Telescope Reviews: Re: Update: Very Short Test on Vixen 16x80 Binocul

Mike, you can't really tell anything by those measures. There is no way of knowing what the focal length of the eyepieces is, and they could very well be different, even though they have the same eye lens diameter. The entire line of Radian eyepieces all have the same eye lens diameter.

Based on the dimensions given in the spec sheets, it looks like they use the exact same body for the 12x, 16x and 20x, possibly even the 30x with a slight extension. That would mean only the eyepiece changes.

Binoculars are almost always around f/4 and an 80mm binocular may be longer, at least f/4.5. So a reasonable assumption is the focal length is about 350mm. Using eyepieces of varying focal lengths is all that is required to change magnification. Varying the Afov, or the field stop diameter, of the eyepieces, will change the field of view or Tfov. So they could use eyepieces such as 29mm, 22mm, 17.5mm and 12mm.

Several things happen when you increase the field stop in an eyepiecce. You get a wider field of view, but you also generally increase the aberrations in the outer field of view, including coma, spherical aberration and astigmatism. This is not just binocular eyepieces I'm talking about, this is eyepieces in general. Most 70° eyepieces on the market today have pretty poor outer field performance. It takes a very well corrected eyepiece to eliminate those aberrations, and it cost considerable money to make such eyepieces. So generally what we see in binoculars that have 70° Afov is pretty poor performance in the outer field of view, unless perhaps you are using a $300-$400 pair of eyepieces.

Few binoculars have well corrected wide fields of view. The Fujinon 16x70 is one that is fairly well corrected. It has a true measured Tfov of 4°, therefore the eyepieces have an Afov of 64°. It's well corrected to within 20% of the edge, or 80% out, maybe 85%.

You estimated sharp view of the Vixen Ark to within 5% of edge. Likewise you estimated the 12x80 to have a sharp view to within 5% of the edge. Frankly Mike, I can think of only one or two binoculars that might even come close to that. The Pentax 16x60 was one, it is sharp out to 90% of the field, but it has orthoscopic eyepiecs with a 42° Afov, not wide angle eyepieces. That's about what I would expect from orthoscopic eyepieces. It is not what I would expect from 70° eyepieces.

Two prior reviews of these vixens, one a 16x80 and one a 20x80, report loss of usable field in the outer 20% or more of the field. None of the prior reports gives us any true measure of the field of view.

edz

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