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 raising the sensor and opening the shutter.
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. So there is no mirror, and no viewfinder prism. The camera also has no light sensor anymore. The imaging sensor is the light sensor.
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 the cameras using a Panasonic GH1. The recording was done at 50fps (to get the most detailed timing measurement), and at 1/50s exposure, so that I would not miss the flash firing.
Using this setup, I video recorded four cameras doing the flash exposure: The Panasonic GH2 and GH3, Pentax K10D, and the Canon EOS 400D. For all the cameras, I used manual focus when taking the test exposures, so that there would be no autofocus delay. I also set the maximum aperture, to avoid the delay of the camera stopping down the aperture.
During normal indoor lightning
Here are the tests, as recorded by the Panasonic GH1:
And the results:
|Pre-flash delay||460 ms||120 ms||80 ms|
|Main flash delay||580 ms||220 ms||180 ms|
Here, we see clearly that the GH3 improves upon the GH2, however, the results are still fairly similar. The GH3 achieves quicker flash activation, which I believe is because it has a shutter that operates faster. The GH3 also takes more pictures per second during continuous drive mode, another indication that the shutter operation is faster.
During very dim indoor lightning
Here are the tests:
And the results:
|1st pre-flash delay||340 ms||120 ms||60 ms|
|2nd pre-flash delay||none||none||160 ms|
|Main flash delay||400 ms||220 ms||260 ms|
The Canon EOS 400D was the slowest, taking 0.4 seconds from the shutter was pressed, until the main flash exposed the image. The GH2 was the fastest here, however, that was just because the GH3 decided to do two pre-flashes. This was probably done for better flash exposure accuracy. I would guess that it normally uses only one single pre-flash, in which case it would have been the fastest in the test.
Avoiding the pre-flash
The pre-flash can concievably be a problem. It can cause your subject to blink, for example. As long as you use TTL flash metering, be it with the on-board flash or with an original external flash, there is no way to avoid this.
However, if you have an external flash, it is very likely that you can overcome this by using the so-called "auto" mode. Auto mode for a flash means that the flash has a light sensor, which measures how much light is reflected off the subject, and shuts of the flash when there is sufficient. So only one single flash is needed.
To be able to do this, you need to tell the flash what aperture and ISO rating you are using. See this article for some examples of how to do this using older, legacy flash units. If, on the other hand, you have an original flash for the Four Thirds or Micro Four Thirds format, then the flash will read the aperture and ISO settings off the camera automatically. See my review of the Panasonic FL360 for an example.
The downside of this method, is if your subject is unusually light or dark. The flash has no way of knowing this, and will give you a wrongly exposed image. If you see the image come out too bright, for example, adjust the aperture rating (on the flash) up to a larger aperture (smaller f-number).
The GH3 has an impressively fast response here, only 0.06s delay from pressing the shutter until the camera fires the first pre-flash, twice as fast as the GH2. And the total delay is around 0.2s. On top of this, you will mostly add the autofocus delay. However, since you have most likely focused during composition anyway, this is just a matter of the camera confirming that the focus is ok. Using a lens with a fast AF motor, this will take a very short time, probably around 0.1s
In the second test, the GH3 did a second pre-flash, probably at a different exposure level than the first. This was done to probe how to best illuminate the subject. As a subject, I had a wall with very little contrast, so a normal subject will probably not require a second pre-flash.
When talking about cameras like this, most enthusiasts would say that you should not use the built in flash at all. The reason is that it is fairly low powered, meaning that you cannot expose people at a long distance when taking pictures indoor. Any distance larger than about two meters might be problematic, depending on the lens you use, of course, the larger aperture the better. Also, the flash is located quite close to the lens, which gives you a quite flat lightning
However, my experience when using the Panasonic GH3, is that it does a good job when photographing people indoor using the flash, both for portrait closeups (less than one meter distance) and groups of people. Of course, using a proper flash, like the Panasonic FL360, or the more recent predecessor Panasonic FL360L will give you much better flash images. But when you travel light and happen to need the built-in flash, don't be afraid to use it.