Do you avoid the TTL (Through the Lens) system on your speedlight because you never know what the result will look like? It’s a simple system that intends to calculate the flash output painlessly, yet constantly photographers are frustrated by the seemingly random nature of its results. Here’s a few tips on how to increase the accuracy of your TTL system.

How TTL Functions

First, you’ll need a  little background on how TTL works. TTL sends out a pre-flash to expose the subject, and then uses that data to determine the final power to use during the exposure. This 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 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.

TTL EV Compensation

The most important thing to know is that TTL is always aiming for an 18% gray midtone result, just like a camera does in automated modes. To control this, you use the flash version of EV Compensation. When you have a subject that is darker than 18% gray, you must tell the camera to reduce the TTL output. For brighter subjects, you want to increase the TTL output. Most off-camera flash equipment allow for -3 stops to +3 stops of TTL EV compensation.

Metering Mode

If you are in matrix/evaluative metering, be aware that the camera is considering the entire scene when choosing the appropriate flash value of TTL. This is where I see a lot of photographers getting overexposed results on their subject, especially when in a dark environment. The speedlight is selecting a power setting to get the subject and background to “properly exposed.” By the time the background is properly illuminated, the closer subjects are overexposed or even blown out.

Spot metering uses the smallest area to determine flash power between 1-5% of the total viewfinder area. This metering mode is typically the best option for achieving desired results with TTL. This is because the metering area is tied to your focus point, and your focus point is almost always on your subject. Linking and unlinking your spot meter area to your focus point is an option in some cameras so if you see your spot meter area away from your focus point, search for that setting.

Partial/center-weighted metering gives mixed results depending on what is in the center area of your image. This works well if your subject is in the middle of the frame. But if your subject is outside of the center, then your TTL value will not be well-balanced with your subject. 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.

Flash Value Lock

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 — cameras have 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). Simply put your subject in the center of the frame, focus, and press the FV-Lock button. The TTL pre-flash will fire and store the power level, and you are free to re-compose and snap the shot. 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.

Setting Concerns

The last thing to consider is strong ambient light or reflections. Back-lit situations will force the speedlight to be under-powered, sometimes to the point where maxing out 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 confuses the TTL system causing it to under-expose. 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. I feel anyone using flash should be comfortable with manually controlling the power level in case TTL is ineffective.

Article and photos by Robert Hall