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FOV and AFOV ? (1 Viewer)

Chris223

Well-known member
The aparent field of view (afov) is : magnification x Field of view.

So does it mean that for your eyes looking throughout a 8x bino with a FOV of 8° give the same "windows size" feeling that using a 10x bino with a Fov of 6.4° ?
 
The aparent field of view (afov) is : magnification x Field of view.

So does it mean that for your eyes looking throughout a 8x bino with a FOV of 8° give the same "windows size" feeling that using a 10x bino with a Fov of 6.4° ?

Yes, roughly and relative, but the formula is an approximation so the figures you get from the formula might differ with the actual specs/values you can find for many binoculars.
 
Let us put aside the "feeling" of what we are seeing for a moment which seems to me to be what AFOV is about.

Does AFOV mean that one will see a FOV covering more area with a 10x with a 6.4º actual field than one will see using an 8x with an 8º actual field? Because that doesn't make sense.

In reality what one will see will be larger in the 10x with a 6.4º actual field than it will be in the 8x but one will also see a field of view covering a smaller area than one would see when using an 8x with an 8º actual FOV.

That is how I understand it and it is also how I see things when looking through binoculars. I don't really give AFOV a thought.

Bob
 
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Yes but perhaps it is an optical illusion rather than a feeling.

Typo's remarks, and the other comments which he mentions in his Meopta Meostar B1 HD 12x50 review, were interesting:-
http://www.birdforum.net/showthread.php?t=333798
" I’ve mentioned a couple of times that I get the impression that the magnification is higher than 12x. There is nothing wrong with the specification. It’s at least in part an optical illusion which I’ve seen before with higher powered, wide view, sharp edged binoculars in particular. It’s something that has been mentioned by others on the forum before. If I narrow the view by positioning my eyes further away than the eye relief distance the enhanced magnification effect vanishes. It’s a rather seductive illusion all the same, if it works for you."

Not sure if the following images are a correct representation but, if they are, then perhaps they illustrate the illusion where, especially between the sharply-outlined wider-field '6 degrees at 10x' image and the narrower '5 degrees at 10x' version, parts of the subject might just appear to be larger.
 

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In reality what one will see will be larger in the 10x with a 6.4º actual field than it will be in the 8x but one will also see a field of view covering a smaller area than one would see when using an 8x with an 8º actual FOV.


Bob


Off course....what i mean is the perceived size of the tube you are looking througouht..

Tunnel effect vs more opened view (aka "windows") effect.
 
Chris223, I took it that AFOV expressed the relative size of the actual window, taking into account both magnification x Field of view, and was surprised to learn that it was an approximation. In practice funny things seem to happen to one's perception but I reckon it is a fair guide.

I reallize it is contradictory to say that, when concentrating on e.g. a particular bird, to me a smaller window (narrower field) sometimes also seems to make that bird seem bigger |:S|

PS I reckon the tunnel effect goes away by extending the eyecups until the image matches the margins of the view
 
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I don't really give AFOV a thought.

Bob

Bob
I don't either. If you use binos predominantly for looking at landscapes I can imagine AFOV being useful for giving you an idea how b.i.g the view through the binos will seem.

If you are more interested in how much land or sea or sky will be in view through your binos then the field of view as measured at 1,000yds or 1,000metres is the most useful for me. And it is more useful still if you treat this linear dimension as the diameter of the circle of view and calculate its area using the pi*r(squared) formula. This will often reveal that what looks like an ignorable couple of metres or feet difference in linear fov makes a significant difference to the area of the field of view.

Lee
 
Let us put aside the "feeling" of what we are seeing for a moment which seems to me to be what AFOV is about.

Does AFOV mean that one will see a FOV covering more area with a 10x with a 6.4º actual field than one will see using an 8x with an 8º actual field? Because that doesn't make sense.

In reality what one will see will be larger in the 10x with a 6.4º actual field than it will be in the 8x but one will also see a field of view covering a smaller area than one would see when using an 8x with an 8º actual FOV.

That is how I understand it and it is also how I see things when looking through binoculars. I don't really give AFOV a thought.

