Mine should be here in a few days.  My only problem is that I have a cheap 270EX and an *old* vivitar manual flash.  Should work fine in controlled conditions.

Bob

In your review you mention the absence of GN reference in the Speedlite book.  I wouldn't miss it since it seems to be so less useful than it once was. For starters the advertised GN is not comparable between different brands as they report GN using different focal lengths.  Also consider for a moment sensor size. Canon reports GN flash figures for full size sensors except for the 270ex that reports based on aspc. Don't know about the newest 320ex as I haven't checked. Nikon does the same thing with their junior flash. It would be nice if competitors Nikon and Canon would use the same focal length when they advertise GN  but sadly they don't. Canon is less conservative as much is reported at 105 mm. Talk about a poor system... today's GN reporting is certainly not a bust but it could only confuse trying to do much with it in a guide that I assume is meant to be useful for apsc and full frame..

I had not heard of that book and appreciate your review and endorsement. Like the others I am going to order one from Amazon.

There is no direct relationship between GN and sensor size. The GN is exactly the same.

Where sensor size could come into play is with a zoom head. It would be possible to achieve a higher GN for smaller sensor cameras by concentrating the flash more, Since from a given distance with a given lens, small sensor cameras cover less area (assuming the same lens and distance from the subject), you could focus the flash tighter.

This works via the same principle that zoom flash heads have a larger GN at longer focal lengths.

However if the GN of a flash is 40m at ISO 100, it's 40 for 35mm and for APS-C, assuming the flash zoom is the same or the flash has no zoom function.

I think the latest generations of Speedlites do just that. For current crop sensor cameras they zoom out further.

As far as I know the 430EX and 580EX (and the "II" versions) do compensate for sensor size when used on current EOS DSLRs. That means I would have expected your 580EX on a 7D to zoom the flash a little more. Whether it reports this with the compensated focal length displayed on the LCD I don't know. I would guess it should but I'm still using my old 550EX (which doesn't adjust for senor size).

I believe the 580EX shows a little graphic icon next to the zoom setting to indicate that it has detected a crop sensor and compensated for that with the zoom setting.

Here is what the Speediliters handbook says:

(page 88)
"When paired with a compatible EOS lens, the 580 and 430 series Speedilites will move the flashtube backward or forward so that the spread of light matches the viewing angle as closely as possible.  The usable range is 24mm to 105 mm on a full-frame DSLR.  If you are shooting on an APS-C camera, the usable range is 15 to 65 mm. 

You will know that the lens and Speedilite are talking through the camera when you see the [little box] symbol on the Speedilite LCD."

I would have expected it would require a compatible camera, i.e. one that tells the flash the sensor size. As long as the lens reports focal length (and as far as I know all Canon and 3rd party AF lenses for EOS do report that) to the camera and the camera passes that information along to the flash, there's nothing else required. Of course if you are using some old manual focus zoom lens on an EOS body, the lens won't talk to the camera and the flash won't zoom anyway, so in that sense the lens does need to be compatible.