Guide number

The output of an external flash is indicated by its guide number (GN).
The larger the value, the stronger the flash output and the further the light will reach.
In the past, the maximum flash output in catalogues was shown using ISO 100 and 35mm-equivalent values. In recent years, however, the maximum value is matched to the narrow angle of the flash head when set to the telephoto end. So, if ISO 100 isn't standard on your camera, you won't necessarily understand the output just by looking at the guide number.

Synchonisation method

In addition to manual, where the photographer selects the preferred flash output, there is also auto, which consists of both external sync and TTL sync. External sync uses the synchronisation sensor on the front of the flash unit. It fires the flash in accordance with the set aperture value and stops firing at the appropriate output.
TTL meters via the usual method within the camera -- firing a preflash just before the actual flash to determine the flash output. The actual flash is then fired for capturing the image.

Wireless shooting

This is used to fire the flash remotely without attaching the flash to the camera's hot shoe. Your photos get a more three-dimensional feel if you can light the subject from a position away from the optical axis of the lens.

Bouncing

High-end external flash models have movable flash heads. Firing the flash toward a white ceiling or other white surface causes the light to bounce off the surface, providing a softer lighting of the subject.

Zooming

The angle of illumination can be changed from wide angle to telephoto according to the focal length of the lens.
For example, you can cut the peripheral illumination during telephoto shooting to focus the light on the subject for more efficient lighting.
The FL-900R is versatile because it's compatible with 24–200mm (35mm equivalent) lenses.

Auxiliary light

As well as illuminating the periphery of your subject with a bounce flash, you can add auxiliary lighting from the front for a flattering effect. The FL-900R/600R has LED light for video.

TTL Auto

TTL Auto is one of the flash control modes that provides outstanding performance and superb results thanks to the communication system between an Olympus camera and the flash unit. TTL stands for "through the lens". With TTL Auto mode, the camera measures the light through the lens and automatically calculates and emits at a flash level that best illuminates the shot.
By simply pressing the shutter button once, the flash fires (preflash) to measure the light levels in the shot and then fires again, based on the measurement, as the camera takes the photo.

The OM-D E-M1 Mark II has been updated to provide an even more accurate TTL Auto performance when used with the FL-900R.

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