Hello, Brian!

Stacking Composites is generally more accurate than using several separate sag surfaces.  The reason for that is the location of the ray intersections.  If you stack the surfaces but don’t designate them as Composites, then during a raytrace, the ray encounters each surface separately at the XY location where that ray strikes each surface.  Before Composites, for example, I would often use a separate surface to add a sag error to an off-axis mirror; this method isn’t ideal, because the ray doesn’t encounter the sag error and the original surface in the same XY location.  For cell phone lenses with extreme surface shapes, this method doesn’t work at all due to those retrace errors.

Composite sag, on the other hand, is added *before* any raytrace, and the rays only see the final total surface sag.  This is both more accurate and faster than other methods of stacking surfaces.

Pros of Composites: 

• Can be used to perturb any surface shape.
• Quick and accurate.
• Gives added control of separate terms during optimization.  (See also the new DSAG|SSAG, DCRV|SCRV, DSLP|SSLP operands.)
• Enables automatic tolerancing of power and irregularity for any surface type.  (TEZI has been updated to use Composites, for example.)
• Enables automatic tolerancing of optics with off-axis apertures.  (We’ve included a button that calculates the tilt and decenter at the vertex of any off-axis aperture.)  

   

• Enables more accurate addition of measured interferometric data on any surface.
• And we have more plans for the future…..

I can’t really think of any cons, but there are some limitations right now.  For example, we don’t yet support adding diffractives to another surface.  Also, to reduce complexity, we made the call that all Composite surfaces in a stack will use the same off-axis coordinates, so you can’t position Composites in different locations.  Enabling composites was a big effort, especially in terms of testing, so if you find any unexpected behavior, please let us know ASAP!