NOTE: For this post I am assuming regular flash sync - Not Auto FP High Speed Sync
NOTE: On some speedlights, including the SB400, SB700, and the Pop-Up flash, there is no selector for iTTL and iTTL-BL. To switch between these two modes on these speedlights, you switch the camera metering mode. Matrix and Center-Weighted force the speedlight into iTTL-BL mode. Spot metering forces the speedlight into regular iTTL mode.
DETERMINE THE AMBIENT LIGHTING CONDITIONS:Use your camera to measure the light! Here are steps:
1) Camera in Manual mode
2) Flash turned OFF
3) Fixed ISO 400 (not Auto ISO)
4) Aperture: f/4.0
5) Aim your camera at the area you want to measure
6) Adjust the shutter to zero the meter
The resulting Shutter speed then will indicate the ambient lighting condition you are in as follows:
1) Low Ambient: Shutter 1/30th or less
2) Medium Ambient: Shutter 1/30th to 1/250th
3) High Ambient: Shutter above 1/250th
CAMERA AND FLASH SETTINGS FOR EACH LIGHTING CONDITION
LOW AMBIENT:In low ambient conditions, your flash will be primary and essentially the only light on the subject. The ambient will contribute only to the background exposure.
Recommended Initial Settings: Camera Manual, Flash iTTL, ISO 400, f/4.0, 1/80th shutter.
The flash system will control the exposure of the subject no matter you do to the ISO, Aperture, or Shutter (within the range limts of the flash).
The shutter will primarilly control the background exposure. Increase the shutter to stop ghosting at the expense of a darker background. Decrease the shutter to brighten the background, at an increased risk of ghosting.
The aperture will primarilly control depth of field. Widen the aperture to decrease depth of field, increase background exposure, and increase flash range. Narrow the aperture to increase depth of field, decrease background brightness, and reduce flash range.
In high ambient conditions your flash will be adding FILL. This means the flash will be secondary to the ambient light in creating your images. The flash will brighten the shadows on the subject's face and clothing. The power of the flash must be adjusted to balance with the ambient light to make the subject equal brightness to the background ambient. You should use iTTL-BL flash mode to allow the flash to adjust itself automatically to balance the subject with the ambient.
If direct sunlight is hitting the subject's face, move the subject into the shade. If that is not possible, turn off the flash to avoid overexposure (blow out) of the subject's face.
Recommended intitial settings:
Camera P mode, ISO 200, iTTL-BL
This is the simplest setting I recommend at first until you more fully understand everything. These settings will add nice Fill to your shots.
Medium ambient is by far the most difficult situation to use your flash. It is also the time when if you use your flash right, it will greatly improve the quality of your images, but if you use it wrong, it will destroy your images.
In medium ambient you have to make several more decisions to determine the initial settings. You have to decide how you want to shoot:
Do you want the flash to be primary?
CAN you even make the flash primary?
Or do you want the flash and ambient to balance on the subject?
Or do you want the flash to be only light Fill and the ambient primary?
Or do you have to turn off the flash and shoot available light?
Then you might ask, why would I want the flash to be primary?
Well, the most frequent reason would be when the ambient light was some funny color, like bright mercury vapor lamps (gymnasium). The flash is the color of daylight, so you would then have a weird multicolored image since there is no single white balance that can be chosen. The only way to get proper colors in this situation is to either turn off the flash and use a white balance that matches the ambient or eliminate the ambient by making the flash primary.
A second reason you might want the flash to be primary would be if you are shooting a subject that is moving quickly (dancers, runners). You can eliminate ghosting and blur if you make the flash primary, since the flash will then freeze the motion because its duration is only about 1/1000th sec.
HOW TO MAKE THE FLASH PRIMARY:
To make the flash primary, you have to make the flash overpower the ambient light. This must be done with the flash in iTTL mode by decreasing the exposure by three stops or more with shutter and ISO from the ambient setting you measured with the camera meter above.
Example 1: Assume the results of your metering step above came out to f/4.0, ISO 400, and 1/60th. You would then recognize that you were in Medium Ambient Conditions, so you must immediately change the ISO to 200. This decreases the ambient exposure one stop.
