External Monitor attached to macbook is blurry

I have a Macbook 13" (purchased new 2012) which I want to use with my existing monitor when editing photos.

Monitor is ASUS HDMI VH222 attached via HDMI cable with Mac adaptor.

The fonts are not clear (not a major issue as I mainly surf directly on MacBook) and photos are slightly out of focus (big problem as I am using monitor to edit photos!)

Resolution is 1080P.

Can you give me any advice re what I need to adjust. (Note I am somewhat clueless so please type slowly and assume complete ignorance!)

The monitor is perfect with my ancient Windows laptop (attached via VGA).
Thanks

Sounds like the monitor is not set for its native resolution. Open the Displays preference panel and change the external monitor's resolution to the highest available.