Flash units are built into most digital cameras and often used when there is insufficient light to capture an image. The intensity of a flash unit is measured as Guide Numbers or GN. Therefore, a flash with a higher GN will indicate it has a more powerful flash ouput.

Flash and camera manufacturers determine the intensity of a flash using this formula.

How this GN value is derived is a complex process. All GN values can only be determined after conducting numerous flash exposure experiments. To arrive at these GN values, the subject must be well exposed within a fixed parameter. This is done by taking into account the distance between the flash and subject as well as the f/stop used on the camera.

Flash exposure is also dependent on the laws of physics. To obtain optimum flash exposure results, you must first understand the inverse square law that governs flash illumination. As you can see from the diagram below, light is spread out and objects furthest from it receive only a small proportion of the illumination.

The light from a flash falls off with distance.
When you double the distance, you get one quarter as much light.
This relationship is called the inverse square law

Simply put, the power of light diminishes rapidly because it spreads and thus a smaller amount of light hits the object. This also helps to explain the difference in exposure between objects or people near the camera and those further away.

The rate at which light falls off is described as the inverse square law. This law states that if the distance between the flash and subject is doubled, only one quarter the amount of light will reach the subject.

Guide numbers are usually calculated based on a setting of ISO100. But flash to subject distance can be expressed in either metric (meters) or imperial measurements (feet).

Flash Usage and Red eye

On traditional film and compact cameras, in-camera flash fired up close usually results in "red eyes" in portrait photos. This happens because light output from the flash unit occurs too fast for the iris of the eyes to close the pupil.

In low light and ambient lighting, the iris is dilated to allow more light in. When a sudden flash of light occurs, some of the light entering the eye will bounce off the blood vessels lining the retina. Once the light is reflected back to the camera, red eyes are often the result.

To remedy this, 'pre-strobe' or 'pre-flash' is often used, which is usually a series of flashes fired quickly just before the shot to try to close the pupils of your subject and thereby lessen the red-eye effect.

On the , red-eye pre-strobe by default is turned OFF and should you require this function, you can turn it on using the basic menu. Red-eye is only apparent when using in-camera flash. At a post-production level, you can use Sony’s Picture Motion Browser to remove red-eye in your subjects but the best way to eliminate red-eye altogether is to use an external flash positioned away from the axis of the camera lens when shooting.

Flash Options on

The comes with three flash options which are user selectable under the flash control menu and these modes can be activated for both in-camera and hot-shoe external flash.

ADI Flash Feature

Most lenses are equipped with a distance decoder or ADI (advance distance integration) feature. This enables the camera to accurately measure and transmit current subject distance information to ensure correct flash exposure in a variety of shooting situations. ADI works in tandem with the light metering system of the to measure both the pre-flash illumination as well as the distance to subject as recorded by the lens ADI decoder. Only five lenses in the system range do not have an ADI decoder. These are the 16mm f/2.8 fish eye (SAL16F28), 20mm f/2.8 (SAL20F28), 28mm f/2.8 (SAL28F28), 135mm f/2.8 [T4.5] STF (SAL135F28), and 500mm f/8.0 Reflex (SAL500F80).

In ADI metering, information from the distance decoder in the lens is combined
with the light metering information for more accurate flash brightness control

The ADI feature is extremely versatile and more accurate than traditional TTL metering as it takes into account the exact distance between the subject and camera before making the appropriate light output requirements.

ADI flash metering makes it easy to obtain beautifully exposed photos even when shooting against highly reflective backgrounds such as windows and mirrors or illuminated backgrounds at night such as cityscapes or sunsets. For the ADI flash metering is set at default whenever using in-camera flash. You can also use ADI with dedicated flash models from Sony such as the HVL-F56AM and HVL-F36AM.

Manual Flash Control

For maximum creativity, manual mode offers the most control over your exposure. In this mode, you can set the intensity of the flash by varying the power output. With the in-camera or external flash unit GN being an indicator of the flash's power and range, you can manually use the GN to calculate aperture setting and subject distance to get the desired exposure.

Manual mode is often used in a studio setting, where it allows a photographer to control the flash output in steps, from 1/1 for full power, ½ for half power right down to 1/32 for off or in-camera flash output.

Pre-Flash TTL Feature

Through-the-lens (TTL) flash metering was widely used in film based auto focus SLR cameras and today, nearly all DSLR cameras have this feature as a standard.

It works in a similar fashion to ADI flash with pre-flash fired just before actual image capture to determine the amount of light reflected back into the camera lens with one key difference—there is no distance information exchanged between the lens and the camera.

The camera's light metering sets the exposure based on the reflected light.

P-TTL vs. ADI Flash Metering. When shooting light colored subjects, against a light-colored or reflective background, P-TTL flash metering produces slightly underexposed results while ADI meter produces beautifully exposed pictures

Objects or subjects that are too bright or too dark can often fool the TTL metering into underexposing or overexposing your images. The photos above is a good example on how this can happen.