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# Increasing Field of View in Binoculars.

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### #1 Art Bianconi

Art Bianconi

Aurora

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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

EdZ

Professor 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

Dennis Menace

Sputnik

• Posts: 27
• Joined: 14 Oct 2007

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.

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

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

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

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