It involves creating a middle grey card to check against your exposure settings. Well, I tried and failed. My card (highlighted), isn’t the grey value I was expecting. I don’t know if I’m doing this right. My setup is quite simple. I have a HDRI at normal strength, and flat materials. Wondering if it’s because the HDRI is an overcast? I just tried another background image, same result.
You’re half way there. Now you need to adjust your exposure (or hdr strength) up or down until that card appears grey. And to be sure, middle grey should be 0.18.
I prefer to adjust the hdr stregth when calibrating. This gives me my base exposure. Then I use the exposure for further adjustments to my liking.
And keep in mind, this isn’t a rule, but a guideline. There’s no 1 true magic exposure. It’s all relative. It’s the same in photography. Depending on the lighting, you wouldn’t always want to expose for a mid grey card.
What is “normal strength”? I use the photographer addon which lets me set exposure as real values. Using sun lamp & sky texture (~400 and ~40 strength), checking against sunny 16 rule (anything outside in front sunlight should be correctly exposed).
If I now replace my sun&sky with an HDRI, I bring up the background strength until I get the “feeling” I want from it. If I know the HDRI shows an overcast, I’d change the exposure to 1/100s, ISO100, f/8. Note that many (most) HDRI have a clipped sun. If the shadow contrast is too little, you may want to supplement with an actual sun lamp, where you use a reflective sphere to watch where your sun is compared to the way toned down HDRI’s sun.
I’m not using a grey card at all. I check false color for blown out, and usually allow a bunch of stuff to be blown out (sky, lamps, strong reflections etc). If not, I’d adjust the light strength of what is causing the blowing out, keeping the main subject in the green (or under or overexposed if that is on purpose). I’m not a fan of “do nothing to the lighting when we can in CG but keep everything completely unclipped using local adaption” HDR tonemapping techniques.
But everything cgCody said holds true. Just wanted to express my way of thinking about “how to expose” from a photographer point of view.
the advantage to use a grey card or grey diffuse 0.18 RGB material,especially with filmic is,that if you calibrate the HDR strength or exposure to this grey,then mostly, if not all PBR materials are fitting to the light then.
the only variations are expected with different light dynamics with different HDRs, or self made extreme lighting setups.
But you can also apply the lessons of the “gray card” without actually trying to render such a thing. First, read up on Ansel Adams’ “Zone System,” then consider the following tips.
Start with the histogram tool. The shape of the curve should be loosely bell-shaped and centered. It should not have "spikes"within the curve, and neither the left nor the right ends of the curve should be pronounced. (Blender provides several video-analysis tools, and you should familiarize yourself with all of them and use them regularly. With them, you can quantify what you see, and exercise statistical quality-control over a long project.)
The area that you would pick as your “gray card” should be in the center (and so, the peak) of the curve. Ansel would this center area, “Zone 5.”
While the properties of video, film, and print are all different, the essential lesson is the same: “light” and “dark” are all relative to this mid-point, and it’s not far at all from this to “featureless black” and/or “blown-out white,” both of which must be avoided.
The human eye is capable of resolving more than 20 so-called “f-stops” of brightness because your eye actually scans the scene to assemble your perception of it in your brain’s visual cortex. All “media” must approximate this effect within their extremely-limited gamut of resolving power – in the case of film, about five stops total.
Ansel took the complex science of densitometry and reduced it to easily-applied rules of thumb that are just as applicable today as when he articulated and applied them. Others have specifically elaborated on his work in the context of video (additive vs. subtractive color) and as the capabilities of video evolved over time. These are things that will make you a better photographer, whether your scenes are synthetic or not.