Currently there are 10 native Macro Prime lenses for Sony A850 and all of these lenses are autofocus (AF) type.
A850 also has a sensor based image stabilization system which means that all of these lenses will be stabilized when used on A850.
Since Sony A850 has environmental sealings (Weather Sealing), you may like to couple your camera with one of these 2 Macro Prime lenses with Weather Sealings.