People love saying TTL. It has a nice ring to it. Say it with me – TTL! 🙂 But what is this elusive TTL? First of all, TTL is an abbreviation for Through The Lens. According to Wikipedia: In photography, through-the-lens (TTL) metering is a feature of cameras whereby light levels are measured through the lens that captures the picture, as opposed to a separate metering window. This information can then be used to set the correct exposure (average luminance), and control the amount of light emitted by a flash connected to the camera. Basically it means that the strobe power is adjusted automatically for correct exposure every shot. In reality and underwater, things tend to get a bit more complicated. Because we are using a third party flash and cable (either sync or fiber optic), there are more factors to consider and things that might go wrong.
What is TTL?
In underwater photography, TTL is always referred to Flash TTL. The general idea of flash TTL is that the camera tells the flash to fire and in realtime, senses the exposure of the image through the lens. When the image is exposed correctly, it tells the flash to stop. The result is that no matter which setting the camera is on, the image is always correctly exposed by the adjustment of the flashes firing length (which is ultimately what we call its power level). This process is commonly done a bit differently in compact cameras. Instead of sensing the exposure in real time , a pre-flash is used to sense the exposure and then the flash power level is already known when the main flash fires. A good test for TTL function would be to take the same photo with same settings and different apertures to see if the exposure remains the same.
TTL in compact cameras
TTL in compact cameras (ones that do not have an external flash hotshoe) is implemented in the built in flash. When we use a strobe underwater that supports TTL, this strobe is imitating the firing profile of the built in flash and acting the same, thus firing at the correct level for a correctly exposed image. S-TTL and DS-TTL are different methods of supporting this technique and the strobes today can identify one or more pre-flashes and cooperate with the camera as if they were a larger , more powerful built in flash.
More about optical TTL and how it works HERE.
TTL in DSLR cameras
TTL in DSLR cameras (ones that have a hotshoe connector) can be done electronically. The hotshoe is connected via a special bulkhead using a 5 Pin Nikonos sync cord (still the same cord of the famous Nikonos cameras) to the supporting strobe and this signals the strobe at what level to fire as if the strobe was a Nikon or Canon flash connected to the hotshoe. The problem is, the strobes do not have the same circuitry as the Nikon and Canon flashes and a TTL converter is required for TTL to work. Another problem is that Nikon uses 5 Pin and Canon uses 6 Pin, the converter is used to accept both connections and convert them to a 5 Pin strobe connector. Ikelite is one of the companies that decided to create their own standards and they have a built in TTL converter in their housing which only supports their strobes. They also use a sync cord that is unique to them.
Shooting TTL vs Manual
As you probably already know , in order to achieve good results underwater , one must shoot manual, or at least control some of the cameras features manually. Shooting manual , especially when you are diving is not a simple task. When using 1 or 2 strobes , controlling their power as well makes it even more complicated. For that reason , TTL is very helpful. It allows you to close aperture to decrease ambient light or open the aperture for a more blueish background while not worrying about the strobes power which will always be adequate to make sure the main subject you are shooting is exposed correctly. The challenge is that many cameras ( i.e. Canon compacts) do not support TTL in manual mode. This requires using Av or Tv modes and that is very limiting as when closing the aperture , the camera decreases shutter speed to compensate for the lack of light and then the required effect is not achieved. The way to use these methods is to use Flash Exposure compensation on the camera or on the strobe if it supports it. This will tell the camera to fire the strobe stronger than it “thinks” or weaker than it “thinks” thus forcing the camera to modify the shutter speed and aperture accordingly and achieving the desired effect without using manual mode. For a beginning photographer , TTL means 9 out of 10 correctly exposed shots when coming back from a dive and this is priceless.
Strobes that support TTL
Sea & Sea YS-01, YS-D1, YS-250Pro Inon S-2000, D-2000, Z-240, Ikelite AF35, DS51, DS160, DS161 SeaLife Sea Dragon It’s important to mention that TTL is not a MUST. Many photographers don’t mind using manual strobes and some even prefer it over TTL. When shopping for a strobe, it’s important to consider other aspects such as recycle rate and max/min power, which might be more important than the TTL feature. Contact us for advice on choosing the best strobe for you.
Visit his personal portfolio at www.ranmorphoto.com.