Point Lights & Caustics
Presto MC cannot handle point lights (Sun, Omni, etc). In order to generate caustics, a light needs to have a surface (mesh light).
Using Bigger Lights
Lights that have a very small area, tend to create sharper caustics that are difficult to solve.
Caustics with HDR images
Caustics can also be generated using an HDR image in the Environment Illumination Channel.
The 2nd Image has been rendered with a smaller emitter. However, even if we are using the same Sampling Limit, the noise has increased.