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Binocular Apparent Field of Vision

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#1 TCW

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Posted 21 April 2014 - 08:53 PM

Most binocular specs give actual FOV but few state the Apparent FOV which to me is very important. The specs on some astronomical binos give the Apparent FOV in the 40's which to me is just not acceptable.

For a person looking for their first astronomical binoculars can anyone suggest binoculars in the 10x63 to 70mm range that have good APOV and a reasonable price?

#2 GlennLeDrew

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Posted 21 April 2014 - 10:41 PM

The ubiquitous 15X70 Skymaster meets those two critera of reasonably large AFoV (60-64 degrees) and cost. The matter of collimation is often an issue, though.

I'm firmly in the large AFoV camp; my homemade 'dual' 50/60mm bino uses 100 degree AFoV telescope eyepieces.

#3 TCW

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Posted 21 April 2014 - 10:52 PM

I am hoping to find something in a lower magnification as I want to use them hand held. I am thinking anything much over 10x would be too much.

The Astro Physics 10.5x70 look promising although they are more than I plan on spending for a first pair. No specs on apparent fov though. Why is it that binocular sellers leave that one out?

#4 Man in a Tub

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Posted 21 April 2014 - 11:13 PM

Hmmm...IMO, the United Optics BA8 Series 15x70 is sold at reasonable prices. AFOV = ~66°. (Oberwerk Ultra, Orion Resolux, Astrophysics Premium, etc.)

However, is TCW only interested in 10x? Don't most 10x from 60mm to 70mm only have AFOVs in the low to mid 50°s?

#5 Man in a Tub

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Posted 21 April 2014 - 11:16 PM

I got a telephone call while replying. The Astrophysics Premium like its brothers under the brand name are ~52.5° AFOV. Simple formula 10.5x5°.

It's also quite heavy for hand holding — 5.5 pounds!

#6 TCW

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Posted 22 April 2014 - 12:42 AM

I have heard that binocular makers skimp on prism size which limits the effective aperture of some binos. I suppose the lower apparent FOV for the low power binos is because the eyepiece diameter is too small.

That formula is handy - thanks.

#7 GlennLeDrew

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Posted 22 April 2014 - 02:35 AM

Indeed, prism aperture is the determinant for the maximum useable eyepiece field stop. Many, if not most, handheld binos these days use 21mm aperture prisms, whereas until 20-30 years ago 24.5mm was much more common.

In general, a higher power at given aperture usually translates to a larger AFoV. This is because, relative to the shorter eyepiece focal length, a given field stop presents as an angularly larger 'frame' in the eyepiece. And so it's common to see 10X50s with at least near the same TFoV--and virtually universally larger AFoV--of the 7X50s.

At given magnification, a larger objective usually means a smaller field. This is because the longer focal length requires a commensurately longer f.l. eyepiece, with a given field stop appearing angularly smaller through the eyepiece. A 10X70 usually has a 5 degree field, compared to a fairly common 6.5 for a 10X50.

Unless you enjoy really dark skies, I recommend an exit pupil no larger than 5mm. This somewhat higher power per unit aperture makes it easier to obtain a larger AFoV.

#8 Jon Isaacs

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Posted 22 April 2014 - 04:29 AM

Most binocular specs give actual FOV but few state the Apparent FOV which to me is very important.



To state the obvious, multiplying the TFoV by the magnification gives a pretty good estimate of the AFoV.

If the TFoV is given as feet/1000 yards, simply divide the field of view by 3000 and multiply by 57.3 to get degrees..

10x50s with a field of view of 341 feet at 1000 yards have a TFoV of 6.5 degrees and an AFoV of something like 65 degrees.

Jon

#9 JustaBoy

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Posted 22 April 2014 - 06:38 AM

That's one I've never heard of Jon, but I just tried it and it comes out the same as what I use.

I just divide the FOV in Feet by the Constant of 52.5 to go directly to degrees.

In your example 341/52.5 = 6.496°

I'm just a Simpleton:-)

#10 TCW

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Posted 22 April 2014 - 01:42 PM

I do have fairly dark skies (dark blue zone) and would mostly use binoculars on the milky way although I would use them occasionally during the day. The next time I go to the eye doc, I plan on asking him to measure my dilated pupil diameter to see what it has shrunk to! :p

#11 Man in a Tub

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Posted 22 April 2014 - 01:51 PM

The next time I go to the eye doc, I plan on asking him to measure my dilated pupil diameter to see what it has shrunk to! :p


If your dilated pupils shrink, you're in serious trouble.

#12 TCW

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Posted 22 April 2014 - 03:01 PM

With age most if not all people have reduced pupil dilation.

#13 Man in a Tub

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Posted 22 April 2014 - 03:45 PM

:gramps:

Oops! Yeah, that's true. I just didn't read it that way. Without "qualifiers," I sometimes forget that most of us here are old[er] folks.

