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Beamwidth control in Canon Speedlite flash units
Many Canon Speedlite flash units offer a "head zoom" feature, by which the beamwidth of the flash unit can be varied to match the field of view of the camera with its current lens, usually on a automatic basis, if desired.
Until a few years ago, the beamwidth was always set to match the field of view that the current lens would afford on a camera with a full frame 35mm format size, even if the camera in use had a smaller format size. Thus, on cameras with smaller format sizes (sometimes called, to my distress, "crop" cameras), the flash beamwidth was set to a larger value than required, leading to lower energy efficiency and shorter maximum "reach" (in some case) than should have been available.
It was always assumed that Canon could have readily improved upon this situation by the simple expedient of having the camera report to the flash unit not the focal length of the current lens (and zoom setting, if applicable) but rather the full-frame 35-mm equivalent of that focal length.
The Canon Speedlite 580EX flash unit incorporated a new feature, intended to have the flash unit optimize its beamwidth for the actual field of view given on the specific camera in use by the the current lens. EOS bodies from the 20D on were equipped to do their part in this scheme.
But the algorithm in the flash unit was bungled, with the result that in may situations the best available beamwidth as not put into play - in some cases, the beamwidth put into play was less appropriate than without the feature in operation.
With the assistance of numerous colleagues on the dpr forum, a few years ago I conducted an extensive investigation of this matter. One discovery was that Canon in fact was aware of the problem, and in later production of the 580EX flash units had corrected it (mostly). And it turned out that some Canon Factory Service Centers were prepared (read: willing) to retrofit older units having the bungled algorithm.
But Canon never made an announcement about this, and often owners had to conduct elaborate presentations (I supplied a script) to convince the local Center that there was in fact an anomaly.
Later models (such as the 580EX II and the 430EX) have clean implementations of the scheme (with a few peculiarities, of course).
The while story is presented in detail in my technical article, "Improved Beamwidth Control in Canon Speedlite Flash Units", available here:
It has recently been updated, in part to include the information to be described next.
Operation with the Powershot SX20 IS camera
I have recently acquired a Canon Powershot SX20 IS compact camera, which accommodates EX-series Speedlite flash units. I recently did some testing to find out how the automatic beamwidth control scheme works out on those cameras. The brief story is that it works properly (although not ideally). The latest issue of the article mentioned above has an appendix discussing this situation.
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