Quote - Is there an advantage to using true HDRI lat maps on our environment sphere, VS a jpeg of the same image? I can see the benefit of HDRI for textures that interact with the HDR IBL light, but the background photo or environment sphere is not (or should not be) affected by the light ( turn the diffuse value down to zero in the main shader tree). so other than for reflection, is there a gain? since the light is actually coming from the HDRI BL, & the environment is there to match light & any reflections.  Please correct me if I'm on the wrong foot here

 

Actually you answered your own question. Of course your direct view of the image is pretty much the same as a LDR copy. But reflections of the environment sphere matter because you are showing a fraction of the image intensity.

Suppose the brightness of the sky is 1, the clouds near the sun is 8, and the sun itself is 50. An LDR copy of this is 1, 1, 1. So the reflection of sky, sun, and cloud would come out the same and look really dull. Whereas, the HDR panorama will give you the true reflection. Suppose your reflection is from something soft like leather, with a value of .02. Then .02 times 1 (sky) is .02. .02 times 8 (cloud) is .16. And .02 * sun (50) is 1. You really would notice the missing clouds and sun reflections in your object with LDR, because all three would be at .02 - nearly invisible.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)