[Roughcollie]

I have a Pentax 645n and I am looking to buy a Macro lens for it,can anybody recommend an Ideal lens and focal length for this type of photography.

[blackcloudbrew]

There really is only one and that's the A or FA 120mm macro lens. Both are excellent. You could go with extension tubes but the 120 macros are THE lenses for the 645 system.

[robthebloke]

The 120mm macros (A or FA) are very highly regarded (and is the only focal length available in the 645 mount - aside from the extremely expensive 90mm dfa)

An alternative would be the 100mm or 135mm macros for the 67 mount (you'd need an adapter, and most people rate the 100mm as a better lens, so the 135mm is the cheapest option - assuming you can find a cheap 67 -> 645 adapter, which can be done, but it may take you a while)

A cheaper option would be some extension tubes, or better yet, a helicoid extension tube).