The Olympus 100-400mm IS lens size is 72mm, please reconfirm the number mark as "ø" (diameter) symbol, if the number is 72, just need to order same 72mm size filter ,thanks. 

Unfortunately this is for adapting lenses to Olympus' modern digital Micro 4/3 cameras. It won't work with the Pen FT or similar half-frame film cameras, and you most likely won't find anyone making adapters for these. 

It’s a standard 1/4"-20 Female to 3/8"-16 Male Screw Adapter Reducer Bushing Converter. This unit comes with one in the box, but fair warning. It tends to stick on the original 3/8” tripod head. I’d suggest possible gluing the nut in the unit or at least waxing the original head before use. 

hello , we are the seller, you could use an adapter to step the 52 down to a 49 , there is a adater ring kit https://www.amazon.com/dp/B00JL4HGAI, 

There are 6PCS adapter ring included in this flash, here is the size:
52 55 58 62 67 77mm, as long as your lens size is one of them, then could be used,
Here is the way to check your lens size:
Your camera's lens thread size will be marked somewhere on the lens barrel or printed underneath your lens cap. This number is always preceded by a "?" (diameter) symbol. For example: ?52 = 52mm lens thread size.
If your lens size is none of them, please tell us, we may include the size you need but at this case, this item must be shipped from China so that we could pack with this adapter ring you need.