ATM caustic test questions

[Astropaulo@aol.com]

Let me run a question by you experts. I've been using the Caustic test for 
the first time and I have noticed that for some values of Y the distance X is 
about right but the shadow is a bit to far towards the inside of the zone 
being measured. If I push the value of x higher (wire farther from the 
optical axis) the wire shadow disappears before it's in the center of the 
zone for it's value of Y. Does this mean that the focal length for that zone 
is to long?  

Any help would be greatly appreciated.

Paulo