As a photographer when I talk about soft light I am referring to the effect the light has on cast shadows in the scene being photographed. In a portrait that is usually the shadow cast by the nose onto the cheek. This is as opposed to self-shadows, such as where the curve of the cheek turns away from the light into shadow. For the purposes of this discussion, I am limiting to cast shadows.
A hard edged cast shadow indicates a hard light. A soft gradient cast shadow indicates a soft light. The control over the range from hard to soft edge shadow transition is controlled by the size of the light as seen by the subject. And that means that the size is relative to the distance between the light and the subject. You can have a large softbox, say 3 feet by 4 feet (90x120cm) at a great distance and it will be a small light source in relation to or as seen by your subject. You need to bring that light in close to it is large relative to your subject. Too often I see people starting out with studio lighting get a large softbox or umbrella and set it up10 10 to 15 feet (3 to 4.6m) away from their subject and wonder why they are not getting a soft light.
I am sure that you have heard the above time and time again from a multitude of instructors, so it should be firmly ingrained now and we can move on.
Today I want to talk about soft light when a manufacturer of light modifiers mentions it in their product descriptions. No, not for big softboxes and umbrellas. We know they are soft when used properly. I want to talk about small modifiers that are described as "soft" in their marketing and advertising. I am not picking on the manufacturers or items discussed here. I am a fan of both Godox and Rogue and purchased all of the items being discussed. I have not talked to anyone at either company or received any equipment from them for this article.
I will start with the Rogue Diffuser Dome's description:
"Soften and Diffuse the Light from your Camera's Flash
"Rogue Flash Diffuser Dome: Snap the Rogue Flash Diffuser Dome to your camera flash to soften the light whenever you're shooting wedding and event portrait photography. The soft white dome helps to diffuse the harsh light from your flash, and the semi-spherical shape of the dome makes it easier on you when bouncing flash."
And the Godox Diffusion Dome, which is part of their AK-R1 accessory kit for round head flashes, such as the AD2000Pro that I am using for all the testing in this article. It isn't listed separately on the Godox website, but on the box it says, "Soft and bright light from various of directions."
So here we have two very similar items claiming soft light. Neither of them makes the light source larger than it already is. So how do they claim soft light? The dome diffuser is interesting in that unlike a flat diffuser, such as the Godox diffuser plate or a sheet of diffusion material that widens the output of the flash to cover the area of a wider angle lens, the dome does not make the output of the flash wider. Instead it softens the transition of the edge of the stamp of light from the flash. It has no effect on the softness of the light on the subject. The shadow cast by the nose is still a hard shadow from a small light source. But if the photo is wide enough to see where the light falls off around its edges you will see a smoother transition from light to darkness. So, yes, it softened the light. But not in the sense that most people would expect from the term softening.
So now you might be wondering, as I was, how the various accessories in the AK-R1 kit compare to each other. What does the Dome Diffuser do differently than the Diffuser plate for example. Following is a series of photos using the Dome Diffuser, Diffuser Plate, Four-wing Reflector (Barn Doors), Honey Comb (grid), and Snoot from the kit along with a couple of additional snoots. In all of the photos the flash head is 24 inches from a white wall that is about 5 feet wide. You can see the light pattern or light stamp of each modifier here. I am using the standard rectangular speedlight head (J), the included bare bulb head (A-I) and the optional H200R round head (K-W) with their various modifiers. This set of photos will be followed by another set showing some of the modifiers effect on a subject.
Some key things to notice here:
Compare K and M to see what the grid does to restrict the light pattern. Then look at K and O to see what happens when you put a diffuser in front of the grid. The diffuser negates the effect of the grid. And though you cannot see it in the photos, it also cost 2 stops of light. Don't put diffusion in front of a grid and expect to see any effect.
Compare K with P and Q to see that the Dome Diffusers light pattern is not any wider than the round head by itself, but the edge of the light stamp is a bit smoother. I have to assume that this is the "softening" of light that is referred to in the advertising for these products.
Adding a grid before the Godox dome (T) has the light fall off at the edges faster than the grid and Rogue dome (U).
You cannot see it in the comparison photos above, but the rubber snoot (L) has a great warning message on it.
The Rogue dome is much warmer in color temperature than the other modifiers.
The "diffuser discs" that come with the 5 inch dish do not spread the light much, if at all. Note that I tested these with the smooth side towards the flash tube and the textured side towards the wall.
None of the restrictors (snoots, grid, or barn doors) provide a sharp edge cutoff because they are all too close to the light source. To create a hard edge the light restriction (flag or gobo) with a hole in it needs to be far away from the light source and closer to the subject. The light source should have some restriction like a snoot or grid to prevent light from bouncing around the studio, too. The further away from the light source the restriction is the harder the edge will be. Experiment with different distances to find what works best for your situation.
Lighting the Subject
So, now on to the effect of modifiers on the subject. Still using the Godox AD200Pro as the light I picked a few of the modifiers from above and added in 3 softboxes for comparison.
Photos A, B, and C show the softboxes. In each the light stand remains at 48" from the subject so we can see a few things happening. A is the larger softbox and is closest to the subject due to its depth from the light to the front diffuser. The highlight on the lips and the catchlight in the eyes are larger and not as bright as in B and C. As the light source gets smaller and farther away the hot spots get smaller and brighter and more noticeable.
It might sound counter-intuitive, but the closer and larger the light source is to your subject the more control you have over bright highlights. These three photos also show that the closer the light is to the subject the darker the background will be. Comparing A, C, and D we see what happens when we go from a large close light to a small distant light. 
Then we move to the small light modifiers at 48 inches from the subject. There isn't much to say about them. D-K all have the same hard nose shadow because with or without the diffusion domes, the diffuser plate, or the grid the size of the light is the same. If there is no change in the size of the light there is no change in the shadow edge transition.
Moving the light in closer to 24 inches from the subject for L-S again shows no real difference on the subject. None of these compares in any way with an actual larger light as provided by a softbox or umbrella.
Diffusion Confusion
Diffusion spreads light out in a wider pattern. It does not soften light unless the diffusion is much larger than than the light source it is placed in front of and thereby becomes the new larger light source. Diffusers spread the light over a wider area to to work with a shorter/wider lens and they can interact with the environment to control the contrast in a scene. Contrast or shadow density or how light or dark the shadows are is controlled by the reflective surfaces around the subject. The walls, floor, and ceiling plus any v-flats or other reflectors around the subject kick light back into the shadows to make them lighter and if the surroundings are not neutral in color, they will also contaminate the color in the shadows. By spreading the light in a wider pattern, the diffuser lets the light reach more of the surroundings. Of course, if the surroundings are dark the shadows will be dark. Black cards or v-flats are used as negative fill to make shadows darker in a light environment.
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