580 EX II
The external automatic sensor option with the 580 EX II flash is one that many photographers will want to experiment with, at least from time to time. 580 EX II will certainly prove useful in some circumstances, such as removing a flash from the camera to light up backgrounds or other subjects from an angle. And some users may find they prefer its exposure characteristics in general to those of E-TTL in ordinary indoor candid situations.
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580 EX II
E-TTL is still going to be vastly preferable for balanced-fill flash outdoors, or for balancing ambient light and flash indoors in Av and Tv modes. And the external sensor auto mode, as discussed previously, will not work at all with Canon’s Wireless E-TTL flash system. Whether you own the latest EOS cameras or not, external sensor auto flash is easy to experiment with using the Canon 580EX II , and adds a new versatility that EOS users have requested for some time.
One thing for experienced users to keep in mind is that the external sensor on the Canon 580EX II measures a relatively narrow 20-degree angle of view. This never changes, regardless of the lens that’s on the camera (a fundamental advantage of TTL flash metering). The 20-degree view means the flash measures an area that’s about the same as that covered by a 135mm lens (on a full-frame camera). This may be a “tighter” metering area than what users may be accustomed to with previous-generation “thyristor” 
flash units. Keep this in mind when evaluating flash exposures in external sensor mode.
Canon 580 EX II
The Canon speedlite system is a source of confusion to many new (and quite a few old!) Canon EOS users. This has been made somewhat worse with the introduction of DSLRs as they are not really compatible with early "EZ" series Speedlites. DSLRs require newer "EX" Series speedlites in order to provide a reasonable degree of automation and allow automatic exposure.
"EX" series speedlites allow the use of E-TTL (and E-TTL II) which are flash metering systems which use a preflash and the camera's multi-zone evaluative metering system to determine optimum flash exposure. Earlier "EZ" series speedlites did not use a pre-flash and determined flash exposure by measuring the amount of light being reflected from the film surface onto special flash exposure sensors during exposure. [Note that "EX" series speedlites default to the older "A-TTL" mode of the "EZ" series when used with older EOS bodies which don't support E-TTL.] Since DSLRs have no film and they are anti-reflection coated, this metering system can't be used and so "EZ" series speedlites are incompatible with auto exposure on Canon EOS DSLRs. They can be used in a fully manual mode, i.e. basing the aperture setting on the camera on the guide number of the speedlite, the ISO setting and the subject distance.
ETTL-II is very similar to E-TTL, except it uses a slightly different algorithm to calculate flash exposure and it can also factor in the distance to the subject when required. You can read a bit more about it here - ETTL II
"EX" series speedlites do provide the pre-flash which E-TTL (and E-TTL II) metering requires. TAs of August 2008, there are currently three "EX" series speedlites in production (plus a couple of specialized macro ringlites). They are the Canon 580EX II, the 430EX II and the 220EX (with the 420EX and 550EX discontinued and the 430EX soon to be phased out). They differ both in flash power and in features. They all support FP high speed flash sync (at reduced power). There are also two specialized macro speedlites, the MR-14EX and the MT-24EX.
Based on Canon's earlier 580EX Speedlite, the Canon 580EX II adds weathersealing, a stronger metal mounting foot and has a 20% faster recycling time. The main functional differences are that the 580 EX II also has an external metering sensor for non-TTL automatic flash exposure control and a PC socket for use with non-dedicated slave triggers.
Where to Buy Canon 580EX II
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