TTL Flash

TTL flash automation works by firing a brief pre-flash before the main exposure. The camera meters the light from the pre-flash through the lens, evaluates the scene brightness, and calculates the flash power required to achieve a correct exposure. The main flash then fires at the calculated power level during the actual exposure. The entire sequence happens in a fraction of a second and is invisible to the subject. TTL allows the photographer to shoot in varying lighting conditions and at changing distances without manually recalculating flash exposure for each frame.

Each camera manufacturer implements TTL using a proprietary protocol. Olympus and OM System use the Olympus TTL standard, sometimes labelled O-TTL or MFT TTL. Panasonic M43 cameras use a compatible protocol. Third-party flash manufacturers like Godox and Nissin produce M43-specific flash units that implement the Olympus TTL protocol, labelled with an O suffix. Using a flash with the wrong protocol will typically result in the flash firing at full power or not at all, rather than correct TTL exposure.

TTL flash exposure can be adjusted using flash exposure compensation (FEC), which shifts the flash output up or down in stops relative to the TTL calculation. Positive FEC brightens the subject; negative FEC reduces flash contribution. FEC is useful for fine-tuning the balance between flash and ambient light — for example, reducing flash to -1 stop to produce a more natural fill light rather than the full TTL output. Most M43 cameras set FEC independently of the main exposure compensation, allowing the two to be adjusted separately.