The Tiffen, Raynox, Sakar and Soligor are about the same in quality.

The big problem is finding the correct adapter to connect them to your camera.

The Olympus WCON-07 is considered the gold standard when it comes to wide angle add-on lenses.
Again you need the correct adapters to make it fit your camera.

http://www.olympus.co.uk/consumer/198_WCON-07.htm

Note:
Any good wide angle lens will have a large diameter.
This means it can obstruct your view when using a viewfinder, and it can obstruct your flash from reaching your subject.

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