This guide will seek to provide you with an explanation of the various concepts and terms which are used which respect to Canon EOS flash photography.

Flash and Strobe

What is the main difference between a Flash and strobe is just whether you are using UK or US English. We refer a strobe as a flash unit in the US whereas the British uses the word as meaning anything which emit blinking lights. Other commonly used terms which cause confusion in flash photography are the words “Speedlight” and “Speedlite”. Speedlight is actually the brand name of Nikon series of flash units while Speedlite is the brand name of Canon series of flash units.

Guide Number

This number refers to the maximum distance which a flash is range for. This number is irrelevant when you are using the automatic flash metering system. However the number is crucial if you are involved with manual flash settings.

EV (Exposure Value)

Exposure Value or EV is the rating used for determining the sensitivity of the Auto Focus (AF) system or exposure metering for a lens or film type. This number is just a combination of aperture and shutter speed. For example, f2 and 1/125 = EV 9 which is also equal to f4 and 1/30.

Dedicated and Non Dedicated Flash Units

Most Camera manufacturers also manufacture their own line of flash units to complement their camera models. These “own” brand of flash units are known as dedicated flash units. The Canon line of flash units is known as the Speedlite series. They are able to work together with the camera proprietary system to give more precision photography. Non dedicated Flash units are those flash units which are manufactured by third party manufacturers. Usually generic in nature, they cannot communicate with the camera proprietary system.

Shoe Mount

The rectangular slide-in socket which is usually located on top of the mirror or prism housing is known as the shoe mount. This mount is usually used for affixing the external flash unit to the camera. Sometimes it is called a Hot Shoe Mount as it has an electrical trigger to initiate the flash unit once the external flash is fixed to the camera. The Canon EOS Cameras have 4 additional contact points in the shoe mount which helps to transmit flash data from the trigger.

The Redeye Effect

A common occurrence with flash photography is the “Red Eye Effect”. This happens when the light from the flash unit is reflected back into the camera from it bounces of the red blood vessels of the eyes. The situation tends to occur in environment with low ambient lighting causing the pupils of the eyes to dilate bigger to allow for more light to enter the eyes.

Redeye Reduction

To reduce the problem of Red Eye, one can use several methods. The most effective way is to distance the flash away from the lens or bounce the flash light onto the subject as opposed to flashing directly at the subject. The other way is for the subject to observe a bright light source just before taking the subject picture. This will make the pupils smaller and thus lessening the reflective capability of the retina.