How to Measure AFOV (?) (1 Viewer)

I was reading this old thread: http://www.birdforum.net/showthread.php?t=146768

The AFOV is then: 2.tan^-1 (d1/2.d2).

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And http://www.birdforum.net/showthread.php?t=260248

John Russell said:

An alternative method is to mount the bin perpendicular to a wall with its eyepieces facing the wall. A laser (ideally green) is then shone through an objective and the extremities of its reach, left and right, are marked on the wall.

The AFOV is then 2.arctan D/2d, where D is the measured distance on the wall and d is the distance from the exit pupil to the wall.

John

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I have been trying to figure out how to use the formula but dont think I'm doing it right. I put my binoculars on a tripod, with the eyepiece facing a wall. I measured 25" to the wall from the eyepiece glass. I then put a light through the objective, and got a 27.5" wide circle on the wall. Could you explain how would I enter Johns forumla in my calculator? My binoculars (Bushnell legacy 10x50 6.5') should be close to 65' AFOV.



I have also done another method, where you face the objectives to the wall, and look through one eyepiece, while looking at the wall with your other eye. You then mark on the wall where you see the edges of the field stop, and note the distance between them, and the distance from the wall to the eyepiece. In my case, it would be 25"D to the wall, and 30.5"W between the points on the wall.

The formula for this is AFOV = 2*Atan(W/2/D)= 62.7'



But what I'd like to know is how to use the the formula for the light projection method.

Dan,
2.arctan D/2d

A very long time since I last did this stuff but I think this works. First do the bit in the brackets. (27.5/(2 x 25) = 0.55.
To get Arctan on the laptop's Scientific calculator click the Inv button followed by Tan which should give you 28.8 degrees.
2 x 28.8 = 57.6 degrees = AVOV if I've got it right.

David

I think how you enter ARCTAN (sometimes marked TAN-1) on a calculator depends on the model. Mine (old Texas Instruments TI-30Xa) requires punching a yellow button marked "2nd" and then punching the TAN button.

Plugging in your measurements of your Bushnell binocular to the formula yields an AFOV of 57.6º, however the measurement to the wall should be from the eye relief distance behind the eyepiece rather than the eyepiece glass. If the Bushnell eye relief spec of 18mm is subtracted from your 25" measurement that changes the AFOV to about 59º.

Edit: I see David posted first and gave you most of what you need except the subtraction of eye relief.

typo said:

Dan,
2.arctan D/2d

A very long time since I last did this stuff but I think this works. First do the bit in the brackets. (27.5/(2 x 25) = 0.55.
To get Arctan on the laptop's Scientific calculator click the Inv button followed by Tan which should give you 28.8 degrees.
2 x 28.8 = 57.6 degrees = AVOV if I've got it right.

David

Click to expand...

That might be right and I'm just measuring at the wrong points, but I dont think. Considering the AFOV should be closer to 65', and the eye method gives me 63'. I've done these with a few pairs of bins, and the light method (at least how i'm doing it) gives lower AFOV than expected for all of them, while the eye method is almost the same as the expect AFOV. So that's why I dont think this is how it's done.

henry link said:

Plugging in your measurements of your Bushnell binocular to the formula yields an AFOV of 57.6º, however the measurement to the wall should be from the eye relief distance behind the eyepiece rather than the eyepiece glass. If the Bushnell eye relief spec of 18mm is subtracted from your 25" measurement that changes the AFOV to about 59º.

Edit: I see David posted first and gave you most of what you need except the subtraction of eye relief.

Click to expand...

Perhaps the eye relief needs to be taken into account, but that still yields smaller AFOV than what you would expect using the old "Mag x FOV = AFOV" formula. Hopefully John will see this thread, I really want to hear his explanation. There isnt a way to PM him.

Here is a previous thread from 2011 about this issue: (Perhaps the "ultimate" thread? Or at least one of them. )

http://www.birdforum.net/showthread.php?t=198292

Bob

You could try the ISO 14132-1:2002 method described on the Nikon site:
http://www.nikon.com/products/sportoptics/how_to/guide/binoculars/basic/basic_08.htm

If the 6.5 degree FOV is accurate then the AFOV is 59.2 degrees which agrees with Henry's distance adjusted value.

David

The ISO method for calculating AFOV is only correct for a condition of zero rectilinear distortion. Pincushion distortion causes the AFOV to increase above the standard. Barrel distortion causes the AFOV to decrease below the standard.

I notice that Dan reported seeing barrel distortion causing a "fisheye" effect with this binocular on another thread. That would certainly result in an apparent field narrower than the ISO calculation, but genuine barrel distortion is rare in binoculars. A more likely condition would be one with almost zero rectilinear distortion. That causes the infamous 'rolling ball", which resembles a "fisheye" effect and might be what he is seeing. If that's the case 59º is about right for a 10x binocular with a 6.5º field. Of course it really hasn't been established that the magnification is exactly 10x or that the FOV is exactly 6.5º.

Henry