Question
When I do a distortion plot on my optical system, there seems to be a very high peak near the axis, but not on the axis, that diminishes, and then trails off. This seems erroneous. Is this a math artefact or real distortion?
Synopsis
Distortion analysis (distplt) in a tilted or decentered system
Symptoms
Large distortion error near center of the image plane (near surface vertex), and the system contains tilts or decenters.
Solution
What you are seeing is an artefact of the math, but it also shows that you have an underlying problem in your system because of the way classical distortion is defined. First a definition: Gut Ray. This is the on-axis chief ray. In an on-axis system, this coincides with the optical axis and is meaningless. However, in a system that has tilts and decenters (such as yours), the gut ray has real meaning. Note that your gut ray does not hit the vertex of your image surface. You can see this by clicking on REF in the Text window toolbar or by typing "SOP 0" on the command line. The resulting gut ray error is the value of YC and will be non-zero. The way per cent distortion is calculated, this creates an INFINITY of distortion at the center of your field (and the graph does not display this INFINITY quantity correctly - hence the math problem). To correct your system, you need to decenter the image surface by the error of the gut ray. Close your surface data spreadsheet and try the following text on the command line: SBR;SOP 0;DCY IMS SSB(3,1); Now try the distortion analysis again and you will notice a vast improvement. Also note that in a tilted and decentered system such as yours, the +Y and -Y field points will not have the exact same distortion. Try an analysis with your field specification (image height, field angle, ...etc) to be negative of what it was originally. With a tilted or decentered system you might notice a big difference in the result.
