I went to the FlashBus yesterday, an around-the-US tour by two of the big names in small-flash photography: David “Strobist” Hobby and Joe “Numnuts” McNally. David is a manual shooter and Joe is primarily a TTL shooter. The dude sitting next to me, and several others at the seminar didn’t seem to get the distinction between Manual and TTL flash usage. And unfortunately this about the only part of the entire day where I felt the explanations were weak.
Manual is what it says: everything is manual. You have to tell each flash how much light to dump when it is triggered. You can adjust the flash by changing its power directly on the device, usually by fractions of power like 1/128, 1/64, etc or by using a CLS remote unit but telling each flash to be manual. The Elinchrome Skyport system also lets you adjust Big Flashes in manual mode remotely. You’re just telling each flash exactly how much light to dump when triggered. How that light looks, is entirely up to you. Want more fill? Increase your fill flash a stop from 1/32 to 1/16 and see.
How do flashes get triggered? There are a couple of ways – you can PocketWizard each of them. You can also optically slave them so that a trigger flash, say a 1/128 speedlite on your camera, will cause the other flashes to fire. Nikon calls this “SU-4” mode. A CLS commander can also trigger the lights.
TTL, on the other hand, has a back-and-forth conversation between the camera and the speedlight that figures out how much power each flash will dump when triggered. You configure the lighting ratios on the commander: the key light is 1.0 (baseline), the fill light is -1.0 stop, the kicker light is +1.3 stop, and so on.
When you press the shutter release button the camera, a complex dance happens. A preflash is emitted. The speedlite looks at a message encoded in the flash, and then fires itself. Each of the groups fires in turn, the camera sees how much light is being produced by them, by using a sensor inside the camera getting the light Through The Lens (hence the name TTL). In conjunction with the ambient light, the camera’s exposure settings, and an N-thousand database of exposure scenarios, the camera figures out on the fly what the absolute power each flash needs to dump. “Group A, you’ll fire at 1/4. Group B, you’ll fire at 1/16. Group C you’ll fire at 1/2.”.
Once each flash is told what power they’ll fire, they’ll wait until they get an “OK! Go!” signal from the commander, and then dump their light on to the scene.
In manual you have complete control over every light. You could have 37 lights each with a different settings. It is more labor and mentally intensive configuring each light, but because you have complete control, there are no surprises from shot to shot.
In TTL you give up a measure of control to the computer brains in the speedlites and the camera in exchange for convenience. You just specify the ratios of light between different groups. You can move from dark lighting conditions to brighter, and system will compensate by telling the flashes to dump more or less light. The exposures will generally look pretty good. You spend more time taking pictures than configuring lights. But then you also get to debug things when things go weird if TTL gets confused.