Sigma do makes some four thirds lens mount for your Olympus. Because the 4/3s CMOS sensor is very small (2x reach) unique to Ollies; you've probably discovered that their lenses costs a lot more compared to other main stream brands
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For example to get your camera to reach 70-200 mm equivalent, you would need 35-100 mm. Now throw in a wide aperture of f2-2.8 (automatically will mean $$$$), in your case the 35-100mm f/2.0 Zuiko E Series costs around $2,049.95.
Now in answer to your question; to get 50 mm on your camera - the Olympus's 25mm f/2.8 Pancake Lens is what you're looking for. Because 25 mm times 2x equals 50 mm. The price goes for around $250 USD new. I don't think anything on the market will go past an F2-2.8 aperture for Oly's. If there is, expect $$$$ consider the example above for 35-100 mm.
For cheaper prices, second hand is the way to go.
www.keh.com is the place to look or the used department at BH, Adorama, or Calumet are good options too.
Often times Oly owners will buy adapter rings for their cameras to make other 3rd party lenses (tamron and pentax) mount. Though you can still focus to infinity, you will lose auto focusing; manual focus only. Metering can also be effected with certain adapters fyi.
A lot of parents at the gym that I shoot for owns Olys'. That's how I happen to know about them
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Hopes this helps
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