In the early 2000s, the 420EX represented Canon's midrange Speedlite offering. It was replaced by the 430EX, and subsequently, the 430EX II. While the Canon 420EX is discontinued, it remains a capable tool for use with Canon's digital SLR lineup, including the 40D.
Function
The 420EX is similar in size and capabilities to later Canon models, such as the 430EX and 430EX II. According to the Canon USA website, the 420EX offers E-TTL ("evaluative-through-the-lens") metering, which will work with the 40D to adjust for proper exposure when using the flash.
User Interface
The one obvious difference between the 420EX and successive units is the user interface. Instead of an LCD panel, the settings on the 420EX are displayed in lights on the back of the body. For someone using a recent digital SLR like the 40D, or used to using a more recent flash like the 430EX II or the 580EX II, this may take some getting used to.
FEC
In a review of the 430EX on "The Digital Picture," a number of differences between the 420EX and the later model are listed. Of note for 40D users is the lack of FEC, or "flash exposure compensation," on the body of the 420EX. In order to increase or decrease the power of the 420EX, 40D users must press the button on the top labeled with the "flash" symbol and "+/-," then use the selection wheel on the back of the camera, in order to set the exposure compensation level.
Tags: between 420EX, exposure compensation