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# Calculating Binocular Focal Length

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### #1 Patrick

Patrick

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Posted 17 July 2005 - 08:01 PM

Hi all,

Is there a way to calculate a binoculars focal length (and f/ratio) from the information typically supplied with a binocular (aperture, magnification, exit pupil, field of view at 1000yds, TFOV)?

If there is, it's eluding me.

In the past, I've read where some have mentioned measuring the length, but that's kind of hard to do when only looking at a picture of one.

Thanks for the help.

Patrick

### #2 EdZ

EdZ

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Posted 17 July 2005 - 08:15 PM

Is there a way to calculate a binoculars focal length (and f/ratio) from the information typically supplied with a binocular (aperture, magnification, exit pupil, field of view at 1000yds, TFOV)?

NO, it is not. You must measure some lengths, unless the f# is given. For instance the BT100 f# is given as f/6.2. Most fixed binoculars are closer to f/4.

Read the article found through the "Best Of" links to "Testing and Measuring Binoculars."

edz

### #3 Patrick

Patrick

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Posted 17 July 2005 - 08:27 PM

NO, it is not.

Yeah, that's what I thought, which is kind of a pain. I recall now reading that article some time ago. Sorry for the redundancy. I was hoping for a different answer this time! It just seems like the focal length is a relevant piece of information manufacturers should be supplying.

Patrick

### #4 EdZ

EdZ

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Posted 17 July 2005 - 08:33 PM

Well, you don't need to know it in a fixed power instrument.

In any instrument where you can vary the power, you need to know the focal length.

edz

### #5 Patrick

Patrick

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Posted 17 July 2005 - 08:54 PM

Well, you don't need to know it in a fixed power instrument.

It might be pertinent information if I was SELECTING a binocular. I might want to choose a binocular with a longer focal length vs a shorter one.

Patrick

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