I posted a bug about this, but still am waiting for a response so what the heck:

Has anyone else had the experience where using a custom lens shader seems to change the size of the ray derivatives quite a bit? We wrote a fisheye shader that we're using for planetarium productions.

One example: vm_geofilterwidth needs to be increased 2-4X to match what a standard ASAD lens looks like.

Another example: displacement dicing seems to be a lot more memory intensive, probably because the dicing width in screen space is smaller.

Anyone?

Bueller? Bueller?

Turns out it was a bug after all!

Fixed in Houdini 16.