Bob

Hi Bob,

Maybe apparent FOV needs a new name. After all it's the real, not apparent, angle of the light cone projected onto the retina when the eye is aligned with the exit pupil of a binocular. Isn't that angle, not true FOV, the relevant one for sorting out whether a binocular should classified as a wide angle instrument? I think all of us would consider a 10x binocular with a 70º AFOV and a 7º true FOV to be a wide angle binocular. Would anyone classify a 7x binocular with the same 7º true FOV and a 49º AFOV as wide angle?

Henry
 
...I took it that AFOV expressed the relative size of the actual window, taking into account both magnification x Field of view, and was surprised to learn that it was an approximation...

It is an approximation because binoculars do not necessarily magnify the whole true field of view an equal amount across the field, and radial versus tangential magnification may differ across the FOV.

--AP
 
Bob
I don't either. If you use binos predominantly for looking at landscapes I can imagine AFOV being useful for giving you an idea how b.i.g the view through the binos will seem.

If you are more interested in how much land or sea or sky will be in view through your binos then the field of view as measured at 1,000yds or 1,000metres is the most useful for me. And it is more useful still if you treat this linear dimension as the diameter of the circle of view and calculate its area using the pi*r(squared) formula. This will often reveal that what looks like an ignorable couple of metres or feet difference in linear fov makes a significant difference to the area of the field of view.

Lee

Lee,

On the matter of "tunnel effect," I always see that in every binocular I use but I have learned to ignore it. The view is circular but there is solid black space surrounding it. I concentrate on what I am focused on and I brace the eye pieces up against and just underneath my eyebrows to help steady the binoculars.

Bob
 
Off course....what i mean is the perceived size of the tube you are looking througouht..

Tunnel effect vs more opened view (aka "windows") effect.

The "tunnel effect" is a good term to use for explaining the "feel". Most people don't like a view that feels like looking down a drain pipe. Too high AFOV might cause problems though, as you might not see the whole FOV without moving bins around a bit, especially if the eye relief is short or using glasses.
 
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Hi,

in my opinion the terms true and apparent field of view are well chosen - the true field designates how much of the landscape, sky or whatever objects of interest can actually be seen through the instrument while the apparent field is how wide the image looks in the eyepiece.

To get back to the o.p.question - yes, the afov is the "windows size" in your analogy, but the 10x pair will get your window quite a bit closer to the target, so you actually see less of it inside the window opening.

Joachim
 
Compare AFOV with normalized subject size. Let's say you want a bird in a nearby tree to appear the same size in both 8x and 10x binoculars. With an 8x binocular with a 7° FOV, you might have to stand 24' away, but with a 10x binocular with a 6° FOV you stand 30' away. Because your distance from the bird is smaller in the 8x, you see less background area (AFOV=56°) than you do with the 10x (AFOV=60°).
 
Steve,

What you describe is exactly the idea behind the simple/approximate definition of the AFOV, which is:
AFOV=magnification x FOV.
The current definition of the AFOV, which is used by most manufacturers, is based on an ISO standard and involves the tangent of the FOV [in deg]:
AFOV=2arctan(magnification x tan(FOV/2))
It generally yields figures that are smaller than those given by the approximate AFOV formula. An even more precise definition of the AFOV should take the edge distortions into account and I believe is used only by a handful of manufacturers (possibly by Zeiss and a few others).

Peter.

Appendix:

The derivation of the ISO formula is easy for those with elementary knowledge of trigonometry.
To see how the approximate formula is related to the ISO formula use the fact that, for small x,
tan(x)≈x
and similarly
arctan(x)≈x
Using these approximations in the ISO formula we get:
AFOV_iso≈2arctan(magnification x FOV/2)≈magnification x FOV=AFOV_approx
The approximation becomes worse as the magnification or the FOV increases. For a 10x bino with a FOV=6.5*, the approximate formula overestimates the AFOV by about 5*. As another example, for an 8x bino with a FOV=8.8* (such as the EII 8x30), the AFOV_iso≈63* whereas AFOV_approx≈70*.
 
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