I created a basic rainbow gradient background in cycles with the colour ramp set to constant. With the standard camera this appears as expected.
But when I switch to orthographic camera I only get one colour.
I can tweak the Z location in a mapping node to move the gradient around and the changes are represented on a glossy Suzanne correctly. but the background is only one colour, at a time.
I change the position of the colour ramp stops and even using 6 decimals points of precision can not get more than one colour to appear on the background at a time.
I get that the orthographic camera is flattening the background and causing a greatly reduced angle of view but should I be able to set up the background to to be a very small colour ramp rainbow?
I also thought about using the compositor, but found the images in this answer to low-res to read. How to create a gradient backround in the compositor?