Flash photography basics

Using the flash is combining ambient and flash lights. Flash photography is easier in the dark and is more complicated with lots of ambient light.

Flash as main source of light - iTTL mode

Dim room with little ambient light (home at night). Using the camera manual mode, ISO100, f4 and 1/80th, a picture without flash is dark. With a subject 3m away and a background 9m away, a straight iTTL photo (no iTTL-BL) with the manual settings as before gives a properly exposed subject and very dark background. Using 1/10th, the subject is exactly the same brightness but the background is brighter (and blurred if you didn't use a tripod). As the subject was lit by the flash (typically faster than 1/1000sec), the subject is sharp. The background was lit by ambient light. So in a dim room, the flash exposure of the subject is controlled by the flash metering system and the background exposure is controlled by the settings on the camera. You can adjust them separately! This is the key behind using the flash in TTL mode and the camera in Manual mode.

iTTL system

The Nikon TTL flash metering system is independent from the camera one, even if they share the same sensor and they both need some other information from the camera. The TTL-BL system, in contrast, IS NOT. On iTTL mode, the flash handles the subject exposure and the camera the background. The Camera metering system does not take into account that the iTTL flash will add to the exposure of the subject. In S mode with a fixed 1/80th speed, the camera choses the same f/stop independently of whether the flash is on or not. So if the ambient light is already good enough for the subject, the flash will overexpose it. The Flash metering system just fires the preflashes and looks for the reflected light in a center weighted fashion regardless of which camera metering system you are using or what the camera f/ stop and shutter is set to. Whatever light reflects back from the preflashes plus the ambient from that area determines the strength of the main flash. So, you can reason that the ambient light reflected from the subject is essentially added to the exposure once by the Camera metering system and once by the Flash metering system and overexposure is often the result if the ambient light is strong. So when the ambient light is bright, it is usually best to turn down the flash using flash compensation to about -1.7 ev to start with. This will usually avoid overexposure on especially the subject's face.

The borderline

When the ambient lighting is a little brighter than this, you can still use this technique if you stop down the camera aperture enough to make the flash the primary light on the subject, which takes the ambient light out of the equation. But then your background becomes dark. And if you try to brighten it by decreasing the shutter speed, then the bright ambient light will begin to affect the subject brightness too, and you begin to risk overexposure of the subject. In this situation, shutter, aperture, and flash power all affect the brightness of the subject. It's usually best to simply live with a dark background in this situation and make sure the flash is primary on the subject.

Ambient light as main source of light (Fill flash, outdoor in daylight) - iTTL-BL

When there is a lot of ambient, the background and subject are no longer able to be adjusted independently, and camera manual mode becomes difficult to use. Switch the flash to TTL-BL and the camera to one of the automatic modes (S, A, or P). The camera will then measure the ambient light to set its aperture and shutter and send this information to the flash, after which the flash metering system will set the power of the flash to make the subject the same brightness as the background. If the ambient is extremely bright, like direct sunlight, then it's usually best to select P or S mode (and iTTL-BL), so the aperture will automatically stop down to avoid overexposure. In A mode, the shutter speed will increase to reduce the exposure, but it will be limited by the flash sync speed of 1/250th (D200) and that is often not high enough for bright light if a wide aperture is selected. However, if you really want the shutter to go more slowly, like on a tripod, for instance, all you have to do is switch to 'slow sync'. Then, the shutter will go as slow as it needs to to allow a normal exposure from the ambient. The subject must be darker than the background for TTL-BL to work properly. The flash can only brighten the subject to balance it with the background, it can't make the subject darker. If the subject is brighter than the background to begin with, then you probably are best off not using flash.

iTTL-BL (Through The Lens Back Lit) Fill Flash

Fill Flash is needed any time the background is brighter than the subject. Outdoors on a bright day, if you look closely at the subject's face, you will see shadows in the eye sockets, under the nose, and under the chin. When you shoot this type of shot, it will look much much better if the flash is used to 'fill' the shadows. Or if the background behind the subject is very bright, like sky. Again, flash should be used to 'fill' the subjects face and make it balance with the bright sky. Or indoors during the daytime, to shoot the subject in front of a window with a beautiful brightly lit scene behind, the flash should be used to brighten the subject's face to balance with the bright scene behind. What happens behind the scenes? Situation: Flash in iTTL-BL, Camera in P mode, matrix metering, and AF-S focusing.

1. Half push: Automated focus and camera measurements.
2. The data from the camera measurement are sent to the flash metering system. If the flash head is straight (i.e pointing as the lens) and no diffuser is used, the focal distance from the lens is also sent (only used with no flash modifiers).
3. Full push: Pre flashes for flash system measurements weighted heavily to the center where the subject should be.
4. The flash metering system (which resides in the camera) compares both sets of data and the focal distance if any to determine the amount of power required to make the subject's brightness equal to the overall scene brightness. The flash never tells the camera what power it chose, so for the camera the flash doesn't exist (except for the shutter speed limit).
5. The calculated power for the main flash is then modified by flash compensation that is set on the camera or the flash. This includes the dedicated flash compensation button on the camera, the camera ev setting, and the compensation setting on the flash itself.

For this to work the subject SHOULD NOT be brighter that the background, the flash should be its only light. Fill flash is supposed to be subtle, and when looking at the print it is usually best that you cannot even tell that flash is used at all. So it is usually best to turn down the flash compensation by -1.0 ev to -1.7 ev, so the fill is just enough to lift the darkest shadows on the face without looking obvious. Changing the camera exposure ev while using TTL-BL can produce some strange results, because camera ev affects both the background brightness and the power of the flash. If you don't like the brightness of the background, it is usually best to switch to camera Manual Mode, and set the f/ stop and shutter to give you the background brightness you want. A rule of thumb could be TTL for dark backgrounds, TTL-BL for bright ones.

Lifting shadows in daylight

The slight shadows on the subjects face can be 'lifted' with a very slight amount of flash in TTL mode. Don't use TTL-BL for this.

via: Nikon CLS practical guide