The EZ model reads flash exposure off the film while the shutter is open and quenches the flash when the the film has recieved enough light for a correct exposure. The EZ models will not give correct exposure with digital sensors as the surface of the sensor is far more reflective than the film emulsion. The EX series is designed to work with digital. Thats it in a nutshell
" The ex series works with Type A canon bodies and the ez line works with Type B. Basically the EZ line works with the older canon A-TTL. don't buy one and expect it two work on anything new than a Elan II. Starting with the Rebel 2000 all canon AF bodies use the EX line"
Actually, all EOS film cameras may use both EX and EZ series Speedlites. Why? All film EOS cameras--both type A & B--are compatible with TTL, A-TTL and E-TTL flash. I used my 430EZ with my EOS 3 and Elan 7E camera many times. However, the DSLR are only compatible with E-TTL, hence EX series is the best way to go if a DSLR is in your immediate future.
TTL takes into account only the flash; this mode is supported by EX and EZ flashes.
A-TTL takes into account flash and ambient light. It is supported by EZ flashes and older bodies.
E-TTL takes into account flash and ambient light. It is supported by EX flashes and newer bodies.
EX flashes determine the balance of ambient light and flash light using a visible preflash. They allow for flash exposure lock (FEL). Also, they support highspeed synchronization at all shutter speeds. Finally, some EX flashes have wireless capabilities.
(There seems to be a new mode E-TTL-2 introduced with the 1Dii and the EOS 30v. As no new flash has been released I reason that it can be used with EX flashes)
I used to own a 540 EZ. Although designed for the older bodies, it worked well with my EOS 50 in TTL mode (not in A-TTL).
With the EOS 300D, the 540 EZ did not work. It did fire only in manual mode but id did not fire in TTL mode. Consequently, I sold it and got a 420 EX.
The 540 EZ is a great flash. But unless you have a old body, go for a EX flash. The 420 EX is nice but lacks important features of the 540 EZ (manual zoom, flash exposure correction, ...) is less powerful an recharges slower. A 550 EX was not in the budget for me, unfortunately.
