johnortt

FWIW I'll add one more practical point that can get lost in the shuffle: An 85mm f/1.8 on a FF sensor may be the closest FF comparison to what you get from the 56mm f/1.2 on Fuji, but that doesn't mean the Fuji 56mm f/1.2 and Canon 50mm f/1.2 lenses shouldn't be compared at all.
 
If you choose Canon but you want the size/cost benefits of their APS-C cameras (i.e. 7DII) then the FF lenses are your only choice for all but a couple of their cheapest primes. There's no way to get a small, cheap 35mm f/1.4 for Canon, you HAVE to buy the huge, expensive FF L lens even if it's going on your Rebel. This is because Canon treats their "crop sensor" lineup as a discount brand, rather than as a valid tradeoff of size/cost v. quality. 
 
Fuji on the other hand treats their APS-C cameras as top of the line, so their APS-C primes are smaller and lighter than what you'd have to carry to get the same effect on a Canon APS-C camera. 
 
The day Canon comes out with fast awesome primes in EF-S (crop sensor) or EF-M (their half-assed mirrorless format) this may change, but I doubt they'll ever come out with anything comparable to the 56mm f/1.2 or 16mm f/1.4 lenses designed specifically to take advantage of APS-C sensors. 

Thanks Black Pearl.  
 
I think I understand it about as much as I am going to 
 
I'll have to get my hands on a FF Camera and Lens to fully figure it out so I can do some side by side comparisons and compare.
 
Thanks again,
 
John

Sort of...
 
A full frame lens throws a circle out the back onto the sensor with a diameter of around a 43mm. On a 35mm camera or a digital camera with a sensor the same size (referred to as Full Frame) you will be using all of that circle - see below - with a cropped sensor you simply use a smaller part of the circle. 
 

 
As you can see from the diagram below a FF lens produces a circle large enough for a FF sensor as well as working fine with smaller sensors which just crop off the edges - a lens designed for a cropped sensor produces a smaller circle that isn't large enough to cover the larger FF sensor. What you can also see is the subject - the mountains in this case - are always the same size as the focal length of the lenses in question are the same. The ONLY thing you change is the field of view due to how big (how much crop) the sensor is. You are not increasing the focal length of the lens by putting it on a crop camera and regardless of which camera you put it on a lens will ALWAYS be the focal length printed on the barrel - same goes for the aperture, it will always be the one you select.
 

I have just bought myself an X-pro1 today on offer for £499 with the 27 & 18 lenses and the Fuji Leather case.  
Needless to say it will be a long while before I will be considering the X-Pro2!
I have to confess it was the XT1 which drew me into the Fuji world coming from a Canon 60D but the price of the Pro1 was too good to pass up on.
At this price I have two lenses I wasn't expecting and I can still afford another really nice lens.
I expect my next Fuji body will be an XT1 once the XT2 comes out and the price drops but we'll see.