/3.8 4x32 51°/13°

f/4.2 16x70 64°/4.0° needs a 37mm eyepiece to make it 8x70. Max Afov would be 42°. It would be a f/4.2 8x70 42°/5.4°

f/4.5 20x80 64°/3.2° needs a 36mm eyepiece to make it 10x80. Max Afov would be 43°. It would be a f/4.5 10x80 43°/4.3°

f/4.8 25x100 60°/2.4° needs a 38mm eyepiece to make it 12.5x100. Max Afov would be 40°. It would be a f/4.8 12.5x100 40°/3.2°



edz

#3 Dennis Menace

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Posted 19 October 2007 - 01:48 PM

If, as an example, I were to change the exit lens on a 10 by 40 thus reducing the magnification to 5X, could I expect to see a proportionate increase in the field of view?


Good question, that's what I thought, too. After all, that's the way a zoom lens on a camera works.

I bought 5x eyepieces to replace 10x eyepieces in a microscope. Was hoping to increase FOV by decreasing magnification.

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The image completey fills the 10x eyepiece. FOV is 18mm.

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Much to my chagrin, the FOV is still 18mm in the new 5x eyepieces. It's like looking thru a straw! :mad:

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I'm still studying EdZ's answer and trying to understand it all......


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