Thank you for this post Joachim, I have a good interest in optics but sadly no formal training and little detail to my understanding. I think/thought I understood that in binoculars, the field stop is generally between the prism and the ocular lens in one way or another, and that in cheaper / simpler instruments it might inherently be the prism or it might be another essential lens, but in better instruments it will generally be a purposefully placed stop that helps control stray light further? Apologies if my question is more confusing than any possible answer. But that one way or another, the field stop is there, either explicitly or implicitly.
Among many things I don't fully understand is the effect of focal lengths in binoculars... there is some variability there in the design as I understand it, and you trade off TFOV for control of chromatic aberration and certainly other things that are beyond my understanding thus far! IE - it seems that, say, the 42mm SF models have an empirically longer focal length than compact 42mm bins like the Monarch HG or the UV HD. This should imply that it is harder to get a large FOV but easier to control CA, and should result in less depth of field. But there are also those who state as a maxim that depth of field/focus depends only on magnification. As I thought I understood it, this cannot be the case - it must be dependent on focal length as well?
In any case, it seems that a "magic" number looking at the market, for 6x bins is 30-32mm, several with very good AFOV, but a good FOV could be achieved down to smaller objective sizes. There are 5x25 bins with very good AFOV as well. I guess the formula you have presented would suggest that 5x25 might be the limit of low magnification and small objective diameter that can still, more or less, achieve a wide field. Smaller than that it becomes a trade off and a low magnification + wide AFOV implies either a larger instrument or, as you said, outsized prisms and eyepieces that do not make sense for the objective size.