Does the diameter of openings on photographic lighting grids affect brightness?

Question

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?

Answer 1

Yes, with the black straws, it is going to absorb most of the light that does not go directly through the straws. The smaller in diameter the straws, the more direct the light has to be traveling to get through and the more light will be absorbed.

If the straws were white, it would actually get brighter as the reflected light would be more focused, but it would also have more light reflecting from the edges, so it would be less co-linear and thus softer.

Also, be careful about getting the straws too close to the speedlite. At close distances, speedlites can get very hot and could actually melt the plastic.

Update: Ok, going back and seeing that the question was actually about not only diameter but depth as well. The closer to co-linear the light needs to be, the less light will get through. This will happen with length or diameter. Smaller diameter is going to mean slightly more material blocking light from entering at the entrance, though it's a fairly small amount of additional light being blocked.

Answer 2

You're right to a degree. If you consider only one of the cells of the grid, there is less source light available to be projected onto the target — the target "sees" a much smaller part of the light source through each of the cells if the cell diameter is smaller. At the same time, though, you are increasing the number of cells visible at the centre of the target by the same proportion when you make the individual cells smaller (discounting, for a moment, the effect the wall thickness of the cells will have), so the additional light loss at the centre of the projected pattern is almost nil for a very small source (like a speedlight or a typical 6- or 7-inch "grid reflector" for studio strobes). With larger sources (a tightly-packed grid on a very soft source, like a tightly-gridded beauty dish or the Kino Flo VistaBeam 600), a subject in the centre of the pattern may lose sight of the source through the cells at the edges.

The main difference between longer tubes/cells and narrower tubes/cells, from the photographers' point of view, is bulk. (Longer, wide tubes/cells also produce a slightly more ragged edge falloff and waste an almost insignificantly larger amount of light.) Once the grid is deep enough to be mechanically stable, there's not a whole lot of sense in making it take up more room than it has to, and it makes the manufacturing easier if the edge rings/frames are all the same for all of the different projection angles. The unit you are describing is essentially the same as the Honl commercial product, and it uses smaller straws (1/8" rather than 1/4") in the tighter grid mostly so that the surround and the velcro bits can be the same on both models.