We know that the quality of light (softness vs. hardness) is based on the apparent size of the light source compared to the subject. Soft light, with blurred shadows or no shadows at all, results from a large light source. Hard light, with a well defined shadow, results from a small light source.
It is frequently said that in order to get soft light, the light source has to have a larger apparent size than the subject. It's hard to disagree that soft light would result in such circumstances.
What happens though if the light source is larger than a point source but is smaller than the subject? Will there be any softening of light? Initially, it may seem that with a small modifier, the shadow will be smaller but just as hard as the light from a point source:
However, the above analysis is actually incomplete because light from opposite edges of the modifier will also illuminate the subject. As a result, even if the modifier is smaller than the subject, the shadow edges will appear softer:
SG-3IR panel to avoid affecting the results). Here is the setup shot:
Test shot 1: bare flash at 105mm zoom. As expected, the shadow looks hard. The bit of blurriness at the edge is because the TV's bezel has a glass edge (which presumably caused a bit of refraction).
Test shot 2: bare flash with the built-in diffuser. As expected, the shadow looks just as hard as with the zoomed flash because diffusion by itself doesn't soften light.
Test shot 3: flash with the 19.5" reflective umbrella. The shadow edges have become a little blurred, evidence that the light has become softer.
Thus, we've confirmed that a light source that is larger than a point source can create soft light even if it is smaller than the subject. Of course, the smaller a modifier is, the less of any softening effect it will have
I'm not saying every small modifier out there is useful. There are a number of modifiers out there that are simply too small to have any material softening effect for portraits. Moreover, there are modifiers that are designed primarily for diffusion - but the problem is that diffusion by itself (without a material increase in the size of the light source) has no softening effect whatsoever. There are also some modifiers that do cause a slight increase in the size of the light source but are no better (in my opinion) than using a bare flash. Finally, please consider that although light quality is important, other aspects of light such as its direction and the highlight-shadow contrast may have a bigger impact on your photos.
Looking for a small modifier? You may be interested in a double bounce card, or better yet, these small umbrellas for a handheld umbrella.