To get an idea of the effect of stacking, I mounted a Godox V1 flash on a D70s body with a 19-35 Tamron lens set to 19mm. I then photographed a blank ceiling from a distance of about five feet. This sequence was shot with one mounted grid (upper left), followed by a second grid (upper right), until I reached a total of four grids (lower right). No effort was made to adjust the exposure because I was only interested in seeing the shape of the highlight and the quality of the shadow edge. It is not as tightly focused provided by a fresnel lens, it is nonetheless a very smooth and symmetric highlight, a useful capability to have tucked into a lighting equipment bag.
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If you're wondering about the apparent size of the highlight, you need to remember that the flash to background distance is shorter in the selfie sequence than that of the ceiling shots. I imagine that when I start using these grids in the field, there will be a lot of adjustments to the location and the size of the highlight before I get the desired results. I anticipate a lot of both chimping and head-scratching if I am to get the subtle splashes of light I can see in my mind's camera.