If you have the Canon 100mm Macro L IS Lens then you do need to buy the Canon 67C Macrolite Adapter when using the Canon MT-24EX Macro Twin Lite Flash, it seems that flash was made for a 58mm thread, and of course because its a L series lens that adapter is the dearest of the lot..

I also have a Tamron 90mm macro lens and if I want to use that Canon flash I was told all I need to get is a step-up ring 55-67mm probably got one anyway.. I ended up ringing the place where I buy a lot of my lenses from to find out this information as its nowhere on the Canon websites.

I ordered one yesterday and its on its way through one of the US sponsors here on the site I am lucky they have that adapter size as they are very scare in Australia there could even be none here due to that terrible earthquake in Japan recently..