Why It Works — When It Doesn't
Text and Images by Tony L. Corbell
TTL. Through The Lens. What could be simpler than working with a
flash system that is automatically controlled through the lens, on
the film plane where the image is formed?
The question arises because some photographers: (a) do not know or understand how TTL works; (b) cannot seem to make it work for them; (c) are afraid to rely on it, especially for important jobs; or (d) all of the above.
Let's first examine what happens when your camera/flash system is in the TTL mode and how the exposure is made. When the camera fires, a sync cord triggers the flash when the lens is open. Then the magic takes place, all rather quickly.
The output, or power, of the flash unit is controlled by the amount of brightness the camera's sensor reads off-the-film (OTF) during the exposure. It takes an average or mid-tone reading of the scene within the sensor's area and turns off the flash when enough light is "seen" or reflected off the subject to give an 18 percent reading.
The subject within the camera's TTL sensor area of the film will
dictate the brightness or amount of flash that will be used, based
on the ISO of the film selected. In other words, as long as the
subject and background are of a mid-tone or average brightness and
the ISO is dialed in correctly, the TTL flash system works
beautifully without compensation.
Although accurate for mid-tone applications, the TTL system will not work perfectly in extreme situations and needs input from the photographer. Under those circumstances, the photographer must adjust the ISO setting on the camera to produce perfectly exposed images. That is why some people see this as an unreliable system.
Fact is, the TTL system is the most accurate available in flash photography. It is also the most worry-free, despite the need for attention from time to time, particularly during a wedding shoot.
Think about the subjects in wedding photography. Brides dressed in white cut white cakes in front of white windows. Grooms dressed in black tuxedos shake hands with groomsmen with black tuxedos, while standing in dark-paneled offices in churches. These situations are good examples of the TTL system needing adjustment. Mid-tones or 18 percent average do not exist in these scenarios and probably will not work accurately with standard TTL operation.In the bride example, what would happen if no adjustment were made? The sensor would shut off the flash before full exposure was achieved, trying to render the white or light areas in the scene as a mid-tone on film. This would underexpose the image.
To compensate for this, it would be necessary to change the setting of the film speed or, in some systems, adjust the exposure compensation dial on the camera to "fool" the system. By making the system think there is slower film in the camera, it follows that the exposure will require more flash. This basically increases the overall exposure in the scene, thereby making the bride's gown and cake white, not middle gray.
NOTE: When the TTL mode is selected on your flash unit, all other functions of the flash unit, automatic and manual, including the ISO dial or selector, are overridden by the system. You cannot change the ISO on the flash while in TTL. It has to be done on the camera. Photographing the groom would have the opposite effect. The darkness of the scene would lend itself to overexposure from the sensor, again trying to make the scene brighter than it is in reality. To compensate in this instance, you would want to increase the ISO setting, which would lessen the exposure on the negative.
These two scenarios; white is approximately one stop from middle gray, black is also about one stop in the other direction. Make the ISO adjustments accordingly.
TESTING THE FILM
In addition to the brightness of a specific scene or subject, one more area of control might need attention: the film itself. The reflectance value, or shine, from emulsion to emulsion may vary slightly, so it is important to test each of the films you use with the TTL system. It is not enough to dial in the film's ISO without testing it for accuracy in its reflectance value.
For example, a color negative film with a 400 ISO may not render an average scene properly unless a slight adjustment is made. This is why you need to test the films yourself. Rating a 400 ISO film at E.I. 250 (+2/3 stop) does not indicate there is a problem within your camera or flash system or even the true ISO of the film. It could simply be the reflectance value of the film base and how the TTL flash sensor reads the amount of flash reflecting from its surface.
So if your lab tells you that your work is a little under- or overexposed, don't automatically assume something is wrong with your flash, your camera, or the lab. It simply may be the reflectance of the film itself causing the problem or possibly the tonal range of the scene without adjustments.