Do you avoid the TTL system on your speedlite because you never know what the result will look like? It's an extremely complex system that intends to calculate the flash output painlessly, yet constantly photographers are frustrated by the seemingly random nature of it's results. Here's a few tips on how to increase the accuracy of your TTL system.
First, a little background on how TTL works. TTL (Through-the-lens) metering determines the value of flash output by sending out a pre-flash to expose the subject, and then uses that data to determine the final power to use during the exposure. This all happens extremely fast, which is why the gap between flashes may not be visible. There is some variety to the process, based on models, however this is the current standard of TTL for major companies. Much like the ambient light meter in your camera that you use to determine your exposure settings, the flash aims to expose your subject at an 18% gray value. Some systems also incorporate the subject distance based on the auto-focus information to determine where the subject is in the scene.
It sounds like there is plenty of information for the TTL to choose an accurate power setting, so why are the results all over the place? There are a number of things to consider, and thankfully most are controllable by the photographer.
The most important thing to realize, is that the system is always aiming for an 18% gray midtone result, just like a dSLR does in automated modes. To combat this, you use the same technique as when compensating for light and dark subjects in natural light. When you have a subject that is darker than the 18% gray, you must tell the camera to reduce the TTL output (essentially the Exposure Value compensation of a speedlite). Conversely for brighter subjects you want to increase the power.
Next, you have to consider the metering mode. If you are in matrix metering, be aware that the camera is trying to light the entire scene evenly through the use of the flash. This is where I see a lot of photographers getting overexposed results on their subjects, especially when in a dark environment. The speedlite is selecting a power amount to get the subject and background to it's desired exposure. By the time the background is properly illuminated, the much closer subjects are overexposed or completely blown out. Partial metering will give you a variety of results depending on what is in the center area of your image. This generally works well, until you are creating a composition where the subject is off-center. This goes for re-composing a shot after locking focus. While you retain focus on the subject, your meter is now making a completely different calculation. I will address this more later. Finally, spot metering chooses the smallest area to determine flash power. Based on the autofocus selection, the speedlite will aim to properly expose only 1-5% of the viewfinder area. As with Partial metering, re-composing an image will also change the flash value.
There is hope however, to still get accurate flash results when re-composing an image after achieving focus. When shooting in an automated mode (such as Aperture priority), the camera has an option called AE-L to lock in the exposure setting for a specific scene or area, prior to changing the position of the camera. There is also a flash version of that called Flash Value Lock (FV-Lock). This enables you to focus on the subject, hold the FV-Lock button to secure that information, and then adjust the composition. This works perfectly in scenarios where your subject is off-center or out of the focusing grid area. The main challenge that the FV-Lock presents is the speedlight will send out the pre-flash when you first lock the flash value, sometimes confusing subjects into thinking the image has been taken.
The last thing to consider is strong ambient light or reflections. Back lit situations will force the speedlite to be under powered, sometimes to the point where maxing out the increased flash compensation still won't be enough. This happens similarly when the flash hits a highly reflective surface such as a mirror. When the flash bounces off the mirror and into the lens, it thinks it has achieved a proper exposure much sooner. It is best to keep your flash from being visible in a mirror or reflective surface to ensure a proper exposure. Even after being fully aware of all this information, TTL may still be ineffective in certain scenarios. If this is the case, simply swap to manual. I hope this gives a greater understanding of how you can make TTL work for you in a more consistent manner.
Robert Hall is a freelance photographer in Southeast Michigan. His work primarily consists of weddings, commercial and editorial. He is constantly improving his skills through trading of techniques and critique with fellow photographers. Robert is always looking for new connections on social networks!