olympus vf-1

I believe the framing was meant to complement the 17mm lens (probably passable for the 20 too).

Some of the old style viewfinders (and ones built into rangefinder cameras) were adaptable with different focal-lengths - you'd dial them in as required 28, 35, 50, 90mm etc.

 

To expand on what phigmov said, it is an optical viewfinder with framelines for the 17mm lens. You could use it with other lenses, but you wouldn't be able to frame accurately.

 

It's a bit tight for the 17 but fits the 20 almost perfectly. Don't use it for anything closer than 2 or 3 metres if you want to avoid parallax errors. The brightlines are really clear and not prone to flaring out like some (e.g. Ricoh). The only thing I don't like is the plastic foot. I'm worried about breaking it. If you want something a bit cheaper and with more frames have a search for a Helios multifinder. They're 3:2 though.

Sent from my HTC Desire S using Mu-43 App

 

I just received a VF-1 to use with my EP3; the bright lines frame the 20mm 1.7 perfectly but you can also use the entire view to frame for the 14mm 2.5 as well.