I was recently asked what kind of diffuser I would recommend for a Speedlight. Rather than just giving a quick answer, I thought I would give a bit of background information and instruction on the the subject. And rather than just email my response back to the person asking, I thought perhaps it was better to post it here so that others may benefit from the information as well.
Here is my response in a Q & A format:
Q. Why do people think that flash is "harsh light"?
A. Two reasons: 1) because of the "sharp shadows" it creates, and 2) often because of the location of those shadows - creating unflattering shadows on the subject due to the position of the flash on camera.
Q. What makes light harsh? (sharp shadows)
A. The "apparent size" of the source of light. The smaller the source of light, the sharper the shadows. I say apparent size because think of the sun. It is the largest source of light we have, yet it is harsh light because of how far away it is. So distance matters. The further away the light source is, the sharper it gets. But in general, a 5' Softbox is softer light than a 2' softbox which is softer than a flash with no softbox. (and, of course, a cloud covered sky is the largest softbox of all)
Q. Does the diffusion panel on the flash (or any other type of small diffusion accessory mounted on the flash) make it less harsh?
A. Not by itself, it does not! It can however .... if diffusing the light causes it to bounce off of walls, ceiling, floor, etc, thus creating a "larger source of light". But in wide open spaces (especially outdoors), a diffused light source that has nothing to bounce off of is still small and is nothing more than a weak, but still sharp, light source.
Q. So, does diffusing the light while it's on camera help?
A. Maybe a little. It's still coming from a bad angle. But if it causes some extra light from nearby walls or ceiling to bounce into and fill in shadows it can be helpful (so let's just say "better than nothing, but not by much").
Q. Are there better ways to diffuse light?
A. Definitely! Rotate the flash head and point it at the wall instead of at the person. The wall will turn the source of light into a much larger source of light. And it will be side light rather than straight-from-the-camera. And therefore will produce much more pleasing 3-dimensional shadows.
Q. Does a flash accessory such as the Gary Fong Lightsphere help?
A. Yes, with caveats (see above). Ignore their sales pitch. If you are outside it does absolutely nothing except waste light (and battery power!). If you are inside with nearby walls or ceiling to bounce the light off of, then it can be helpful because the Lightsphere sends light in all directions and those nearby walls and ceiling can, indeed, create a larger source of light. But ... consider the direction of your light. And, consider that the walls and ceiling may be colored (thus affecting the color of the light on your subject).
Q. What would you recommend instead?
A. Get the flash off camera. Have someone hold it or put it on a light stand. This gives you a more pleasing direction of light. Then use a softbox or umbrella to create a larger source of light. (for umbrellas, a shoot through umbrella is easier for me, but reflective works too)
Q. What is your favorite flash diffuser?
A. My current favorite flash accessory is the Lastolite Ezybox. It comes in two sizes, a 24x24" softbox or a 15x15" softbox.
Q. Why do you prefer it over an umbrella?
A. Well, I have to admit that an umbrella can create a much larger source of light, and that umbrellas are ultimately very portable (and the two biggest reasons why I shot umbrellas for many years). But the biggest problem with an umbrella is that it catches wind like no other. The tiniest breeze can knock over a light stand. I've broken dozens of umbrellas over the years. And sadly, I've also had several speedlights break (usually the umbrella falls on itself and cushions the fall, but every now and then the crash is hard enough that the speedlight breaks, usually off the hotshoe connector).
The biggest reason I resisted going to a softbox for such a long time was time and portability. It takes time to put together a softbox (all of the rods need to be inserted into the base and the diffusion panel put into place, etc). Such a time consuming process (compared to simply opening a closed umbrella) that I rarely take my softboxes apart. But a softbox that is assembled is generally not very portable (again, as compared to a folded umbrella, the assembled softbox occupies a lot more space and is harder to transport to an off site location).
Q. Why is the Lastolite Ezybox different?
A. Because, just like the fold up reflectors that you probably already have in your camera bag, it folds down flat and pops-up in an instant! No assembly required!!! You pop up the softbox and attach it to the mounting bracket (which I leave my flash attached to at all times) and violá, in mere seconds I'm up and running with a mini-studio regardless of location.
Q. Are there advantages to a softbox over an umbrella?
A. There is generally less "wasted light" because all of the light is moving forward. When using a shoot through umbrella, some of the light bounces out the back (I like this fact, and still use umbrellas in a room where the backward light can hit walls or ceiling and create a still larger source of light), but by having all of the light going forward the softbox can be more efficient, reducing recycle times and increasing battery life.
A softbox with internal diffusion is generally softer light than an umbrella which will still have a hot spot in the center. I typically use the diffusion panel on my high end strobes, but don't use the diffusion panel for my speedlights because outdoors I generally need all the light I can get (and my strobes are only used indoors and are more powerful / faster to recycle anyway). The Ezybox comes with the internal diffusion panel and you can choose to use it or not (I might use it if I'm indoors and not as concerned about recycle times).
Q. Are there any advantages that an umbrella might have?
A. Yes. The umbrella is much cheaper and, as mentioned above, can produce a much larger source of light. But did I mention wind and broken umbrellas? Yeah, the price difference will disappear quickly if you are shooting outdoors.
So I still use both - umbrellas and softbox. But if I had to pick one over the other, I would pick the Lastolite Ezybox.