I did some testing of Honl 1/8" grid: Honl Speed Grid The tests show that the gird provides 24-25 degrees angle of view, which differs depending on flash zoom setting.
Grids allow you to control/constrain the spread of light. For example, in the shot provided, it's lighting just the model/mannequin's face. Without the grid, it might be lighting up the whole head/hair. There are endless applications where you would want to tighten the spread of light.
Thanks, Mike. We use the same principle in X-Ray (my profession), but I've never heard of it being done in lighting.
Many flash units have a zooming head, which is how they control the spread of the light. The primary purpose is so that you don't waste light by spreading it out farther than the lens's field of view. That's why a modern dedicated flash will zoom automatically as you change the focal length of the lens. The newest ones even know when they are on a crop sensor camera and adjust accordingly. One way to get creative with your lighting is to override the zoom and use a tighter beam that the lens can see. Or better yet, when using off camera lighting, you can control where & how large the spread of light can be. Grids are just one way to constrain the light. Another method is to use a snoot or barn doors. It doens't even have to on the light...you can use something to just block off some of the light...usually called a flag or Gobo.
I'll get the grid soon enough, i decided on the snoot though, because it can double as a bounce card.
You could also consider these as a cheaper alternative. I have them and they work just as well as anything else. Honeycomb Airflow Straightener
Depending on where you are look on Craigslist. We had a couple of stores go out of business and, they were selling some.
