Warning! Here's another boring optical thread, so read no further if that's not your cup of tea. 
It's seldom necessary to measure the true field of view of a binocular, but I was interested in another aspect which I'll explain later. Binocular manufacturers' specifications are usually quite reliable.
Amateur astronomers using various telescopes and eyepieces are often interested in angular fields of view and sometimes measure them with the transit times of a star across the field.
However, I think accurate results can be obtained terrestrially more easily and binoculars with their short objective focal lengths can be measured indoors.
To do so, a stable base (tripod) for the binocular is required and a long ruler or tape measure at a moderate distance. It's easy to find the maximum FoV by panning (vertical measure) or tilting ((horizontal measure). View through just one barrel.
The distance, "d" is measured from the binocular's objective lens to the target and if "w" is the measured width on the scale, the field of view at 1000 m would be 1000.w/d.
To get the value in feet just multiply by 3. Some marketing people have been known to convert metres into feet overlooking the fact that the FoV is then quoted at 1000 yds. and not at 1000 m!
If the angle of view is required that woud be 2.arctan (w/2d). For scopes with a narrow FoV, (360.w/2.Pi.d) would give a very close approximation.
To obtain the multiplication factor of a barlow lens it was merely necessary to divide the numerical FoV without the barlow by the FoV with the barlow using the same eyepiece.
On most Porro binoculars close focus is achieved by racking out the eyepieces but as few of them have very close focus capabilities one would not expect much influence on FoV. However. the objective lens of a 6x18 Zeiss monocular can be extended by 4 cm for a close focus of about 25 cm. The magnification is then around 9x with a corresponding loss of angular FoV.
On most roof prism binoculars the distance between objective lens and eyepiece is fixed, so close focus is achieved by an internal focussing element to reduce the focal length of the objective. Consequently I would expect a slight reduction in magnification and a slight increase in angular FoV at short distances.
Two of my binoculars, an 8x33 Kowa Genesis and a 10x42 Swarovski EL SV have close focus capabilities of about 1,5 m and I measured a REDUCTION in angular FoV.
Doubting the accuracy of my measurements I repeated the tests with the Swarovski (spec. 6,4° or 112 m @ 1000 m) and achieved 6,44° at distance and 6,28° close, corresponding to 112,5 m and 109,7 m respectively.
Both binoculars btw have negative focussing elements that shift backwards when close focussing.
Explanations anyone?
I know that one member here has a dynameter with which he can measure magnification, so it would be interesting if the the prediction of a reduction in magnification of a close-focussing roof prism binocular could be confirmed.
Lastly, for those interested in measurement of apparent FoV, here is an old thread A simple and precise method of measuring AFOV
John
It's seldom necessary to measure the true field of view of a binocular, but I was interested in another aspect which I'll explain later. Binocular manufacturers' specifications are usually quite reliable.
Amateur astronomers using various telescopes and eyepieces are often interested in angular fields of view and sometimes measure them with the transit times of a star across the field.
However, I think accurate results can be obtained terrestrially more easily and binoculars with their short objective focal lengths can be measured indoors.
To do so, a stable base (tripod) for the binocular is required and a long ruler or tape measure at a moderate distance. It's easy to find the maximum FoV by panning (vertical measure) or tilting ((horizontal measure). View through just one barrel.
The distance, "d" is measured from the binocular's objective lens to the target and if "w" is the measured width on the scale, the field of view at 1000 m would be 1000.w/d.
To get the value in feet just multiply by 3. Some marketing people have been known to convert metres into feet overlooking the fact that the FoV is then quoted at 1000 yds. and not at 1000 m!
If the angle of view is required that woud be 2.arctan (w/2d). For scopes with a narrow FoV, (360.w/2.Pi.d) would give a very close approximation.
To obtain the multiplication factor of a barlow lens it was merely necessary to divide the numerical FoV without the barlow by the FoV with the barlow using the same eyepiece.
On most Porro binoculars close focus is achieved by racking out the eyepieces but as few of them have very close focus capabilities one would not expect much influence on FoV. However. the objective lens of a 6x18 Zeiss monocular can be extended by 4 cm for a close focus of about 25 cm. The magnification is then around 9x with a corresponding loss of angular FoV.
On most roof prism binoculars the distance between objective lens and eyepiece is fixed, so close focus is achieved by an internal focussing element to reduce the focal length of the objective. Consequently I would expect a slight reduction in magnification and a slight increase in angular FoV at short distances.
Two of my binoculars, an 8x33 Kowa Genesis and a 10x42 Swarovski EL SV have close focus capabilities of about 1,5 m and I measured a REDUCTION in angular FoV.
Doubting the accuracy of my measurements I repeated the tests with the Swarovski (spec. 6,4° or 112 m @ 1000 m) and achieved 6,44° at distance and 6,28° close, corresponding to 112,5 m and 109,7 m respectively.
Both binoculars btw have negative focussing elements that shift backwards when close focussing.
Explanations anyone?
I know that one member here has a dynameter with which he can measure magnification, so it would be interesting if the the prediction of a reduction in magnification of a close-focussing roof prism binocular could be confirmed.
Lastly, for those interested in measurement of apparent FoV, here is an old thread A simple and precise method of measuring AFOV
John
