I recently got a Nissin Di622 (my first speedlight) and I've been been using the flash off-camera in slave mode (wireless optical triggering). However it doesn't always work, so I've been trying to figure out how to get it to work more reliably. I haven't done a lot of testing yet.
What factors affect performance? The sensor is the big red transparent panel on the front, right? I guess that points at the subject... Should I expect it to perform reliably as long as the distances between the speedlight and the camera flash and the subject are all within, say, 10m, and we're not in direct sunlight?
I saw this product:
What does this do that the speedlight doesn't already do? Is it more sensitive?