I recently purchased a bundle of Honl modifiers for Speedlites (grids, snoots, and gels), to use on my 600EX-RT flashes. I wanted to determine the optimal zoom setting for them (from the available 20-200mm zoom settings on the flash head), and took the opportunity to test them out, along with a couple of diffusers.
The setup was a 600EX-RT on a light stand, 5 feet from a neutral-colored wall (measured distance between front of flash head and wall surface). Images were captured with a 1D X and 24-70mm f/2.8L II at 35mm, 1/200 s, f/8, ISO 200. Ambient light was nil, and the flash was triggered with an on-camera ST-E3-RT. Flash power was set manually, 1/32 power in most cases, but higher power was used for the diffusers (1/8 power for the Sto-Fen OmniBounce, 1/16 power for the Honl Heavy Frost filter). The dots on the wall are spaced 12" apart, so beam spread can be determined from the images.
First up is the bare flash. The 14mm setting is the built-in pull-out diffusion panel.
The Sto-Fen provides a pretty even distribution of light, and is able to effectively diffuse even the 200mm zoom setting.
The Honl Heavy Frost diffuser (part of the Color Effects Gels kit) is weaker than the Sto-Fen, and has some fall-off at the higher head zoom settings.
The shape of light from the snoots is obviously quite dependent on how you connect the Velcro bits and how you shape the end of the tube. With the 5" 'shorty' snoot, at 70mm and narrower zoom settings the pattern of the flash zoom head is apparent within the area projected by the snoot itself.
The 8" snoot gives a tight beam with no effect at long zoom settings, although light lost to the zoom mechanism is apparent wider than 70mm.
Unlike the snoots, the honeycomb grids deliver a nice, circular beam (although the grid must be flush and tightly attached for the beam to porject straight). With the 1/4" grid at zoom head settings wider than 70mm, there is a horizontal pattern evident in the beam. One online comparison that I found shows a smooth beam with no pattern; I think this is because those shots were focused on the flash itself (shot from behind) and not the wall onto which the flash beam was projected. At 80mm and narrower, the circular shape is affected by the shape of the light exiting from the zoomed head.
With the 1/8" grid, the horizontal pattern is evident at 35mm and wider, while at progressively longer zoom settings the falloff is faster and spill is reduced even further.
Get out there and shape the light!