Just wanted to get your thoughts about using zeiss 12mm f/2.8 on a full frame sony body. Price and size wise of the 12mm is very appealing than the 18mm. Since 12mm in apsc sensor is 18mm in ff...appreciate your thoughts and comments.

Thnx.

If that were me I'd stick with the correct lens for the camera in use. FF or APSC, match the lens to the sensor. 

I don't know which body you have, but on a7 cameras you can switch to apsc mode.  But I think you are making a wrong assumption: 12mm full frame is 18mm on apsc, you are thinking it is the other way around?  A 12mm apsc ( normally the mm are stated for full frame) is only equivalent to 8mm on full frame.  And you will have black corners. 

Err how does your math work? 12mm focal length is 12mm focal length, no matter what sensor you use. If you put a 12mm APS-C lens on a fullframe sensor it will still be 12mm, just with black corners because the sensor is bigger than what the lens is designed for.

On the other hand, if you put a 12mm lens (be it the 12mm APS-C lens by Zeiss or the 12mm fullframe lens by Voigtländer) on an APS-C body (or fullframe body in crop mode) the field of view will be identical to an 18mm lens on a fullframe sensor. Thus photome09's question is valid, but nonetheless a bit silly as it foregoes on the very reason to get a fullframe camera in the first place, which is better image quality due to a larger sensor (and possibly some better DoF-control). Now he intends to crop out all that fullframe goodness just to save a few $$. Might as well sell the fullframe camera and buy an A6000. The resulting picture will be better than with your fullframe camera in crop mode and you save even more $$.