#14 TCW

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Posted 22 April 2014 - 03:49 PM

Sad but true. My night vision when young was just shy of cat. Now it is not quite as good.

Fogpatch - is that the coast or the San Joaquin valley?

#15 Man in a Tub

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Posted 22 April 2014 - 04:07 PM

San Francisco/Daly City close to the Pacific.

A long time ago, a guy I knew said that old maps of this area were labeled Fog Patch. I've never confirmed that, but it is an apt description of my environment.

#16 TCW

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Posted 22 April 2014 - 04:14 PM

San Francisco/Daly City close to the Pacific.

A long time ago, a guy I knew said that old maps of this area were labeled Fog Patch. I've never confirmed that, but it is an apt description of my environment.


True - so very true. The worst spot I know of is the Lost Coast north of you a bit. I am about 150 miles east of you.

#17 KennyJ

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Posted 22 April 2014 - 04:51 PM

Regardless of Field Of View, I continue to be puzzled why people looking for 10x binoculars to use hand-held for astro use consider anything larger than 50mm binoculars.

I wonder if the reason is a throw-back to the times when certain sources continued to insist that for "astro- viewing", one must essentialy seek out the largest aperture possible, with the largest exit-pupil?

It's not only higher magnification that (typically) comes at the expense of narrower fields of view, but also larger objectives.

Consider all the 7x35 binoculars that have AT LEAST 9 degrees TFOV, with some up to 13 degrees, then count how many 7x50s you've come across with a TFOV above 8 degrees.

Similarly with 10x50s, and especially 10x42s versus 10x or(10.5)x 70s, and 11x80s etc.

Most 42mm and 50mm binoculars are considerably more comfortable to hold too, due to less sheer bulk and weight.

Why not instead just get the best 10x50, or even the best 10x42 one can afford, with at least 60 degree AFOV?

Kenny

#18 TCW

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Posted 22 April 2014 - 06:15 PM

Some people insist that their 100mm refractor is vastly superior to scopes with 10x or more the light grasp and that is their prerogative. As far as wanting more light gathering capability in binoculars it is not mysterious at all. More light is what it is all about! I never said I wanted a 13 degree FOV but a wide apparent field of view. I don't see hand holding a 70mm pair of binoculars as a problem but I do think that 10x or so is the upper limit for hand holding and keeping them steady.

#19 Jon Isaacs

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Posted 22 April 2014 - 06:54 PM

Some people insist that their 100mm refractor is vastly superior to scopes with 10x or more the light grasp and that is their prerogative. As far as wanting more light gathering capability in binoculars it is not mysterious at all. More light is what it is all about! I never said I wanted a 13 degree FOV but a wide apparent field of view. I don't see hand holding a 70mm pair of binoculars as a problem but I do think that 10x or so is the upper limit for hand holding and keeping them steady.


I think Kenny's post raises some important points, one's definitely worth considering. Here's a few to consider:

- In many/most situations, the difference between a 5mm and an 7mm exit pupil is relatively small, approaching zero. The advantage of increased aperture is most often that you have greater magnification at the same exit pupil..

- For many observers, particular older observers, a pair of 10x70s is a pair of 10x50s..

- 10 x 50s typically weigh about 2 pounds, 10x70s like the Fujinon's or the Resolux are around 5 lbs. Again to state the obvious, Hand handing a 2 pound binocular is much easier than hand holding a 5 pound binocular..

- The TFoV is generally about 1-1.5 degrees larger with the 50mm binoculars.

Bottom line: For many observers, for many situations, if you buy a pair of 10x70 binoculars, what you are getting is a pair of 10x50s that are heavy-heavy and have a relatively narrow field of view.

Bottom line number 2: More light is not what it's all about.. the right tool for the job is what it'a all about. Some/many of us, me for sure, have the capturing more light well covered. Hand held binoculars are about something else.

Jon

#20 Jon Isaacs

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Posted 22 April 2014 - 06:59 PM

That's one I've never heard of Jon, but I just tried it and it comes out the same as what I use.

I just divide the FOV in Feet by the Constant of 52.5 to go directly to degrees.

In your example 341/52.5 = 6.496°

I'm just a Simpleton:-)


Chuck:

It's straight forward.. dividing by 3000 feet gives you the angular field of view in Radians, multiplying by 57.3 (Pi/180) converts it to degrees...

What you are doing is dividing the 3000/57.3 = 52.6 and doing it all in one operation...

Jon

#21 TCW

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Posted 23 April 2014 - 06:13 PM

Is anyone familiar with the Celestron Nature DX 10x56 and 12x56 binos? The FOV is pretty good although the objectives are a little smaller than I am looking for they do look attractive. Celestron does not give a weight - wonder why?






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