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#401593
I'm no expert with this type of render so hopefully some more experienced users will add comment. Some tips I would explore -

- add an artificial emitter where you estimate the HDR sun location is so you can control its intensity to simulate something similar to Maxwell Sky sun,

- the HDR may not be a high quality one?? - it may require rendering to a higher SL level. You can try a reduced size render so it renders quicker (say 640 x 480) and maybe even turn on the denoiser then render to you begin to see something similar to Maxwell Sky?

Hope this helps.
#401597
Your question on HDR as background lighting got me thinking as I felt sure after my last post I had used this in some way so I created a simple model using green frosted glass , a site photo of a building project we did with the sun in the background for reference and ran some quick 640 x 640 res tests with results below. All of our work is on buildings and interiors we design so we don't get down to the finer details of small objects as per your situation.
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Image
Environment = IBL lighting with SKY =HDR Image, REFLECTION = HDR Image, REFRACTION = HDR Image ILLUMINATION = Active Sky
Changing the film Response from the standard Maxwell to Agfachrome CTPrecisa 200 improved the clarity in the green caustics shown on the ground plane. The IBL was also increased to 24x standard photo illumination whilst the camera settings (Maxwell default) were at ISO 100 and Shutter 600, Sun and Sky both at 1.0. For an intermediate user, like myself, this option allows an image background (good for architectural renders) whilst also being able to accurately position the sun, day/date/time etc. whilst also getting the benefits of visible caustics and other lighting effects.
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Image
Environment = IBL lighting with SKY =HDR Image, REFLECTION = HDR Image, REFRACTION = HDR Image ILLUMINATION = HDR Image
Film response changed to Kodachrome 64, IBL at 11111, ISO 1.0 and Shutter 3600, Simulens also on for some effects. Green caustics still visible but faint in comparison to the above scenario. I'm not sure how to accurately locate a sun's day/date/time using this set up.
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Image
Environment = IBL lighting with SKY =Active Sky, REFLECTION = HDR Image, REFRACTION = HDR Image ILLUMINATION = Active Sky
Film Response is Ektachrome 320T (Color Rev), with the Tone Mapping Temp adjusted into the Blue spectrum to increase the depth of blue in the sky. Simulens also turned on showing some lens flare etc. ISO at 50, Shutter at 400 Sun 1.4, Sky 0.5. The green caustics are now even more prominent.
This is a setup we use more often with an example of the subtle lighting Maxwell can produce here - viewtopic.php?f=7&t=45951
This is a first attempt at trying to create more realism in our renders.
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Image
Environment = IBL lighting with SKY =Active Sky REFLECTION = HDR Image, REFRACTION = HDR Image ILLUMINATION = Active Sky
Same as above except Film Response changed to Agrfachrome CTPrescisa 200
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Image
Environment = IBL lighting with SKY =Active Sky, REFLECTION = HDR Image, REFRACTION = HDR Image ILLUMINATION = Active Sky
Film Response changed to Agrfachrome RSX2 200 and Tone Mapping temp back to 6500
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Maxwell works differently to other renderers with most of the "tricks" coming from adjusting the camera settings to match real world situations. I still have a way to go on this front but have found this is where the best realism can be produced. With your render, you might take a look at some of the additional settings available noted above to see if the caustics become more visible. Also, the subtle shadow and lighting effects in Maxwell become more visible the longer the render is left to work.

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