Last renders of the caustic cube series
Some observations:
The Falcon renderer has several default settings relevant for rendering refractive caustics:
Clamp sample = 10
Adaptive sampling = on
Ray depth = 10 ( 3,2,10,8 )
Caustic filter = 0.1
The ray depth here is often too low and should be set higher, especially glossy bounces should equal specular bounces in order to catch internal reflections.
The other settings normally grant clean and speedy renders with adaptive sampling but are coming with restraints:
The caustic filter is blurring caustic patterns. Lowering the value to get sharper caustics heavily increases noise and render times.
Clamping samples is another restriction introducing bias by discarding light (violating energy preservation).
For my taste the default value of 10 is too low regarding light balance (especially in combination with filmic tone mapping).
Increasing the value again causes more noise and longer rendering.
So the default values together are optimized for manageable rendering at the expense of aesthetic quality.
That's ok for scenes with small, not dominant refractive objects.
When the values are changed the noise detection from adaptive sampling starts to struggle.
I often resort to rerendering parts of the imige with a camera mask and denoising in Photoshop.
One more hint:
Clamp settings in the light properties also compromise caustics so I tend to disable them (in HDRI object set value to zero).