So, what are the A and AA modes are on the SB-800 and more recent flashes?
Well, first, there are actually three non-TTL modes that use this technology: 1) A mode, flash in hot shoe, 2) AA Mode, flash in hot shoe, 3) Commander AA Mode, wireless flash in Remote Mode.
These are all 'automatic' modes on the flash that do not use TTL (Through The Lens) technology. Instead, these modes use a small clear window in the flash itself to determine when the reflected light from the subject is correct. Take a look at your SB-800 from the front. You will see the small round clear window on the lower left side pointed straight forward. Newer Nikon speedlights have expanded windows and sensors for further improved accuracy.
A Mode, Flash in Hot Shoe:
The A Mode with the flash in the hot shoe is the oldest and simplest technology used for non-TTL automatic flash control. I first used an 'Automatic' Vivitar flash in the early 70's, and it worked extremely well. This technology still works extremely well today in the SB-800 and newer models.
The SB-800 flash defaults to AA mode, which I will discuss in a moment, so to put the SB-800 flash in A mode you have to go into the menu on the flash and choose A mode. Press ON/OFF to exit leaving the flash in A mode.
The concept of the A Mode is very simple. The main flash pulse fires (turns ON) when the shutter is pushed, and there is a thyristor circuit inside the flash that measures the reflected light from the subject through the little clear window. The flash pulse stays ON until that reflected light accumulates to a certain amount, and then the circuit turns the flash OFF.
Of course the flash needs additional information about the camera settings in order to work. In the older Automatic flashes, you had to enter the ISO and f/ stop manually into the flash. With the SB-800 in A Mode, the ISO is picked up automatically from the camera through the hot shoe, but you still have to enter the f/ stop manually using the rocker switch on the back of the flash.
Then you simply aim and shoot. The flash takes care of the exposure automatically.
One very nice thing about the A Mode on the SB-800 is that there is no pre-flash pulse sequence to cause people or especially animals to blink.
NOTE: If you want to observe the pre-flash pulse sequence separately from the main flash pulse, simply put your camera in flash Rear Sync, and set a long shutter, say 1.6 sec, and push the shutter. The pre-flash sequence will occur at the instant the shutter is pushed, and the main flash pulse will occur at the end of the shutter sequence right before the curtain closes. This is handy for verifying what I am explaining about no pre-flash pulses when using A Mode.
AA Mode, Flash in Hot Shoe:
The AA Mode is the default mode for the 'Automatic' non-TTL flash modes. If your flash is sitting in the hot shoe, and you cycle through all the modes, you will see AA in the list unless you have changed it to A.
The AA mode is similar to A mode except for two things: 1) The flash automatically picks up both the aperture and the ISO that you have set on the camera, so AA mode is really fully automatic, and 2) A pre-flash pulse sequence is used.
To use AA mode, simply select it on the flash, point and shoot. The flash measures the amount of reflected energy from the Pre-flash through the small clear thyristor window. In this mode the flash does not used the main pulse to set the power.
The pre-flash is also used by the camera in AA mode to calculate the white balance.
Another useful byproduct of the fact that the AA flash uses a pre-flash is that you can use the Flash Value Lock (FV Lock) function. This helps with the fast blinkers when taking group shots.
In addition, there is another way use AA mode that eliminates the pre-flash sequence. You simply put the flash in SU-4 Mode, while in the hot shoe, and when AA mode is selected, it works by measuring the main flash through the thyristor window just like the A mode does.
Commander AA Mode, Flash in Remote:
The third 'automatic' flash mode is the Wireless AA Mode. The flash is set to Remote Mode and placed separate from the camera with the small round red IR window facing the commander. Then AA Mode is selected on the Commander menu on the camera. In this mode, the Commander sends a pre-flash pulse to the flash telling it to use its internal AA mode followed by the command to fire the main pulse. The remote flash also fires a pre-flash pulse which is used for setting its own power via the thyristor 'eye' as previously discussed.
So, when would you use these modes?
Flash A mode or SU-4 AA modes are useful when shooting pets. These modes eliminate the problem of pets that can close their eyes so quickly that they are closed by the time the main flash fires. The alternative is to use TTL mode and FV Lock. I use both techniques. Some animals are greatly bothered by the flash, so I use A or SU-4 AA mode just to reduce the flashes the animal has to put up with.
It turns out that Flash A and SU-4 AA modes measure the reflected energy quite evenly across the frame, so it is useful for group shots as well.
Flash A and SU-4 AA modes do a very good job of setting the flash to the correct power most of the time, but these modes still suffer from the same problems as TTL, i.e. the flash power is set a bit too high when the subject is black (or dark colored) and a bit too low when the subject is white (or light colored).
One 'gotcha' item to remember for both AA and A mode, especially if you use them in an umbrella, is that, you must always turn the flash body so the thyristor 'Eye' is pointed at the subject.