At various points in the pages on flash photography techniques, I specifically mention where and how one of the popular diffusion cups that clip over a speedlight, wouldn’t have helped, or would’ve given less satisfactory results.
Firstly, you have to realize that all flash modifiers do essentially the same two things
– they spread most of the light all around / upwards,
– they throw a small measure of light forward (to help avoid shadows under the subject’s eyes.
An important thing to remember is that there simply isn’t ONE light modifier that will give you the best results in ALL situations.
And if you do find one that works well in a certain situation, it should remain exactly that — a specific technique you you use in certain situations where it gives you the best results.
Using a diffuser cup, and the angle that you have your speedlight tilted and rotated at, needs to be an active decision based on the specific scenario you’re shooting under. Putting a diffuser cup on, and tilting your flash head to 45′ should never be a mindless default.
Some thought should always go into considering the direction of light that you want from your flashgun. In the same way that you wouldn’t often place your light source directly above your subject in a studio environment, you’d also rarely want to specifically bounce your light from the ceiling directly above your subject.
I have the diffusion cups that come with the Nikon SB-800 speedlights, and I have several Stofen diffusers for my Canon 580EX speedlights. I do use them, but mostly with a hole cut in the top of the omnibounce diffuser cup. The reason for this is that it allows me more control in where I want the majority of the light from the flash to be bounced from.
So these omnibounce and diffuser cups do come in handy for specific uses. I also recommend that any photographer who uses flash, have one of these handy in the camera bag.
However, there are some things to keep in mind with these diffuser cups.
Most importantly – the diffuser cup does NOT soften light per se.
It might scatter light, but in itself it doesn’t give you softer light.
The reason for this is that your light source is pretty much the same size with or without the diffuser cup. To get softer light, you need to have a larger light source. That is, a larger light source in relation to your subject and taking distance into account. The sun is a phenomenally large light source, but it is a zillion miles away, and hence a pinpoint light source and therefore gives harsh shadows.
Your flashgun is a small light source, and will therefore give harsh shadows if you just shoot directly without any thought. The only way to get softer light from a strobe, is to make your source of light larger either via an umbrella or diffuser panel, or bouncing the light off a wall or something.
If you’re using the diffuser cup indoors and bouncing your flash, the light from the strobe is scattered and bounces back from the walls and ceiling and that makes the flash light appear a bit softer. But, too much light is still coming directly from the strobe head itself, and that is why it will very often look harsher than flash bounced off other larger surfaces.
Outdoors, using the flash with a diffuser cup, makes very little sense, except for when :
– you need a wider spread of your strobe’s light, since you’re using a wide-angle lens.
(This is valid for indoors as well.)
– you’re working so close to the subject that you’re outside the range of which your strobe (sans diffuser cup) can give you proper TTL range. Then the diffuser cup will help in pulling your strobe’s output within a range that it can still give you properly metered TTL exposures.
Outdoors, “bouncing” at 45` with a diffuser cup, makes very little sense, except for this :
– using the flash in a bounce position gives you an additional 2″ height from your lens’ axis.
Maybe this helps you? Dunno.
– you specifically need that light fall-off from your foreground and up.
I honestly can’t think of any other reason to use the diffuser cup outside, especially in a bounce position. Using your flash outdoors in a 45` bounce position just needlessly eats battery power.
If you do so without having given it clear thought as to why, then you’re an idiot for trying to bounce flash off the clouds. There is nothing to bounce the flash off, and you’re just wasting your batteries, and shortening the lifespan of your speedlight by unnecessarily dumping a lot of charge every time.
Re: the 45′ bounce position with the Stofen diffuser – here is what Stofen says on their website:
Q: Why must I tilt the flash head to 45 degrees?
A: In Non TTL models this is necessary to avoid under exposure caused by light from the Omni hitting the external auto sensor of the flash. In TTL models it gives a better feathering wrap around of the light in the range from close to about 15 feet from the subject. Beyond that point with TTL we find straight on works OK for you.
Where I do use the flash in a near -upright bounce position with (and sometimes without) the diffuser cup, when there is no ceiling to bounce from … is when I am shooting indoors with the 70-200 and I want to reduce the risk of red-eye. That extra 2 or 3 inches might just help somewhat there in reducing chances of red-eye, by getting my flash even further away from my lens axis.
I also specifically use a diffusion cup indoors, when I am working close to a subject, and can’t move back – and don’t want my light to be top-heavy like it would’ve been if I had bounced without a diffuser.