I am often asked about diffusers and brackets. Photographers mostly want to know what they do and which ones are best.
Well, both the bracket and the diffuser were introduced into photography to solve problems of the small flash.
The problems of a small flash, when used pointed directly at the subject, are that it makes harsh, flat light, that often causes 'Red-Eye', and, when turned to the vertical, casts a harsh shadow to the side of the subject.
In general, harsh light comes from a physically small 'apparent' source. An infinitely small source is called a 'point' source, because the light comes from a single point.
I use the word 'apparent' because the both the size of the source and its distance from the subject cause the apparent size to change from the perspective of the subject. For example, one of the harshest light sources is also one of the largest - the sun. It is huge - over 800,000 miles in diameter, but it is so far away 93,000,000 miles, that it acts as a point source to a subject here on earth. In other words, its apparent size is very tiny. This type of light casts very harsh shadows and makes facial features appear dull and lifeless.
The solution to harsh light is diffusion. Diffusion is the process of scattering the light from a point source so that it acts like a much bigger source. Under harsh sunlight, a translucent diffuser is recommended between the sunlight and the subject.
Think of the sun on an overcast day. The light still comes from the sun (effectively a point source), but the clouds scatter the light as it falls on the earth so that is becomes some of the softest light you can find. In effect, the clouds have changed the apparent size of the sun from a point source to a source the size of the entire sky. You can make extremely good portraits on an overcast day outdoors, where the facial features have nice texture and seem to come alive.
The shoe-mounted flash is also very small in physical size (about 3 square inches) and is an effective point source beyond a foot or so. Consequently it makes extremely harsh light that is not suitable for portraits without modification (diffusion).
Now, some people think that simply placing a diffuser in front of the speedlight will make it soft, but this is not the case, because that does nothing to increase the apparent size of the flash.
There are two ways to create diffusion when using a small flash.
1) Utilize a physically large 'retransmission' system or …
2) Bounce off large surfaces.
You can also use a combination of these two.
A 'retransmission' system is usually made from a large piece of translucent material that is lit by the flash that then retransmits the light evenly across its surface. The Gary Fong Light Sphere II is such a system. It looks like an inverted Tupperware bowl that is mounted on the flash. Then, the flash is pointed straight up and when it fires the whole translucent bowl lights up. In fact the cross section area of the bowl is about 16 square inches. However, in this case, more important than having about five times larger area than that of the flash itself, what makes it unique and very effective is that it will be using all walls, the ceiling and most surfaces in the room or studio to bounce off the light. This softens the light so that portraits made within about five feet are noticeably softer than those made with bare flash.
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