The merit function should never increase during optimization, so this is clearly not correct operation.

That said, hammer should not be needed for a two-variable system. Can you post your file here, as an archive (ZAR) file so we can see what is happening? I can see you have a POPD operand in the MF at line 15 that is returning a value of zero, and that's where I'd look first. The optimizer might be looking an a design variant where the POP calculation fails. It should back itself out of that case without problems though.

For a two-system file, I'd suggest using the Standard DLS optimizer, or maybe the Orthogonal Descent optimizer

Hey Rob,

That's the process I would advise. Use a ray-based merit function from the Optimization wizard to generate the basic shape of your system, and then use POPD for a final refining pass. I'd certainly not want to start a design from plane-parallel-plates and only use POPD.

That said, the behavior you reported initially is a bug, but support will need a sample file to see exactly where and what it is. 

I can help with that :-)

There are problems with optimizing with POPD from a poor starting point. The optimization can stagnate, which means that the optimizer cannot compute a change vector: every change makes the MF larger. The design may have to get worse in performance terms before it can get better (you are trapped in a local minimum) and a single target in the merit function (POPD) in this case) generates only a single difference.

In contrast, the ray-based methods that the Optimization Wizard uses can generate hundreds of targets in the merit function and each one peforms continuously with perturbation. It's actually a huge achievement to have a merit function that works from a completely-out-of-focus design to diffraction limited without modification! The key is that you have many targets (operands) and so can compute the change vector better.

There's also a more prosaic reason. Physical Optics typically generates small changes compared to the ray calculations: the focal point may shift by 10s to 100s of microns say over whatever the EFL of the lens is. Using POPD to iterate from the ray-based solution to the POP based solution is much faster than using POP to compute the entire history of the lens from initial setup to final design.

So bottom line: always use the ray-based default merit functions from the Optimization Wizard until you have a system that is near to the diffraction limit, or is diffraction limited. Then use POP to identify the beam waist minima/other POPD data rather than the ray-based minima.