Then increase the shutter by two stops to 1/250th (double it twice and pick the closest setting - 1/60th-->1/120th-->1/240th). Notice that the highest shutter speed you can select when in regular flash sync (on a D200) is 1/250th. Now, you have changed the settings so the camera will underexpose the ambient by three stops. This example is right on the edge of where you might have to turn off the flash and shoot available light or switch to TTL-BL and go for balanced fill. In fact, if your camera is a D80 it has a maximum flash sync speed of 1/200th, so you would not be able to fully overpower the ambient in this situation.
1) It is important to understand that when using the flash in regular iTTL mode, the flash metering system does not measure the ambient, so whatever flash power it determines is as if you were in pitch dark conditions. This causes the ambient to add to the flash on the subject and if the ambient is not totally overpowered, it can easily overexpose the subject. So, if you cannot totally overpower the ambient, and you still want to use iTTL mode, YOU MUST REDUCE THE FLASH POWER using the FEC button to avoid overexposure. The amount you must reduce it is a judgement call, making flash in medium ambient conditions that much more difficult.
2) In iTTL-BL mode the flash DOES measure the ambient, and then sets the flash to make the subject equal brightness to the ambient. This will only work well, however, if the ambient light is the same color as the flash (ie, daylight). If the ambient is something light mercury vapor, don't use iTTL-BL. Also, it is important to understand that TTL-BL will only work correctly when the camera is set for a normal ambient exposure. P mode makes this easy, since the camera will automatically set itself for a normal ambient exposure.
Note: On the SB-600, 800, and 900 Speedlights there is a mode selector to put the flash in either iTTL or iTTL-BL modes. ON the pop-up flash and the SB-400 and SB-700 speedlights, you control the selection of iTTL or iTTL-BL from the camera using the metering mode selection. If you select Matrix or Center-Weighted metering, you will get iTTL-BL mode. If you select Spot metering, you will get iTTL mode. It shows this in the display of the SB-700 but not the SB-400.
Example 2: Assume the results of your metering were ISO 400, 1/200th, and f/4.0. You should immediately see that you will have major problems reducing the exposure of the ambient by three stops. You can't double the shutter, because you are already near maximum flash sync speed. That leaves only ISO and Aperture. You can reduce the ISO to 200 and that gets you one stop, but the only way to get the other two stops is to stop down the aperture by two stops (f/4.0 --> f/5.6 -- f/8). But if you change the aperture to f/8, the flash range will become less than six feet or so (using a diffuser), so that may not be an option. In fact, if this is your situation, you are forced to abandon the idea of overpowering the ambient light.
You must now consider balancing the flash to the ambient.
HOW TO BALANCE THE FLASH TO MEDIUM AMBIENT CONDITIONS
If you find yourself in a situation like Example 2) above, you could decide to turn off the flash and shoot available light or you could decide to balance the flash to the medium ambient.
There are two approaches to balancing the flash to medium ambient conditions:
1) In iTTL mode you reduce the flash compensation manually by trial and error until the right flash power is determined. I am not going to discuss this method.
2) In iTTL-BL mode, the flash system will set itself automatically. This is the approach I will now discuss.
Recommended Initial Settings for allowing iTTL-BL to balance automatically with medium ambient:
Camera P mode, Flash iTTL-BL, ISO 200. (notice that these are the same settings as for High Ambient conditions).
This will give you nicely balanced images with bright backgrounds and bright subjects. You might want to apply some negative flash compensation, say -0.3 ev, to keep it from looking like the subject is jumping out of the picture.
You might be tempted to use camera A mode, and it can be done, but you have to be very careful to prevent overexposure. The shutter is limited to 1/250th (or 1/200th) flash sync speed, and since it can't go any higher than that, if you choose an aperture that is too wide, the shutter will bang into the limit and your images will all be overexposed. P mode fixes this problem.
Incidentally, among professional photographers the joke is that P mode stands for 'Professional' mode, because we all use it in bright ambient when shooting fill flash. For some reason, beginning photographers avoid P mode like the plague. P mode has its important uses, and shooting iTTL-BL is one of them (especially in bright ambient).
CONCLUSIONS:1) You must learn to use your camera metering to measure the ambient light
2) Low ambient situations are the easiest to learn to shoot flash. These are also the most common, and occur almost all the time indoors at parties, receptions, family time, etc. Use the recommended settings for exceptional flash shots every time.
3) High ambient light situations are also easy to shoot flash. This situation occurs every day outdoors during daylight. Let the wonderful iTTL-BL mode do all the work for you.
4) Medium ambient light makes flash photography very complicated, but the results are well worth it. This is the situation you have to really study in order to learn how to do it right.