Metering for ambient light setting and metering for flash pulse occurs at two different times of operations. Ambient light metering happens the camera’s shutter is opened. For the Canon EOS cameras, this happens when you press the shutter button down halfway. Flash Pulse metering, on the other hand, is done only when you depress the shutter button fully. There are two main ways which one can meter for flash pulse. The first method is to measure the flash light as it is being emitted by the flash unit. The second method is by doing what we call a pre-flash test. What this means is that you send a low powered test pulse of established brightness before opening the shutter.

These are the two methods of flash metering which Canon uses for their system. The first method as described above is the Through The Lens (TTL) system. This method was first introduced by Canon during the 1980s with the introduction of the T90 model. The system was later adopted by the EOS range of Cameras as a standard feature. This makes the T90 as the only non EOS Camera which uses the TTL flash metering system.

The TTL system works by measuring the light which bounces of the subject matter and entering the camera lens after the flash light pulse has been emitted. In essence, the TTL system, measure the actual light which is reflected off from the film in real-time. This is done by using a OTF (Off The Film) sensor. When enough light has entered the sensor, to get a proper exposure level, the light from the flash will be extinguished. Because Digital Camera does not use film, the Canon Digital EOS cameras do not support TTL.

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. Once the shutter button is depressed all the way down, the internal mirror is flipped up and the film is exposed. Then Power is sent to the flash unit illuminating the area. The duration of the flash burst is determined by the OTF sensor which meter for a normal operational condition. If the picture is taken under bright lighting situation, the auto fill reduction will apply reducing 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.

It is also important to note that as the OTF sensor reflects the amount of light of the film, the composition of the film itself will also affect the amount that is reflected off. But as all camera are calibrated to work with normal film this will not become an issues except with special films like slide films which has different level of tolerances.

Similar Posts:

• TTL Flash Metering System of the Canon Camera
• Canon Camera Basic Guide Flash metering
• Canon Camera Basic Guide Flash metering – E-TTL system
• The Canon Guide to the E-TTL flash metering System
• Explanation of the Canon EOS Flash Photography

Tags: Photography

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