[EDIT] Topic was created about eevee, but having the exact same problem with Cycles, in fact …
Hello there,
I am running some volumetric lighting tests on the last version from the builder (v2.81 alpha)
And render viewport turns black on orthographic mode. On this gif I’m switching between orthographic and perspective :
First that came like a bug for me…
But then I realized that in fact it’s not totaly black, it’s just “much darker”. Like if Volume Scatter of the scene would have crazy high density. And indeed, thinking about that, with some volume scatter, darkness of the scene depends on camera distance (if you move back the camera and zoom in, it’s darker due to more “volume scatter” between camera and surface).
And here it’s like if Blender was computing orthographic camera from very very far. But mathematically, an orthographic camera model is just a projection which can be done on any plane, so we should be able to actually “choose” how far is the viewport camera from the scene…
So I came to this second test : Adding a new camera an setting it to orthographic.
Here it works, and indeed, changing distance changes nothing but darkness :
It means that the only problem is to be able to set the distance of the viewport orthograhic camera… But can we ? 
Here is the question…
See you 
++
Tricotou
[EDIT] Topic was created about eevee, but having the exact same problem with Cycles, in fact …
++