Difference in shadow without caustics

[hrisek]

I made render for few boxes with simple glass shader. If caustics are off then shadows are not so intensive (refraction color is 250,250,250 ).
If turn on caustic - shadow is going to be black (refraction color is the same - 250,250,250).
Is this bug or a physical correct feature?

[Ondra]

This is intended bias to prevent fireflies. You can set the render engine to PT+PT with MSI 0 to get the unbiased result, but it would created fireflies. You can also set Renderer in main tab to Bidir/VCM to get unbiased result, although fakes are not supported in that mode

[Ludvik Koutny]

This is correct behavior. When caustics are disabled, transparent shadows are used instead. Similar to checking "Affect shadows" checkbox in VrayMTL. When Caustics are on, shadows are opaque and caustics are traced in them. Progressive renderer is quite inefficient at resolving caustics though, that's why there is almost no caustics yet, just a few fireflies. You may try Bidir renderer with VCM enabled to render caustics faster.