One Simple Trick for Photoreal Renders

margrave

As anybody who's attempted to use real-world camera and lighting values in Iray knows, it doesn't seem to do a very good job with them. Oftentimes we need to jack up our aperture or f-stop to ridiculous degrees just to make sure we're not looking at a pitch-black screen. For instance, the following viewport render was made with the "Sunny 16" rule. The f-stop was set to 16, the ISO was set to 100.0, and the shutter speed was one stop above it at 125.0. The only lighting came from an HDRI set to its default values.

Looks terrible, right?

But the secret lies in one of the most obtuse sliders on the Render Settings tab, "cm^2 Factor". Per the Iray documentation, this slider controls "the conversion factor between pixel values and candela per square meter".

Simply change the value from 1.0 to 10.0, and voila.

Perfectly realistic outdoor exposure.

So it seems Iray does handle real-world lighting values. It just (for whatever reason) handles them at a tenth of their real-world values. By changing the scalar that converts pixels to light, we can bring them up into the correct range.

It also works for "Scene Only" lighting setups.

The following render was taken mimicking a portrait studio setup. Shutter speed: 200.0. F-stop: 2.8. ISO: 100.0. It was lit with two spotlights sized 18in x 24in and set to 7000.0 lumens apiece.

As we can see, the first image is way too dark. But, if we ramp "cm^2 Factor" up to 10.0...

Perfect range for portrait lighting.

Here's another one with the Beam Exponent turned down to 1.0 (and the Lumens turned up to 8000.0) to get rid of the highlights.

nonesuch00

Thanks, I will remember now that cm^2 Factor to avoid fiddling with all those other parameters.

ed3D

~  interesting subjects  ,   thanx

lamoid_5f20d3e469

Excellent tip. Thank you, Margrave!

Gordig

I did a similar experiment quite a while back, only focusing on cm^2 factor rather than playing with other values.