FUN PHOTOGRAPHY

Metering for ambient lighting conditions and flash metering has totally different requirement. The metering for ambient lighting condition is done before the shutter of the camera is opened. In the Canon EOS camera, ambient light metering is done when you press the shutter button halfway down. Flash Pulse metering however is done when the shutter button is press fully down. There are two ways to meter for flash pulse automatically. One way is by measuring the flash pulse as it is being released. The other way is to do a pre-flash test. This is achieved by a low powered test pulse of a calculated brightness before the shutter opens.

These are the two metering system used by the Canon flash metering system. The Through the Lens (TTL) metering system adopts the first method of metering for flash automatically described above. Canon first introduces the TTL metering system with their T90 model during the 1980s. This feature was later incorporated in the EOS line of camera as a standard feature. As such the Canon T90 is the only non EOS that can use the TTL flash-metering system.

TTL system works by calculating the light which is reflected of the subject and entering the lens after emission of the Flash Pulse. In short the TTL system actually measures the light which is reflected off from the film surface in real time. This is archived by the use of the “Off The Film” (OTF) Sensor. Once there is enough light entering the sensor, the flash will be turn off. As Digital Camera does not use films, the Canon Digital EOS Camera does not support the TTL flash metering system.

The sequence of operation of the TTL system is as follows;

1. By pressing the shutter button halfway down, the ambient metering process takes place. Shutter speed and aperture size is calculated and programmed by the camera or user depending on the mode which the camera is running on. These modes can be the AV, P or M mode.

2. Only when the shutter button is depressed all the way, the internal mirror is turn up and exposes the film. Power is then sent to the flash unit which will emit a flash light to illuminate the scene. The length of the flash pulse is determined by the OTF sensor in the camera which is metered for normal operational conditions. If the picture is taken under bright lighting conditions, the auto fill reduction will take effect and reduce the illumination of the Flash.

3. When the foreground is sufficiently illuminated, the power supply to the flash unit is terminated which in turn extinguishes the flash light. The shutter is opened during the entire duration of its speed run. After that, the shutter then closes and the mirror flips back to it original position. For those cameras which have a warning light to confirm the success of the flash exposure operation, the warning light will then glow to indicate the success of the flash metering operation.

As the OTF sensor measure the reflected light off the film, the chemical composition of the film will also affect the amount of light which will be reflected. In most cases, this will not be a problem as cameras nowadays are adjusted to operate with normal films. This only becomes a problem if you have to use specially coated films with different level of light tolerance.