No... the above responses (and the info at the link provided) are incorrect.
The original question is regarding using a Ringlite with the 100/2.8L IS lens, specifically... And with that lens you cannot directly attach the Canon MR-14EX, MR-14EX II Ringlites or MT-24EX Twinlite flashes.
These Ringlites and Twinlites can be directly fitted to (clip directly onto) any of the Canon macro lenses that use a 58mm filter. These include the 100/2.8 USM (non-L/IS), MP-E 65mm and EF-S 60/2.8 Macro. The 50/2.5 Compact Macro - which uses 52mm filters - also is designed to allow these flashes to be directly attached without any adapter (note, no filter can be used on the lens when any of the flashes is attached).
But to use any of these macro flashes with the EF 100/2.8L IS lens (which has 67mm filter thread) in particular, you will need Canon "Macrolilte Adapter 67" (http://www.bhphotovideo.com/c/product/656030-REG/Canon_3563B001_67_Macrolite_Adapter.html/prm/alsVwD...)
To use any of these macro flashes with the EF 180/3.5L you need Canon "Macrolite Adapter 72C". (http://www.bhphotovideo.com/c/product/112547-REG/Canon_2829A001_72C_Macrolite_Adapter.html)
There is also a Macrolite Adapter 58C (http://www.bhphotovideo.com/c/product/12860-REG/Canon_2365A001_58C_Macrolite_Adapter.html) that's used to allow the Ringlites and Twinlite to be mounted on non-macro lenses with 58mm filter thread size. Or you could use it to fit the Canon macro flash units to non-Canon macro lenses (though you may also need a step ring if the lens has filter threads other than 58mm). This adapter is not necessary with EF 100/2.8 USM (non-L/IS), EF-S 60/2.8, MP-E 65mm Macro or EF 50/2.5 Compact Macro lenses.
So, in summary, to use either Ringlite or the Twinlite with the following lenses, utilizing the "standard" clip-on mounting method, you need the indicated adapter...
- EF 50/2.5 Compact Macro.... no adapter needed.
- EF-S 60/2.8 Macro... no adapter needed.
- MP-E 65mm 5:1 Macro... no adapter needed.
- EF 100/2.8 USM... no adapter needed.
- EF 100/2.8L IS USM... Macrolite 67 Adapter required. (Mfg. #3563B001)
- EF 180/3.5L USM... Macrolite 72C Adapter required. (Mfg. #2829A001)
- Non-macro lenses with 58mm filter threads... Macrolite 58C Adapter. (Mfg. #2365A001)
- Third party macro lenses, choose 58, 67 or 72 adapter that's same size or closest in size to the lens' filter threads. If filter threads are not exactly the same size, use a step ring in addition to the adapter you selected.
If you have never used one, you might try renting a Ringlite or Twinlite, to try out and see how you like their effects on your lens before commiting to buying one.
Personally I'm not a big fan of ringlites type flashes on 1:1 macro lenses. I do use the MR-14EX, but mostly just on the MP-E 65mm at much higher magnifications. It's excellent for high magnification work with that lens.
I prefer the MT-24EX twinlite on 1:1 macro lenses. But I don't use the standard mounting system with it. Instead I have the heads mounted on a Lepp Stroboframe dual macro flash bracket (sorry, it's no longer in production) that allows much more flexible positioning of the flash heads.
Many Thanks Alan,
I bought the lens and ringflash setup a year ago and removed the adapter ring while working in the lab to place a filter. As happens , once these little items are free of the lens they grow legs and runaway. In the middle of the great hunt I then suffered transient amnesia and couldn't remember if this set-up had an adapter or not. Thank you for the part number, I have ordered it today. I agree with your preference for the twin macro lite and use one with extension tubes for close-up work. I do like the master/slave function with a 600 ex-rt and use this for additional lighting, however my fellow co-workers feel I'm being more artistic than productive. There is nothing as sweet as professional jealousy, I might buy a second 600 and some light boxes to up the ante.