The measurement you are talking about is also important, but that isn't the FOV. Field of view is a subjective measure of how much to the right and left and up and down you can pivot your eyes (or see peripherally) you can see in degrees.
Think about this for a moment though. The FOV in cardboard is a measurement of how much of the 180 or 360 degrees are included on the screen. If you change the value in the profile generator from 120 to 60 (or 60 per eye to 30 per eye) you're getting half the picture, quite literally, as the app will adjust the image size/crop (aka FOV) depending on the entered value. That's it. It is easy to check if the FOV value is correct because too low of a FOV value will in many apps result in borders and the image not covering all of the visible screen. I can tell for sure the values in this thread are pointless, because they don't fit Cardboard's standard when I try a similar relative value on my viewer.
The by far biggest problem I have with the FOV expressed in degrees the way we're doing it in this thread, aside from the hard to replicate measuring methods, is that when you need another abstract point of reference to make sense of your abstract point of reference, which isn't relevant to the given standard... it is just not a particularly good point of reference.
The measure you are referring to is a function of FOV, and the focal length of the lens.
The measure I'm referring to is the horizontal field dimension. It is the function of the combination of the focal length and the lens diameter, only expressed in a static value rather than a dynamic value designed to allow for perspective and distance.
In our mostly static medium though we don't need a dynamic value because it only adds confusion. We can however accurately measure the "static FOV" (or horizontal field dimension) with a precision of one or two millimeters. By adjusting the lenses to their nearest distance, with minimum distance between the eyes (IPD), we get the minimum value. Farthest screen distance with maximum IPD gives us the maximum value.
That being said, there is a general rule of thumb you can use. 35mm focal length is about a 5 inch screen. 40mm FL is about a 5.5 inch screen. 45mm is about a 6inch screen.
You can't use that as a rule of thumb for several reasons and you've got the focal length backwards, 35 mm focal length shows more of the field than 45 mm at a given distance.
Lens diameter works the opposite, 45 mm will show more field than 35 mm. The greater the lens diameter, the greater our perceptible FOV at given distance because the sensor or image plane in this case is our eyes and they cover 135 degrees so are obstructed by the lens diameter's relatively telescopic view.
The lens diameter itself doesn't represent FOV though because it's again a combination of focal lenght and lens diameter. You can't remove one from the other.
This means that you could measure the static horizontal dimension and just write down the lens diameter. Why? Because if two headsets both show a visible horizontal view of 120 mm, they will both have the same FOV, but because of the relationship between focal length and lens diameter, the bigger lenses will inherently have a shorter focal length and less of a telescope effect.
Basically doing it this way will allow you to determine the quality of the view in two easy to understand steps: 1) the horizontal tells you if the phone will fit the viewer 2) the visible lens diameter tells you which of the viewers that will fit your phone has the least telescope effect/most immersion.
I suggest you try this for yourself to see if you get any useful values from your viewers:
1) Download the On-screen ruler app and a pure black wallpaper for background.
2) Adjust the lenses to minimum field dimension (lenses farthest from your eyes with minimum distance between the eyes) and then take another measurement with the maximum field dimension (lenses closest to your eyes with maximum distance between the eyes).
3) Open the app. The ruler should calibrate automatically but make sure with a physical ruler just in case. Then open the black wallpaper in the Gallery or whatever else that will give you full screen immersive mode (no soft-key bar).
4) Flip the ruler to a horizontal, central position and with the phone in the headset and on your head as you'd normally view it, align "0" with the outermost left edge of your left eye. Then read the measurement on the outermost right edge with your right eye. Example.
5) Remove the phone and measure the visible dimension of the lenses. You should now have an easy to read value. If the field dimension is greater than the screen can cover and you're seeing edges, you simply eyeball with the help of the ruler and note how much of it is edges. For example, my Fiit VR gives me this value...
Fiit VR: 108-130 mm horizontal (approx 3+3 mm edges at max). 38 mm visible lens.
That's it. Now if someone wants to check if a headset's FOV fits they can simply use a ruler on their phone or a calibrated online service if they're looking to buy. Once narrowing down on the viewers that will give a optimal, edgeless FOV for their given screen size; they can narrow their choice down further with the lens diameter to eliminate telescope effect. Simples.
Now, try doing the same with one of the random FOV numbers. It is just not possible. They are numbers without meaning. You can't enter them into the cardboard profile generator and you can't use them to make an educated consumer choice. In essence...
Q: "What is the meaning of FOV?" A: "42...¯_(ツ)_/¯"
