Film based SLR cameras
For film based SLR cameras, this is usually implemented by having a flash light meter in front of the film plane. The amount of light reflected off the film from the flash is metered, and the flash is turned off when there has been a sufficient amount of light for the desired exposure. See the illustration below.
Film based SLR camera with lens
In this illustration, the mirror is raised for exposing the film.
This generally worked well, at least as long as the subject was not too dark or too light, in which case you needed to manually adjust the flash exposure.
Digital SLR cameras (DSLR)
With digital cameras, this does not work well, since the imaging sensor, replacing the film, is not reflective enough. To overcome this problem, most DSLR cameras fire a pre-flash before raising the mirror, and then fire the main flash after exposing the sensor.
The pre-flash is used to determine the amount of flash needed for the exposure. With this method, the TTL flash meter is no longer needed, the camera's ordinary light meter is used. See the illustration.
DLR camera with lens
There are some DSLRs that still measure the amount of light reflected off the sensor chip, and avoid the pre-flash. The Fujifilm S1 and S3 does this.
As you know, Micro Four Thirds is a mirrorless camera system. The camera has no light sensor anymore. The imaging sensor is the light sensor. So to find the correct flash exposure, a pre-flash is triggered while the sensor is exposed. Then the camera must make the sensor ready for a second exposure, and fire off the flash with the correct amount of light. This typically takes a bit more time than with a DSLR. The DSLR used the separate light meter for the pre-flash, and could expose the main imaging sensor only once.
Here's a basic illustration of a mirrorless camera with lens. It is much simpler, since there is no mirror, pentaprism, or light meter.
Mirrorless camera with lens
Flash and pre-flash timings
To examine the pre-flash and main flash timings, I have video recorded operating the Panasonic Lumix GH1 and GH2 cameras. I also measured the Pentax K10D, which is an older DSLR from 2006. I turned off autofocus, to measure the flash delay only, and not also the autofocus delay.
I used 50fps when recording, which gives an accuracy of approximately 0.02s.
|Time to pre-flash||0.16s||0.12s||0.08s|
|Time to main-flash||0.12s||0.14s||0.12s|
|Total flash delay||0.28s||0.26s||0.20s|
The first timing is the delay from pressing the shutter until the pre-flash is fired. The second is the delay from the pre-flash until the main-flash. The third figure is the sum of the two first: The total delay from pressing the shutter until the main flash is fired.
You can see the recordings here. I uploaded them as 25p videos, so they are not as good for verifying the actual timings.
One could say that the GH2 improves slightly on the GH1 in terms of flash delay. However, the difference, 0.02s is not significant with my way of measuring. So we can only say that they are comparable.
When comparing with the older Pentax K10D, we see that the GH1 and GH2 perform almost as good. The difference between 0.20s and 0.26s is not very large. Probably, the autofocus speed is more important to the average user, and we have seen that both the GH1 and GH2 perform very well in terms of autofocus.
One way to avoid the TTL pre-flash, is to use a flash in Auto mode.
Now, we should not conclude that the extra flash delay with Micro Four Thirds cameras is exclusively a bad thing. As opposed to DSLRs, which have a limited number of light metering sensors, the Micro Four Thirds cameras essentially take one full picture to determine the correct exposure. This means that the camera has at least 12 megapixels of information available. It probably doesn't use all of this information. But what it can potentially do, is to use the information about where in the frame faces are, to enhance the exposure. Also, the camera knows which areas are in focus, and can make sure that these areas are properly exposed. The extra information the camera has can be put to good use to give you a better exposure.