AFOV-Confusion Due To New ISO Standard (1 Viewer)

Browsing through the new Nikon Sport Optics catalogue, I notice that Nikon have now adopted the ISO 14132-1:2002 standard for apparent field of view.
The 8x30 E II for instance has a FOV of 154m @ 1000m, which corresponds to 8,8°. We would up to now have defined the AFOV as 8x8,8°, i.e. 70,4° but the new ISO standard is that the AFOV is equivalent to viewing the 154m from a distance of 1000/8, i.e. 125m.
The angle corresponding to a tangent of 77/125 is 31,6° so the AFOV is now twice this, 63,2°, which is the value now quoted in the Nikon catalogue.
While Nikon's honesty is to be commended, I question whether this really makes sense.
AFOV is an attempt to quantify a subjective impression, much like wind chill factor. We know that values over 60° wil be experienced as wide angle and those under 45° as tunnel vision.
There also seems to be a chicken and egg element in that the magnification is used to define a value expressed in tenths of a degree.
How though is magnification measured? I have been unable to find an ISO definition for it as the international standards are not freely available. There are dozens of them appertaining to optics and each costs about 50 Swiss francs!!
Magnification is probably subject to minor design tolerances and also to variations across the field (distortion) so this new ISO AFOV standard seems somewhat absurd and is going to cause confusion if manufacturers use different bases for their calculations.

John

John Russell said:

AFOV is an attempt to quantify a subjective impression, much like wind chill factor.

John

Click to expand...

No, it isn't. It is defined! And it is no more "subjective" than the APPARENT luminosity of a star which simply does not take into account the distance.

There is no confusion by a new standard.

T

ThoLa said:

No, it isn't. It is defined! And it is no more "subjective" than the APPARENT luminosity of a star which simply does not take into account the distance. There is no confusion by a new standard.

Click to expand...

Tom,
There is confusion if Nikon are using the ISO standard, Zeiss the old standard, and Swarovski a mixture of the two (e.g. EL 10x42, FOV 6.4°, AFOV 60°; Habicht 7x42, FOV 6,3°, AFOV 46°).
I am not an astronomer but believe star luminosity to be a relative value with respect to a reference (Sirius?). A measured value would depend on the seeing conditions.
I am not aware if there are means of measuring AFOV but the values quoted by the optics manufacturers (with the the possible exception of astronomical eyepieces) are calculated values and the basis for the calculation, new or old is an integer (magnification), which may be subject to tolerances. If both values are imprecise we don't need a new standard to confuse us.

John

I agree with John that we are in for some confusion about apparent fields if and when other manufacturers follow Nikon and begin to change over to the ISO standard. To make things easier I intend to completely ignore the new specs and continue to use the simple approximation of multiplying real field by magnification, because I can do it in my head and I have a long established sense of what those numbers mean to me. For instance, using the simple method I know I'm going to be unhappily aware of the field edge impinging on the view if the apparent field falls very far below 60 degrees.

I would add that any calculation based solely on real field will be inaccurate for any binocular with pincushion distortion. That's a problem because most binoculars have pincushion distortion added intentionally to reduce the "rolling globe" effect. Pincushion causes the magnification to increase toward the edge of the field and by an amount that varies in different binoculars. Add to that problem unit variations in true magnification on axis (I think the ISO tolerance is 5%?) and just plain inaccurate specifications.

Fortunately it's quite possible to compare apparent fields using real binoculars rather than specs. Simply hold a barrel of one binocular to the left eye and a barrel of another binocular to the right eye. It's easy to see how the apparent fields compare by overlapping the two field circles. I used that method to compare my Zeiss 8x42 FL to an 8x32 FL. According to Zeiss the 8x32 field is 420" and the the 8x42 is 405". Since both are 8X the 8x32 apparent field should be larger using either method of calculation, but in fact, the two I compared had virtually the same size apparent fields. That's because the 8x42 has more pincushion distortion, which causes its apparent field to inflate from higher off axis magnification, just enough to match the 8x32. That particular 8x32 also had slightly different sized apparent fields in each barrel, not at all uncommon.

