How does TTL flash work in manual exposure mode?

Question

I got the Metz 44 AF-1 flash recently that is TTL capable for my Canon 550D, and have been reading around to get a better understanding of using flash on-camera (Neil Van Niekerk's article on flash exposure compensation was particularly helpful in this regard). TTL seems to be a useful feature and seems to take care of flash power based on the exposure settings. From what I've read, TTL seems to provide fill flash when in the semi-automatic exposure modes (Av, Tv etc).

I was wondering how TTL works in the manual exposure mode, where we can choose to deviate from the camera meter reading for correct exposure - what exactly would TTL be doing in such a case?

The reason why I'm asking the question is that it seems flash photography involves some amount of trial & error if we want to control the lighting, and understanding the way TTL works in Manual Exposure mode would make the approach more methodical.

Note: It seems that Nikon & Canon differ in the way their flash systems & Manual exposure modes work - Canon allows only flash exposure compensation when in Manual exposure, while Nikon allows both Flash & regular exposure compensation.

Update: Found another bit of information that may be useful for metering purposes when going with E-TTL\E-TTL II on Canon -

There's never any "full-frame" flash metering in E-TTL or E-TTL II. In both cases, with EOS cameras that use a 45-point focusing system/21-zone metering sensor, all flash metering is carried out by the 17 metering segments within the Area AF ellipse shown in the viewfinder. Subject matter outside the ellipse is completely ignored in terms of flash exposure control.

Answer 1

When you use TTL flash in manual mode you set your shutter speed, aperture and ISO to values that would result in an under exposed photo if you didn't have a flash, the camera will then use the flash to add the exact amount of extra light needed for the photo to be well exposed.

Example: you are indoors and you set you camera to 1/200s, F/8 and ISO100 (at least in the room I'm in right now without a flash this will result in a black picture) the camera will set the flash power to get a well exposed image - repeat that with F/5.6 (one stop brighter aperture) and the camera will lower the flash power by half (one stop less flash).

The result is that you set your shutter speed to your sync speed (or slower), your aperture to what you want for depth of field and your ISO to whatever you like and the camera will make sure you get a well exposed picture in the settings you selected.

Answer 2

TTL flash works in exactly the same way whether you are using full manual or semi-automatic mode. Regardless of what you set the shutter speed* or aperture to, TTL will adjust the flash brightness to match the metering mode (spot, average, evaluative etc) and the flash exposure compensation setting as it would in shutter or aperture priority.

*as long as the shutter speed is at or below your camera's flash sync speed (1/200s on the 550D) - use High Speed Sync for faster speeds

Answer 3

I just want to add that shutter speed has little to do with the flash power metering if you exclude the ambient light factor (and most of the time you will shoot M mode in low ambient light). If you don't exceed the synch speed, no matter what is your speed setting, the flash power will be the same, because the actual flash is way faster than any camera speed setting. Even I can say, that if the synch speed was not a problem (and it is a problem only due to constructional issues), the speed will completely have no effect on flash power, because a flash speed 1/18000 will properly expose an image in every shutter speed setting, considering that most cameras highest speed is 1/4000.

Answer 4

I've done a lot of indoor photography at nightclubs and the like using a Nikon system. At first I used the TTL and let the camera decide what to do, however after a bit of practice i found out that what I really wanted to do was use bounce or diffuse flash so that I didn't get those nasty sharp shadows and red eyes. The TTL system wasn't able to cope with bounce flash so I switched to full manual.

It's probably the best thing I did to actually begin to really understand how the camera works.