Firefox said:
It acts as a diffuser. By allowing the light from the flash to be projected over a larger surface, it reduces the harshness of the shots taken..
A better unit would be the Lightsphere though... It's got a much larger surface area and further more, by projecting light over a curve surface, the resultant shadows are lighten more due to bounce over more surfaces..
Permit me to elaborate on your explanation.
First of all, some fundamentals need to be understood:
The 'hardness' of the light incident on your subject is dependent largely on the size of the light source in relation to the size of the subject/object being illuminated (this is not all there is of course - but for simplicity's sake, we can assume it is) . All things being equal, the larger source of light will look softer than a relatively smaller source of light when both are directed straight at the subject from the same distance.
So if you illuminate someone for a head and shoulder shot with light from a large umbrella 60 inches across versus say the light coming from your usual hotshoe gun flash, both placed say 1.5 meters away, aimed directly at the subject, the light from the umbrella will look less harsh simply because it's a larger light source.
Before I go any further, understand that distance is a vital factor as well. If you pull the umbrella'd light source say 20 meters away from your subject, you'd discover that the light is now a lot more harsh than when it was 1.5 meters away. Bear this in mind.
Whether a source of light is large or small is relative to the size of the subject as well. A large softbox might be a big light source to illuminate a pot of flowers for a still life. The same lightbox becomes a small light source placed next to a large boat for instance.
Now, with this as basic background knowledge, let's understand how the Omnibounce works:
The first thing that the Omnibounce does is that it modifies the directionality of the light source from your small gun flash. The root word "Omni" basically means "all" and used in this case, it means that it modifies the light from your flash to be thrown in 'all' directions - or more accurately in much wider 3 dimensional range. When you use your flash normally, the light is a lot more directional as you already know. It throws in a much tighter cone.
The second thing to understanding how the Omnibounce works is to know that slapping one on does not change the SIZE of the primary light. Your primary light source (your flash gun) is still pretty much about the same size as it was.
What's critical is to understand that it's the size of the combined INCIDENT light illuminating your subject that's important here. What the Omnibounce does is that in an enclosed space, because light from the flash head now travels in many directions, the surfaces they reflect off form secondary light sources. These, combined with the light coming from your flash form a much larger incident light source than directly from the flash alone.
Hence you can imagine that if you're in a small room with a low ceiling, there are many reflective surfaces for the omnidirectional light to bounce off. When light from all these secondary sources as well as light coming directly from the flash combine together, your subject is illuminated with a light source much larger than light coming from your flash alone. Hence, the appearance of 'softer' light.
With a lesser number of, or less efficient reflective surfaces, or no reflective surfaces at all, the combined incident light 'source' becomes smaller and smaller, and hence less and less 'soft' (or harder) in appearance. For instance if you go outdoors at night to an open field and try taking the same picture, you will now discover that the light from the Omnibounced equipped flash is a lot more 'hard/harsh' since there's no nearby surfaces for the light to bounce off to 'enlarge' the light incident on your subject.
This is a rather simplistic but a pretty reasonable explanation of how the thing works.
The Lightsphere works in the exact same way, giving you omni-directional light, with one difference.
By itself, it's already a larger light source than the Omnibounce (it's bigger in size, it has more surface area). So if you compare the combined sources of light incident on your subject with an Omnibounce and then a Lightsphere, the direct light coming from the Lightsphere comes from a bigger source (more surface area) than the Omnibounce, hence the final effect looks softer. If you repeat the head and shoulder shot experiment out in the open field at night, the Lightsphere will of course give you a slightly less hard effect than the Omnibounce for this reason.
Of course the other thing is that the Lightsphere has a clever trick of allowing you to unscrew it's top (and I think a lot of users employ it this way), so that in an enclosed environment with a favorable ceiling or wall, you can create a big bounce light with direct light from the surface of the Lightsphere.
Just remember these fundamentals:
1. Think of light as 'additive'
2. Think in terms of the total amount of light incident on your subject coming from both reflected and direct sources. How 'big' or 'small' is the combined light?
Try DIYing some of these omni-directional light modifiers. One option would be large Dutch Lady UHT milk bottles. At NTUC, you can get this (with a beneficial nutrition angle - the Banana or Choc ones are yummy) as well as an accordion type water holder for water-colouring which will stretch out to be about the same size as a Lightsphere. Attach some velcro and you should be done. It wouldn't look like a well finished product of course but hey, you can save the money over buying a commercial product, to go watch War of Worlds with your girlfriend, buy popcorn, a bag of chips, drink, and maybe even a knockoff Tom Cruise t-shirt for your friend from Queensway, and still have some change left. You can also just send me a check with the balance for the lesson learnt. I am afraid I cannot accept payment in UHT milk because I am lactose intolerant.