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As we've seen, when shooting with a flash, your camera does all of the things that it normally does when you're shooting. It uses its light meter to determine exposure settings to properly capture the ambient light in your scene. And just like when you're shooting without a flash, in your camera you have access to different metering modes that take different approaches to measuring that light that's in your scene. Like your camera, your flash probably has different metering modes. It has to do a lot of analysis to figure out how much light it needs to output.
And it performs this analysis by measuring the light in your scene and making some decisions about how much light there is, where light needs to be added to the scene, what exactly needs to be done to balance your exposure. Like your camera, your flash has different metering modes that take different approaches to measuring light. You can usually change the flash mode using controls on the back of the flash, but you might also be able to change them from the camera itself. Almost all flashes these days, whether an external flash or a built in flash, offer metering modes that are fundamentally pretty similar to each other.
More than likely the default advanced metering mode on your camera is some version of TTL metering. TTL stands for Through The Lens. This indicates that the flash will use the same meter that your camera uses for measuring its ambient exposure. In other words, the light that your flash is measuring will be measured through the lens of the camera. This is as opposed to the light being measured by some kind of external sensor on the flash itself. Now, this is significant because it means that we're actually measuring the light that will strike the image sensor rather than the light that's striking the flash somewhere up above the camera.
So, when you press the shutter button all the way down during a flash shot, the camera fires a tiny pre-flash just before it opens the shutter. If your flash is in a TTL metering mode, the light from that pre-flash bounces of your scene, comes back through the lens, and is measured by the camera's light meter. Where one flash system will differ from another is in the analysis of the data that is gathered during that process. Different vendors have very different ideas about what kind of analysis yields the best flash exposure.
Some flash systems will take your focus point into account and assume that it's sitting on something that should be your subject. Some flash systems will take distance information from the lens and include that in the calculation. The meter itself is gathering data from all over the frame. How it chooses to average and combine all of those readings, that's another part of this very complex calculation. This is a tricky problem. The flash doesn't know if you want just fill light or a full on key light, it doesn't know how much background you want to preserve, how much ambient light you want visible, how you want those things mixed.
That said, a good flash system can make very intelligent assumptions about these things and you'll have very good results automatically. You're flash might actually have multiple TTL modes. For example, this Nikon flash has a TTL mode that assumes that you want only a keyline on your subject. It makes no effort to preserve the background. But it has another TTL mode that works to balance the flash light with the ambient light in your scene, so that details are well-lit throughout the entire picture. Because TTL modes require a pre-flash, there's a very slight delay between the shutter button press and the opening of the shutter.
What's more, those flashes might, very rarely, cause some people to blink. It's more prevelant in children and animals and you won't encounter it that often, but it is a possibility. Because of this, your flash might offer a non-TTL mode of some kind. Now in one of these modes, instead of using a pre-flash, when you press the shutter button, the flash will be turned on and a meter in the flash will measure the flashlight that's bouncing back off of your scene. And then shut the flash off when it thinks enough light has been added. That eliminates the pre-flash, but it possibly creates less accurate metering.
Your flash might have a number of specialty modes, such as a multiple mode, which lets you configure the flash to fire multiple times during a single long exposure. This allows you to take a moving subject and freeze slices of it in different places through the frame. Finally, there will be a manual mode, which can also be very useful in certain situations, and we'll be looking at manual flash use in great detail later. There are many different flash systems and vendors often, regularly change the way modes work on their flashes.
Covering the workings of a specific system is just beyond the scope of this course. But your flash manual will have detailed descriptions of all of your flash's modes. And you should now know enough about how to decode the basics of what different modes are doing. All flash non-manual modes have to solve the problem of how much flash needs to be added and how much ambient light needs to be preserved. Automatic systems will fail in some circumstances. And throughout this course we'll talk about situations that might challenge your flash system. But for our first forays into shooting, you want to just stick with your flash's TTL mode.
In fact, you might not ever need to switch your flash out of its default TTL mode. Modern TTL systems are good enough that you can very likely rely on them for all of your flash shooting. Using them well, though, can require some thought and a few tricks. And we'll be looking at those throughout this course.
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