The following code can be used to explore the issue. I've not yet dug 
beyond what I tried in this posted scene.

Just something I happened to see while looking into other height_field 
questions of late.

The HF zero (y as image/fnct evaluated) and z HF result should cleanly 
show up no matter scaling! At best it is today noisy.

Looks to be an issue back through v3.7 stable at least. I think given 
the noise it's likely some numerical and/or bounding issue rather than 
the actual HF mesh. We'll see.

Bill P.

// Test scene
#version 3.7;
global_settings { assumed_gamma 1 }

camera
{ // location <1,1,1> * 2
   location <1.5,3,-2>
   look_at <.5,.3,0>
   angle 50
}

light_source { <10,10,10>, rgb <1,1,1> }

height_field {
   function 500, 500 { 0 } // Noisy result at y scale 0.3? Poof 0.5?
//function 500, 500 { 1 } // This is always OK no matter the scaling.
   smooth
//scale <1, 0.3, 1> // Zero Fn val Noisy
//scale <1, 0.5, 1> // Zero Fn val. HF now invisible ?
// and with no scaling also invisible.
   pigment { green 0.8 }
}

//----------

plane{y,0 pigment{rgb .2}}

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