I did a few experiments with Gels last night and I'm very curious about how non-colored Gels are used.
I have the Roscolux gel swatchbook and a DIY gel holder for my 580EX. I've used colored gels and got the expected results, where all the light that passes through the gel becomes that color and you can color a scene, hair light or background this way.
Then I took a series of shots using some of the frosted, spun and silk gels and although some of them appear as if they would diffuse or soften the light I found that was not the case.
My setup was simple but perhaps I just don't understand the use and effect these gels are supposed to have. I used a single 580EX about 45 degrees off the camera's axis and had no other modifiers or lights. I used direct flash so I could get a hard shadow line and some specular highlights. The first shot was with no gel and then took 6 additional shots with various gels.
When I looked at the RAW files at 100% I couldn't see any difference in quality of light or the shadow line and the change I could see was that a couple of them seemed to have a color shift of some sort but that was the extent of the changes.
So my question is... How are these gels used? What is their purpose? What is the expected effect?