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minimum focus question

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minimum focus question

Postby Roger on Thu Sep 09, 2010 7:35 am

It seems there are a lot of trade-offs when it comes to binoculars. For example, eye relief and FOV seem related. Increasing magnification tends to decrease exit pupil and stability.

So my question is, is there any trade-off regarding minimum focus? Do binoculars with a 20 foot minimum focus have any advantages over similar binos with a 5 foot minimum focus?

Re: minimum focus question

Postby Jne_K on Thu Sep 09, 2010 11:52 am

Hi Roger

The difference is more directly a relationship between magnification and field of view, rather than eye relief and field of view. FOV is a matter of both magnification and eyepiece design - that's why you see variation in FOV between binoculars of the same magnification - whereas eye relief is solely a matter of eyepiece design and is not directly related to magnification. You are right in that going to a higher magnification typically means less eye relief. This is because, given the restraints of binocular eyepiece design, it is more of a challenge to keep long eye relief at higher magnifications. It can be done, but there is only so much you can do with an eyepiece the size of which you use in a binocular. Much easier to do in telescopes where you have room to go to more sophisticated eyepiece designs.

As for minimum focus, that is something that is mostly a matter of roof prism versus porro or, more correctly, a difference between internal focusing systems, as you typically get in a roof and external focusing systems, as you typically get in a porro. Just harder to design that much focus travel in an external focusing system. In an of itself, no, minimum focus will have no effect on performance. Manufacturers have gone to shorter and shorter minimum focus in response to birders, where this is an issue, but unless you are a birder (or butterfly watcher), I wouldn't lose any sleep over it. Manufacturers typically keep the minimum focus short so that a given model won't be excluded out of hand by birders, who do make up a significant portion of the market.

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