This flagship of the Canon accessory flash line is the most powerful unit the company offers, with a GN of190, and a manual/automatic zoom flash head that covers the full frame of lenses from 24mm wide angle to 105mm telephoto. (There's a flip-down wide-angle diffuser that spreads the flash to cover a 14mm lens's field of view, too.) All angle specifications given by Canon refer to full-frame sensors, but this flash unit automatically converts its field of view coverage to accommodate the crop factor of the EOS 40D and the other 1.6X crop Canon dSLRs. Compared to the 580 EX it replaces, the Mark II model recycles (inaudibly—no more hum!) 20 percent faster, and has improved dust- and water-resistance so you can use it in harsher environments.
The unit offers full-swivel, 180 degrees in either direction, and has its own built-in AF assist beam and an exposure system that's compatible with the nine focus points of the 40D. Powered by economical AA-size batteries, the unit recycles in 0.1 to 6 seconds, and can squeeze 100 to 700 flashes from a set of alkaline batteries.
The 580EX automatically communicates white balance information to your camera, allowing it to automatically adjust WB to match the flash output. You can even simulate a modeling light effect: When you press the Depth-of-Field Preview button on the 40D, the 580EX emits a one-second burst of light that allows you to judge the flash effect. If you're using multiple flash units with Canon's wireless E-TTL system, this model can serve as a master flash that controls the slave units you've set up (more about this later) or function as a slave itself.
It's easy to access all the features of this unit, because it has a large backlit LCD panel on the back that provides information about all flash settings. There are 14 Custom Functions that can be controlled from the flash or using the EOS 40D's menus. The built-in flash must be flipped up and any external flash attached and switched on for the menu options to become available. These functions are (the first setting is the default value):
C.Fn.-01 Automatic cancellation of Flash Exposure Bracketing (Enable/Disable)
C.Fn.-02 Flash Exposure Bracketing Sequence
(Metered>Decreased>Increased Exposure or Decreased>Metered>Increased Exposure)
C.Fn.-03 Flash metering exposure (E-TTL II/E-TTL)
C.Fn.-04 Slave unit's auto power off time (60 minutes/10 minutes)
C.Fn.-05 Cancellation of slave unit auto power off by master unit (Within 1 hour/Within 8 Hours)
C.Fn.-06 Modeling Flash (Enable/Disable)
C.Fn.-07 Flash recycling on external power (Use internal and external power/Use only external power)
C.Fn.-08 Quick Flash with continuous shooting (Disable/Enable)
C.Fn.-09 Test firing with autoflash power level (1/32 power/full power)
C.Fn.-10 Modeling flash triggered by test firing button (Disable/Enable)
C.Fn.-11 Auto setting of flash coverage to match image size (Enable/Disable)
C.Fn.-12 AF-assist beam OFF (Disable/Enable)
C.Fn.-13 Flash Exposure Compensation setting button (Flash unit Select/Set with Select Dial/Select Dial only)
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Although we usually tend to think of the digital camera as the best thing since sliced bread, there are both pros and cons with its use. Nothing is available on the market that does not have both a good and a bad side, but the key is to weigh the good against the bad in order to come up with the best of both worlds.