Hi Wilson, 

I think it would be worth giving this model a try in OpticStudio. The geometric calculations done by OpticStudio can predict the performance of a system beyond the diffraction limit, and it can report the diffraction limit in its performance analyses. In fact, it's common (and typically, desired) for people to design diffraction-limited optics using OpticStudio. Therefore, OpticStudio would be a good tool to 1. Determine if the Zebrafish eye does hit the diffraction limit and 2. Predict the performance of the eye, whether it hits the diffraction limit or not.  

Two common tools to quickly observe the diffraction limit are the Standard Spot Diagram and the MTF plot. It’s important to note, the MTF consists of three distinct tools (FFT, Huygens, and Geometric), where only the FFT and Huygens options account for diffractive effects, and the Geometric MTF does not.  Whether with the spot diagram, or with the MTF plot, you will be able to determine if your system hits the diffraction limit.  If you need help identifying this in OpticStudio let me know. 

In addition to the Image Quality tools mentioned above, the Physical Optics Propagation (POP) tool allows for surface-by-surface propagation of a beam with full diffractive effects, which is useful for situations where diffraction effects dominate over geometric aberrations.  

Take care,  

Nikki