I was kind of thinking about different grid options considering the depth vs diameter of individual openings and get to a interesting point (at least for me :-) ). Let's consider a very simple home made grid from some straws fitted on Speedlite. Straws are black and do not consider any reflections from its (inner) surface.
While it is obvious that making the diameter smaller (of each straw - or rather using smaller straws, since making it smaller would be problem :-) ) or making the straws longer, both will end up by making the degree of the light narrower.
But here comes the other thing. If you imagine a single opening (straw) with certain diameter, it will produce a circular spot on the wall. And every little point of the spot on the wall will collect brightness from every little point on the speedlite (that is shining through the one opening). So the question is: if I then make the opening (straw) smaller, there will be less little points on the speedlight allowed to shine through that smaller opening and therefore the spot on the wall will be collecting less brightness in total.
Good to mention, that I am talking about only the spot on the wall, that is lit by all little points on speedlight, so I am excluding the parts of the spot, where fading occurs.
So does that mean, that the lower the diameter will be, the less bright the "product of our grid" will be?