Just now I used the same method to compare two Nikon binoculars, an 8x30 EII and an 18x70 Astroluxe. According to Nikon's new ISO specs the 8x30 apparent field is 63.2 degrees (formally 70 degrees) and the 18x70 is 64.3 degrees (formally 72 degrees). The 18x70 apparent field should be slightly larger than the 8x30, but in fact it's slightly smaller and the right barrel is a little smaller than the left. Sloppy specs, inaccurate magnification or differences in distortion (the 8x30 has considerable pincushion, the 18x70 virtually none)? Take your pick.

I think apparent field is a very useful guide as to whether a binocular has a very narrow, narrow, medium, wide or superwide field, something real field taken alone does not reveal, but the exact specification should not be taken too seriously.

If the new standard does not change the rank order of binoculars from widest to narrow in apparent fov, I have to wonder what's the point? I am with Henry. I'll use the old way. If it ain't broke don't fix it.

Steve C said:

If the new standard does not change the rank order of binoculars from widest to narrow in apparent fov, I have to wonder what's the point? I am with Henry. I'll use the old way. If it ain't broke don't fix it.

Click to expand...

Trying to get people to quote an accurate and comparable number?

One can measure AFOV directly (and why the manufacturers don't do this is a mystery). We even have to tools at home now to do this: a digital camera. Digiscope an image though the camera then spend a little time calibrating the camera by photographing a tape measure at a known distance. A little math (involving tan theta ) and there you have it.

You can calculate it precisely too (note that's different than "accurately" - it doesn't include distortions) if you use the correct method (the ISO calculation) rather than the approximate method (the multiply method). The latter worked well enough when the FOV was 5 degrees (how long ago was that?).

Of course the accurate method gives smaller values than the approximate method so the marketing folks are thinking "to hell with that; quote the biggest number". And the way to fix that is for the customer to understand the difference and to ask how the numbers were determined.

The ISO are just trying to get people all on the same page. That's why we have written standards not just weakly applied rules of thumb. It matters rather less to us (amateurs) than it does to the pros who need to know when people are submitting RFPs their AFOV means the same as all the other makers AFOVs. They do that by requiring them to use the ISO methods.

Perhaps it's just easiest to forget about AFOV and compare the FOV and the magnification. You can see in this case of the Euro 3 + Nikon that they're all similar. So the AFOV will be similar. QED.

One other point of AFOV and measuring yourself is to consider the effects of eyeglasses you are wearing. In my case I have -3D sphere and -0.75D cylinder in my left eye and -3D sphere and -3D cylinder in my right eye. That extra negative correction for astigmatism my right eye increases the minification (the opposite of magnification) i.e. makes angular distances smaller in the right eye than the left. I knew this (technically) but didn't experience it until I started testing binoculars where the whole FOV is just visible. In some bins I can't see the whole field stop with the left eye but with the right eye the whole field stop is visible. This effect also lead to some fiddly exit pupil alignment issues or "curved exit pupils" and kidney bean blackouts. If I have a problem with a bin it will always be worst in my right eye.

If you "eyeball" the AFOV in a bin always use the same eye (unless you are sure both give the same magnification).

I have just completed a test after some quick, but not too deep, thought.

It seemed to me that if the exit pupil of the eyepiece were just an image of the objectives focal plane, then the converse would be true. The image in the objective would be the image of the eyepiece’s focal plane.

For an experiment, I took the 8x20 IF Zeiss compacts in my pocket and measured the true FOV at 6.76 degrees. I then turned them around and measured the angle of the exit pupil (AFOV ??) at 53.183 degrees while trying to keep the eye as centered as possible.

This appears to support Henry contention about magnification, 7.87x. I have measured the power of these in the past and I seem to remember about 7.8 or 7.9x but can not find my notes at this time.

It also seems to me that the smaller light cone of either focal length would limit either the FOV or AFOV.

Where have I gone astray with my assumptions?

Best,
Ron

Brilliant bit of lateral thinking, Ron.

At first thought I can't see an error in the logic.

And here was I just thinking that the only reason to look into a bin the wrong way was to look at insects!

it seems to be a timely topic:


http://www.cloudynights.com/ubbthre...1690/page/0/view/collapsed/sb/5/o/all/fpart/1


Maybe there is some cross-fertilization between those threads.

Tom