So Im working on a new scene and were doing some rendering tests in Superfly and I meshed a bit around with the settings and especially the Diffuse bounces settings. Which accordingly to tool tip description control the indirect diffuse light. I have always kept this on 2 usually. But I did a bit of testing and was wondering if anyone can give some examples of why you would use it? and under which circumstances that would justify the extra render time. Because based on the following tests Im not really sure what exactly the huge benefit is from using it.

The test scene is a WIP scene using Superfly materials and all images are rendered using the same settings besides those mentioned under the images.

Test 1:

(No clamp filters have been used in this image)

Its pretty obvious how much noise is in the left image, purely due to the 2 diffuse bounces. However whats more concerning I think is that the render time is doubled.

Test 2:

(No clamp filters have been used in this image)

To increase quality of the render its common to increase the pixel samples as well. However again this will greatly increase render times as well. But again the left image contains a lot more noise than the one on the right. However the greatest visual difference between the images from what I can see, is the green painted iron shelf on the left side of the image. Which seems a bit more flat in the right one compared to the left one. However whether that's due to the noise rather than the diffuse setting, is hard to tell I think.

Test 3:

(Uses clamp filters 1 in all of them)

This shot uses the light from the HDRI map shining through the roof which is the only light source. And it seems like there is a very minor difference in lighting, if you look very careful. However again the render time is much higher than the non diffuse one.

Have anyone done any testing on this and in which cases these diffuse bounces should be used? Because personally for me, double render time based on the visual differences in these test images is not really justified to why this shouldn't be kept at zero at all times.

Would really like to know what experiences others have done in regards to this.