Flash diffusers: The good, the bad, and the ugly
Probably the most overpriced piece of crud in the history of photography, the so called diffuser dome can be found in almost any hotshoe flash
photographers bag. Ranging in price from a couple of dollars to 60 dollars for the "specially crafted" variants, they all share one thing common: people often do not know how to use them.
Let me start first by explaining what it is. The diffuser dome (hereafter referred to as the diffuser) is mounted on the flash, in order to spread light all over the room. This invention was neccesary after photographers switched to flash tubes on hotshoe flashes, where early photographers could just unscrew the reflector to achieve the same. Shooting with a diffuser is sometimes referred to as "shooting bare bulb".
This is also where the diffuser got it's name. It diffuses the light across the whole room. And here is where the chaos starts. Diffuse light means the light is going everywhere (does not have one general direction). However, many photographers use the words "diffuse light" to indicate a certain softness of the light. Here's the catch: light coming from a diffuser is not soft in any way!
Softness of light is dictated by the relative size of the light source. If the light source is small, a subject will cast a very sharp (well defined) shadow. This is called harsh light. However, if light is coming from an infinite number of directions, it will cast a softer gradient from areas of light, and areas of shadow, because more is illuminated.
A diffuser works on this by spreading across the whole room, essentially making the whole room your light source. This of course only works if you have a very big white room (also known as a studio), in which case you probably have something better than a diffuser to light your subjects.
Now we are coming to the most common misconception: a diffuser makes your light softer. No, it does NOT. Take a look at your flash bulb. It's not very big. Now pop on the diffuser and look again. Is it bigger? Yeah, a couple of millimeters. In other words: not field-relevant. What you need is a large light source, like a wall, to become your main lightsource.
So how do we define large? The sun is quite large yes? However, if you read carefully, you noticed I said "relative" a while back. The sun is still a very harsh light source, because it's further away. That's also key to getting soft light. The closer you position your subject to the wall you are bouncing from, the softer it will be, because it's relatively larger!
Very often, I see people with diffusers on their flashes, outdoors, thinking it will make their light softer. Now you know it does not. It actually has
one negative aspect as well. Because the light is spread everywhere, you will lose around 2 stops of light. 2 stops! That means if you are shooting at full power, your actual light output will be 1/4 power! You'd have to use 4 flashes with diffusers, or one without to get the same exposure outdoors.
You're wasting your batteries!
So in the end, are diffusers still worth buying? I'd say yes, but get the cheapest you can find. It can work in white/gray rooms, but remember light picks up the colour of the object it hits. If you use a diffuser in a blue room your subject will become a smurf.
So that's it. Now you know when to use a diffuser (and more importantly, when not).
One final note: you may have seen the lumiquest softboxes for on-camera flash. These are NOT diffusers. They create a bigger lightsource from your camera flash, and we all know what that means: (all together now!) SOFTER LIGHT, yaaay!
And that's our final bottom line. Try to create the biggest light source you can, any way you can. I shoved a large white paper bag over my flash at an event.