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 the 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. 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 a very small physical size 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 putting a diffuser in front of the light 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, or about five times larger than the flash itself. This softens the light so that portraits made within about five feet are noticeably softer than the bare flash.
Another even better 'retransmission' system is a soft box. If you get one that is 36 inches by 48 inches, that's 1728 square inches of area. This is 108 times larger than the Light Sphere, and creates light that just as soft as the light on an overcast day outside.
The problem is portability. How big is too big to carry mounted on your flash. My personal feeling is the the Light Sphere II is about as large as you can conveniently handle under normal circumstances.
As you probably know, the SB-800 flash comes with a small snap-on diffuser. If you own an SB-600, you can buy an after-market snap-on diffuser from companies like Sto-Fen. Just looking at this snap-on diffuser, you can see that it is physically very small and the question is: does it work? It is obviously not much bigger than the bare flash itself, so it cannot increase the apparent size of the flash directly. However, think of this small diffuser attached to the flash and the flash pointed straight up. Then when it flashes, the light scatters to the sides, up, back, and everywhere but down. This means some of it goes directly to the subject, but most of it goes towards walls and ceiling where it can bounce, sending some of the bounced light back at the subject. The key is that the bounced light hits the subject at a different angle than the direct light. This different angle is what makes the apparent size of the flash much larger. So, there is a small portion of the light which is still harsh that goes directly at the subject and the rest that bounces off walls and ceilings is soft. The small amount of harsh direct light is what makes the 'catch-light' reflection in the eye of the subject and is not objectionable.
The reason the Light Sphere II works so well is that it is large AND it scatters light in all directions. This makes it work extremely well indoors where the direct light is much softer than the small snap-on diffuser, and the walls and ceilings all increase the softening effect more than with the small snap-on diffuser.
Now, outdoors, all the light that scatters from a diffuser is lost forever. It never makes it back to the subject. This leaves the subject lighted entirely by direct light from the flash which is still harsh because the apparent size of the flash is still small. So, outdoors, the only thing that happens if you use a diffuser is that your flash batteries run down quicker, because the flash has to flash at a much higher power to light the subject than normal. The bottom line is that outdoors, never use a small diffuser; just use the flash direct. However, the Light Sphere II is useful outdoors due to its large size which softens the direct light out to about five feet.
SHADOWS AND RED-EYE CONTROL
Flash shadows are always objectionable and you should always strive to eliminate them.
Red-eye is also extremely objectionable. It is caused by the flash reflecting off the retina of the eye and back into the picture. Red-eye is best addressed while taking the picture, but can also be somewhat corrected during post-processing. I will discuss how to avoid causing it in the first place.
The solution to the red-eye and shadow problem is to raise the flash higher above the camera while keeping it directly above the lens axis. This can be achieved with either a flash bracket or a flash diffuser that sits high above the flash.
The flash bracket attaches to the tripod mounting hole on the bottom of the camera and the flash mounts on its top directly above the lens. It raises the flash several inches higher than when the flash is in the hot shoe. Then, there is a cable that attaches the hot shoe on the bracket to the hot shoe on the camera, so that the camera thinks the flash is still in the hot shoe.
The main reason for raising the flash higher above the camera is to force the flash shadow down behind the subject where it can't be seen. The LS-II diffuser also raises the flash high enough to fix the shadow problem when the camera is in the horizontal orientation.
Also, by raising the flash higher above the camera, the angle of the direct light causes the red-eye reflection to fall below the camera lens. This simple step elimintes 99% of the red-eye problems. It is important to note, however, that the farther you are from the subject, the higher the flash needs to be raised to avoid red-eye. For instance, if you are using a telephoto lens, and you zoom tightly onto a person's face from 30 or 40 feet away, you risk red-eye.
So, the solution to red-eye is also the bracket, although with a DSLR, the flash is already much higher above the camera than with a point-and-shoot, so red-eye problems are less of a problem.
There is another very important feature of a flash bracket - it fixes the dreaded 'side shadow'.
When you turn the camera to portrait orientation (vertical), and the flash is mounted in the hot shoe, you cast a shadow to the right of the subject on a wall. This shadow is very unprofessional and is to be avoided at all cost. With a flash bracket either the camera or the flash rotates so that the flash always remains directly above the camera. By locating the flash above the camera, the shadow will now fall behind the subject and out of sight.
This is a disadvantage of using the Light Sphere II diffuser. The LS-II still casts the side shadow, although it is less defined as with a bare flash. Often when I want vertical orientation while I am using the LS-II, I simply shoot horizontal and crop to vertical later in post processing.
Another important consideration with a flash bracket is the direction you rotate the camera when going to to vertical. Most professional photographers rotate the camera counterclockwise so that the alternate shutter release button is on top and useable. But some flash brackets for some unexplicable reason force you to rotate the camera clockwise, so you have to be careful when buying. I use a Custom Brackets 'CB Junior' with my D200 and SB-800. It is relatively inexpensive at about $110 and works well.