I've seen product listings online for flash diffusers where they are described as opaque. Is this an incorrect description? Should it be semi-opaque or is that assumed? How does opaque make sense in describing a diffuser?
Opaque generally denotes something that blocks light, rather than letting it through. Definition 1 from linked:
- not transparent or translucent; impenetrable to light; not allowing light to pass through.
The proper term would be translucent. Semi-opaque is a bit of a misnomer, as it tries to augment a rather concrete term in meaning.
Frosted or patterned glass (as is found in shower doors for example) is often referred to as opaque. The light is scattered such that you can't clearly see through the glass, but the correct term would be translucent. Nevertheless, this seems to be common usage of the term opaque. Which, as the others have pointed out, is wrong.
Glass can be opaque to certain wavelengths of light (UV for example), so it could be correct to say that a diffuser was opaque (or semi-opaque) if it only transmitted certain wavelengths, but I doubt this is the case.
You can use a black panel or scrim on a flash to prevent direct light from hitting the subject. The effect is that only diffuse light hits the target.
I've been using white and black craft foam to do this with some of my shots. The amount of light that comes through the white foam is a significant drop - about 2.5 stops less. Depending on the setup, this can be effectively opaque.