Flash-head zoom problem with 50D

I use a Sigma EF 500 DG Super ETTL 11 on my 20D and 30D bodies with no problems but when I use it on my latest 50D body the flash head zoom goes haywire and starts hunting to and fro and finally settles on the wrong setting (usually 105mm).
Cross checking with different lens shows the problem to be with the 50D body but I am unable to discover an answer despite trying altering Custom Functions settings.

I remember that problem, I once had a 50D and tried to use it with a Sigma 500 flashgun, it gave all sorts of strange problems.

When Canon bring out different camera models they tweak various software settings and although their own stuff stays compatible a lot of third party stuff will not work properly, it is possible that Sigma might be able to update the flashgun software to make it work but they only seem to offer this sort of service for their latest models.

I think you may have to buy another flashgun to use with the 50D, either a genuine Canon one or if you are buying used then something like a Metz 48-AF1 or 58-AF that enables you to update the software yourself.

I second that. I have only Canon flashes, and have no problems. But a friend had a similar issue with a third party flash.

My Sigma 500 as worked without trouble on my 300D, 20D, and 50D, but just yesterday I ended up having a fair bit of trouble with it. I'm not sure if some how all my settings have been reset to defaults, but it wasn't keeping the shutter between 1/60s and the max sync speed in Av mode, and the power output also seemed all over the place too. I'll have to have a good look at it in a bit.

Alan