Most speedlights come with a diffusion dome. This small white translucent dome snaps onto your speedlight and diffuses/softens the light. Or does it? Here is a quick guide when to use the dome, and what it really does.
A fundamental lighting principle states that the soft quality light on your subject is directly related to how large the light source is, and how close the light is to the subject. If I use a three foot diameter softbox and place it right next to my subject, I will get nice soft light. Move that softbox 20 feet away and take the same shot, and the light will be less soft with stronger shadows and contrast.
The diffusion dome cap’s ability to soften the light is limited by how large it is. The cap is about the same size as the flash head, so even if I place this near my subject, the light will have strong shadows and not be nearly as soft as a softbox. But the diffusion dome does some important things.
1. fill flash; if you are walking around a market shooting portraits with fill flash, add your diffusion dome. The dome takes the hard edge of the light; not really diffusing it, but takes the directional edge off the light which ‘softens’ the flash.
2. softbox use; if I am shooting my speedlight in a softbox, I use the diffusion dome. The dome will spread the light all through the interior of the softbox. When the light leaves the softbox, it is more even…i.e. no hotspots in the middle.
3. fill the room with light. Occasionally I will need to fill a room with light for an interior shot, and the diffusion dome will help with this task. The dome spreads the light in all directions.
There you have it. The diffusion dome is an important accessory for the speedlight shooter. Just know when to use it.