This general guide will help to explain to you some of the concepts and terms used in relation to Canon EOS flash photography.

Flash units and Strobe

The words flash and strobe has been used interchangeably actually to denote the same thing. It all depends on whether you are saying it the British or American way. The American refers strobe as a flash unit whereas the British uses the term to refer to anything which produces blinking lights. Speedlight and Speedlite are also two commonly used terms which create some confusion among those new in the photographic world. Speedlite is actually the brand name for Canon series of flash units while Speedlight is the brand name of Nikon series of flashes.

Guide Number

The maximum distance which a flash unit can reach with its flash pulse is known as the guide number. But if you are using automatic flash metering, this number is not important at all. You will only need to know this number when you are more inclined to do manual settings for your flash.

Exposure value (EV)

Used for determining the sensitivity of the Auto Focus or (AF) system or exposure metering for film types or a lens, the EV or Exposure Value is a combination of both Aperture settings and Shutter speed. The numbers 1/125 and F2 give an EV value of 9 which can also be obtained from 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

Usually located on the top mirror or prism housing, the shoe mount is a slide-in rectangular socket meant for attaching an external flash unit to the camera. They are also known as "Hot Shoe Mount" as they possess an electrical circuit which will initiate the flash unit once it is attached to the camera. Canon EOS cameras have an additional four electrical points in the shoe mount which transmit data from the flash trigger.

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.