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Re: [OM] OT Binoculars

Subject: Re: [OM] OT Binoculars
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Sun, 18 Oct 2009 06:11:01 -0400
Thanks for the extra info.  I had never considered the extra resolution 
afforded by the larger diameter objectives but should have thought of 
that.  But even so, I don't think I could be persuaded to carry a pair 
of 7x50s very far.  Sort of like there's nothing photogenic more than 
500 yards from the car.

Chuck Norcutt


Moose wrote:
> Chuck Norcutt wrote:
>> The first thing is to understand the size convention which is to
>> list magnification and objective lens diameter.  For example, the
>> common 7x35 (and there's a reason why it's common) means
>> magnification 7X and an objective (front) lens diameter of 35mm.
>> These two numbers are important since together they determine the
>> size of the exit pupil.
>> 
> 
> <big snip>
> 
> Nice explanation of issues I was too busy/lazy to spell out. The only
>  problem is that it leaves out at least a couple of related issues.
> 
> A problem with exit pupils beyond what you have said is that they
> also affect ease of use. With a small exit pupil, careful alignment
> of both optical paths and irises is required. Some people find this
> troublesome, complaining about difficulty getting them adjusted so
> the image is not partially occluded in one eye or the other.
> Particularly a problem for some with folks with glasses and some
> vision problems.
> 
> The whole objective/magnification/exit pupil business interacts with
>  resolution. Assuming equal optical quality and good vision, the 7x50
> in the beach example is capable of resolving more detail than the
> 7x35. Just as with camera lenses, and all optical systems, resolution
> is a function of aperture. This factor is important in birding, which
> is why you often see serious birders hauling around bigger, heavier
> binocs than might otherwise seem needed.
> 
> Moose
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