The True field of view of the binocular is equal to the Apparent field of view of the eyepiece divided by the magnification. Tfov = Afov / mag

If the Tfov is given as feet at 1000 yds., then Tfov in degrees is (Tfov in feet at 1000yds) / (1000yds x 3 feet per yd) = radians. Then radians x 57.3° per radian = Tfov in degrees.

If the Tfov is given as degrees then the Tfov in feet at 1000 is (Tangent Fov degrees) x 3000 = Fov in feet at 1000 yds.

Binocular Tfov Conversion Tables
given the measure usually printed on your binoc, i.e. 51M at 1000M, look up your field of view here.

But not all binoculars will actually measure what is specified or what is printed on the binocular. In fact many do not. So if you want to know the actual real field of view, measure it in the field by observing a star field.

A useful list for Binocular Observer's is this one with many star fields measured so you can determine the actual True Field of View of your binoculars
How To Measure TFOV Of Binoculars

Nice collection of constellation charts with separations printed on the charts is here
Stellar Separations for Determining Field of View
Chart of Sagitarius
Chart of Leo
Chart of Lyra

Southern Stellar Separations for Determining Field
Chart of Ara
Chart of Musca
Chart of Crux
Chart of Triangulum Australe

Bino FOV Circle templates for a wide selection of charts

Wide-angle binoculars can have narrow Afov eyepieces and Narrow angle binoculars can have wide Afov eyepieces. Except at the narrowest and widest extremes of True field of view, there is no standard rule you can follow.

Some binoculars do not actually measure in the field what is stated for Tfov. Usually the higher cost premium binoculars do measure as stated.

Half of all binoculars I measured have Afov eyepieces between 60° and 65°. They cross a range from narrow to wide True field of view. There are some but few wider eyepieces in use.

What may seem like a narrow 4° field of view at one power would be wide for a 15x or 16x binocular. A 3.5° field of view would be wide for a 20x binocular. On the other hand, a True field of view less than 3.0° seems narrow for ANY binocular.

Some wide Afov eyepieces have excellent performance. Fujinon 16x70 and Oberwerk 15x70 use 64° Afov eyepieces and are sharp to 80%+ Tfov. In comparison, Orion Giant 16x80 uses a much narrower 53° Afov eyepiece and its sharpness characteristics are poor in the outer 40% Tfov.

Some 8x binoculars with a 6.5° field of view may be considered the low end of a wide True field of view for that power binocular. This Tfov is produced with a 52-53° Afov eyepiece, not considered a wide Afov eyepiece. They are sharp to 80% of the True field of view.

Most of the 22x and 25x binoculars on the market are advertised as 3.0° Tfov, but are measuring less than stated. If any 22x or 25x binoculars measure over 3.0° Tfov, they are using eyepieces with Afov approx. 66° to 75°.

You will find that your eyes are not capable of seeing the edges of the filed in eyepieces with an apparent field of view greater than 65° to 70°. Some binoculars advertise 'eyepieces with 80° apparent filed of view.' Well the eye simply cannot use all that at once. The eye has limitations and generally for most people that falls in a range between 60° and 70°.

edz

Apparent Field of View vs. True Field of View
Original thread Confusion with Field of view Differences

Find more info at

Original Thread Apparent FOV vs True FOV

A lengthy explanation of True field of view and Apparent field of view resides in this thread. A selection of the main points pasted here:

Q
I've done a search on FOV and still don't really understand the differences between real TFOV and apparent AFOV.

As a more specific example, Pentax have two roof prism binoculars with specs as follows:
Pentax DCF SP 8x42 (TFOV 6.3 degrees, AFOV 50.4 degrees)
Pentax DCF SP 10x42 (TFOV 6.0 degrees, AFOV 60 degrees)

How does the 10x42 which has a narrower True FOV end up with an Apparent FOV greater than the 8x42? Does this mean a wider visual view through the binoculars (i.e. more sky)?

A
The narrower TFOV means it will see less sky. The apparent AFOV relates to the width of the image generated by the eyepieces. Dividing that by the magnification of the complete instrument yields the true TFOV in terms of actual degrees of arc of sky that will be displayed.

A
In your example the Pentax 8 x 42 has (TFOV)R.F.O.V. of 6.3 degrees

This is more usually described as TRUE field of view(TFOV)

What does 6.3 degrees actually mean ?

One degree can also be expressed as 52.4 feet / 1000 yards
Or as 17.4 metres / 1000 metres

What this means is that if you looked through a binocular with a ( very narrow )one degree TFOV , then at a distance of exactly 1000 yards from where you were standing you would only be able to see a stretch of land or object that was 52.4 feet wide . Anything wider than that would be out of the area which you can see through the binocular.

Or , to convert to metric measures , if you looked through the same binocular , then at a distance of 1000 metres from where you were standing , you would only be able to see a stretch of land or object that measured 17.4 metres wide.

So with a 6.3 degree TFOV , to get the equivalent figure in feet per 1000 yards or in metres per kilometre , you would multiply the figures quoted above by 6.3.

So at 1000 yards you would be able to see a stretch expanding 6.3 x 52.4 which = around 330 feet at 100 yards

In metric , 6.3 x 17.4 = around 109m per 1000m.

The Apparant Field of View simply means the TFOV as described above MULTIPLIED by the MAGNIFICATION.

So a 10 x 50 with a 5 degree TFOV provides a 50 degree AFOV

In this case , knowing the requirement is for a 5 degree TFOV with a 10 x power factor , the lens designer selects what is universally accepted as a 50 degree eyepiece of focal length sufficient to provide 10x magnification within that binocular system.

Thus is why telescope eyepieces are always described by their APPARENT field of view rather than by their TRUE field of view.

For example a plossl e.p usually offers around 50 - 52 degrees AFOV whereas a Televue Nagler can offer as wide as 82 degrees AFOV .

The True fields of views offered by such e.ps in a telescope depends on the focal length of the telescope which in turns affects magnification.

The greater the magnification the narrower the TFOV and the lesser the magnification the wider the TFOV.

A TFOV that would be considered annoyingly narrow for say a 10x binocular( for example 4 degrees TFOV )only really appears so because the AFOV would be only 40 degrees ,arrived at by multiplying 10 x 4 .

However if you had say a 16x binocular with a TFOV of "only" 4 degrees -- the AFOV would be 64 degrees(16 x 4), and as such would appear to be perfectly acceptable because the extra magnification is making up for the narrow field.

What is an "acceptable" AFOV depends very much on individual choice and experience. One can and often does "get used to" a particular AFOV.

Kenny

Field Of View
a discussion inthe eyepiece forum regarding the ability of the eye to scan around the Afov of various eyepieces from 30°Afov to 100°Afov. Also discussed is the range of the eye's ability to see resolution, see clearly, peripheral vision and extent of peripheral movement.