There are convex and also aspherical. In North America we generally get a flat driver's mirror and a convex passenger's mirror from the factory.
The convex mirror glasses will distort the reflection across the whole glass, though it's not a huge distortion.
The aspherical is flat for the inner 75% and then convex for the 25% outer edge. This provides a non-distorted reflection for the bulk of the mirror glass and then the outer edge gives you that added coverage for the blind spot. Best of both worlds.

Mirror Glass | Aspherical Blind Spot
You can see the line in the aspherical glass above where the glass goes from flat to convex on the edge.
If anyone has any questions don't hesitate to ask or PM/email:
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Cheers,
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