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The question of whether one should use TTL vs manual flash output is one that many photographers will experience at one point or another in their careers. Both methods have their advantages and disadvantages. The majority of flashes can shoot in manual mode (thought there are some that indeed can’t and there are also flashes that can do both). But not every flash can fire in TTL mode.
TTL communication requires specific pins on the camera hot shoe and flash to communicate and relay information about the exposure to make the two work together.
In general, TTL has been the king when it comes to photojournalism, weddings, events, and sports. But in situations where you are trying to mix ambient lighting with natural lighting, TTL can be a godsend and eliminate the need for specific metering that will need to be done. In my apartment, I sometimes like shooting a subject in front of a window. Evenly illuminating the subject while properly exposing the outside can be tough, but it is a challenge very easily done by using TTL metering.
Manual light output is typically used on editorial, portrait, headshot, commercial, and fine art photo situations where someone can take their time and set a scene up. It gives the photographer specific control over the light to make it look brighter or darker or exactly the way that they want it. In contrast, a TTL system will read your camera meter and adapt itself to deliver a result that you may not necessarily want.
Manual lighting also works best when working with large light modifiers as a TTL light can sometimes not work so effectively based on various parameters like how large a light modifier is and how far it is positioned from a subject.
Keep this in mind when you’re shooting, and be sure to also check out our massive lighting tutorial roundup.