Confusion always results when users of Canon EOS camera tries to use the various modes available without trying to understand how they work. This is because the Canon EOS Camera has different modes like AV (Aperture Priority), TV (Shutter Priority), Program (P) and Manual (M) mode to handle its operations. The differences in they way they handle flash illumination is more obvious especially when the ambient light is low. This guide will give you a brief rundown on the various modes so that you will have a better understanding. We are also assuming that the FP mode of the camera is not turn on.

The P (Program) Mode

With the P mode, The Shutter speed is set to the highest possible speed to let you be able to hold the camera with your hands. You will also not have the problem of motion blur since the shutter speed is set fast enough to compensate for the low ambient light conditions. The P mode also functions when the ambient light is high. Under High ambient lighting conditions, the Camera will assume that the foreground will be flashed filled. Therefore, the output of the flash will be low powered. Conversely when the ambient light is low, the flash will be adjusted to give a higher output. The shutter speed will also be adjusted accordingly. But the picture will result in having a dark background.

The TV (Shutter Priority) Mode

Under this mode the camera will let you decide the shutter speed. It will instead choose an aperture priority to suit the shutter speed to ensure that the exposure level for the background is correct. The duration of the flash pulse will be determined by flash metering. In short, the camera always operates in fill flash mode under the TV mode as opposed to P mode. The Canon EOS system will also warn you if the background is too dim. This will be shown when the maximum aperture value of the camera lens starts to flash in the viewfinder. When this occurs, you should slow down the shutter speed. You will also require a tripod if you wish to avoid motion blur.

AV (Aperture Priority) Mode

Under this mode, you will be able to fix the depth of field by setting the lens aperture. The camera then decides the appropriate shutter speed to expose the background to the right level. The flash pulse duration will be determined by the metering system of the flash. However note that, if the shutter speed is set really low, you will need a tripod to avoid motion blur. As in TV mode, the camera will work in flash fill mode as well. The only exceptions to this are the Canon EOS 10/10s and Canon Elan II/ EOS 50 models. These cameras have a custom function which allows you lock the shutter speed to the X-sync speed.

M (Manual ) Mode

When you use the M Mode, you will determine the aperture and shutter speed of the camera. The exposure level will also be decided by you. The camera will just determine the correct flash duration to illuminate the foreground using the flash metering system.

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 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 calculating the amount of light which is reflected off the subject into the camera after the flash light is emitted by the flash unit. It actually measures the light which is reflected of the film in real time and this done by using an OTF (Off The Film) sensor. When enough light has entered the OTF sensor, the flash will be deactivated. Due to the fact that the Canon EOS Digital cameras do not use films, the Digital range of Canon cameras does not support the TTL system.

The TTL metering system works in the following manner and order:

1. Once the shutter button is depress midway, the metering for ambient lighting takes places. The speed of the shutter and size of the aperture is programmed by the camera or User depending on which program mode the camera is using. For example whether the camera is using 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. Once the foreground has been adequately illuminated, power to the Flash unit is cut off which extinguishes the flash light. The shutter remains open for the entire duration of the shutter’s speed time. The shutter then closes and the mirrors flips back down. Some camera has a Flash exposure Confirmation Led light and if the flash metering operation is successful, this light will glow.

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.

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