You don't need to know exactly what every modifier ever made does, exactly. However it really pays off to know what sort of patterns your favorite modifiers create. It speeds set up time greatly, and we never have enough of that, and helps you pick the right modifier for the effect you want. If you're at a bright location or working with speedlights, it's extra valuable.
Honl 1/8" Speed Grids and 8" Speed Snoots are some of my go-to gear. I carry them pretty much everywhere, and use them all the time. Both create circles of light, with snoot circles having a harder edge, and grids having a somewhat longer falloff. As a side note, I find that snoots do better firing back towards the camera (as rim lights or backlights) than grids, because they produce less lens flare.
For flashes, these days I rely heavily on my speedlights: LumoPro 160s and Canon 430 EXIIs. Both claim their zoom heads can concentrate their light output to match the field of view of a 105mm lens.
But what, exactly is the difference between a snoot circle and a grid circle? And is there a standard definition of a 105mm light pattern, or does that vary from manufacturer to manufacturer? I decide a quick test was in order, and the results are presented visually below, with discussion to follow.
There is a much larger version on Flickr (opens in a new window). From top-to-bottom: LumoPro 160 @ 105mm, Canon 430 EXII @ 105mm, Honl 1/8" Speed Grid, Honl 8" Speed Snoot Gray Interior, Honl 8" Speed Snoot Black Interior (gray, reversed), and Honl 8" Speed Snoot Gold/Silver Interior.
Flash to wall distance is 20 inches in all photos, flash power at 1/64. All were shot with an 85mm lens from about 5 feet behind the flash, with the flash heads always zoomed to the 105mm position. All the Honl modifiers were used on the 430 EXII.
The first thing that jumps out at me is that the LP 160 and the 430 EXII have pretty different ideas of what 105mm coverage means. The LP 160 is a lot more concentrated, with a much sharper falloff. The 430 EXII offer a much gentler transition, and what I would term over coverage - it covers a lot more of the frame, which is actually an 85mm lens 5 feet further back.
Neither is right, or wrong, I guess, though the LP 160 is closer to what I expected from a flash set to 105mm. The important thing is that I now know that if I need a seriously tight beam from an unmodified flash, I'm reaching for the LP 160. If I need a long transition area, I'm grabbing the 430 EXII.
Next up, the differences between the grid and the snoots. Things progress nicely through the next three examples. The grid produces a tight circle with a nice transition zone, the gray interior snoot produces a slightly smaller circle with a sharper transition, and the reversed gray snoot (black inside) produces a circle with the harshest transition from light to black.
Then we hit the gold/silver interior snoot and whoa! The color difference I expected, but not the almost double circle effect. It's a bright center, with a sharp drop to a dimmer secondary ring that has a looong transition to black.
It's dramatically different than the Speed Grid, or the other Speed Snoot variants. This isn't bad: it just means I'm going to be thinking about how and when I use Gold/Silver snoot differently from now on. Previously, I'd treated it as nothing more than a warmer, more reflective Gray Snoot. If I need a sharp drop to black, it'll stay in the bag, or be used reversed. If I need that double circle effect to put details into some shadows around spotlighted area, this baby will do the trick.
In fact, I have a belly dance performance to shoot where that will be awfully handy...
One final thing: notice how little loss of intensity there is in the center of the circles, vs the unmodified 430 EX II. Unlike light softeners like umbrellas and softboxes, these modifiers don't greatly reduce light intensity.