IES And Camera Exposure
Maya, Mental Ray
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To create physically correct lighting we need to do 4 things.
1. Use 32 bit Linear workflow with gamma’d colour swatches and textures.
2. Use physically correct shaders (Mental Ray shaders)
3. Use lights with real world settings, IES profiles or estimate the intensity settings with quadratic falloff.
4. Render with camera exposure settings using the lens shader. This is especially important for dark scenes.
To set up a light colour in degrees kelvin
1. Select the light
2. Map it’s colour to a
– Mental Ray > Mental Ray Lights > Blackbody
The intensity is a multiplier of the regular light intensity leave at 1.
We always work in 32 bit linear workflow for correct light and color mixing. For full info about linear workflow go here.
An .IES file (Illuminating Engineering Society) is a real world industry lighting standard that describes real world lighting data. We can use these to plug into Mental Ray and they’re commonly used for archvis (architectural visualisation) or rendering for architecture.
IES Light Profiles can be downloaded from light manufacturers and various 3d communities. Here’s a good collection of basic manufacturer’s lights from Derek Jenson…
A real camera needs to open the shutter or lower it’s FStop in dark lighting situations, and the same is for us in Mental Ray. It mimics the real world. We can do this in compositing/post to varying levels with our 32 bit images, but it’s best to see this in the maya viewport and do it before compositing.
Creating a Lens Shader
1. Select the Camera
2. Go to the camerashape tab in the attribute editor
3. Scroll down to the mental ray section
4. In the lens shader slot click on the add checkerbox
5. Under the mental ray section go Lenses
6. Click on the mia_exposure_photographic
7. IMPORTANT! With linear workflow we have to make the gamma on the lens shader to 1. I miss this in the video.
This will create the lens shader, the defaults are extremely dark for low light situations.
Tweaking the Lens Shader
1. For dark scenes it’s easier to set the exposure node to “arbitrary or photographic mode” to do this set the Film ISO to 0. Now we just adjust the CM 2 Factor attribute.
2. Up the “Cm 2 Factor” in small increments to change the results. Settings of 15 – 200 is not uncommon for darker scenes.
Cm 2 Factor is Candela/m2, which is a type of math for lights.
This from wikipedia…
“As a measure of light emitted per unit area, this unit is frequently used to specify the brightness of a display device”
I’ve tried to look around for real world intensity for candles with little luck. I couldn’t find an IES file as they seem to be made exclusively for electric lights.
Maya is very sketchy on what it’s intensity values actually add up to, but from the reading I’ve done it seems to equal lux, a way of measuring real world light.
Interestingly a “foot-candle” is a lighting measurement still in use in the USA. It equals 10.764 lux. So that’s a pretty good starting point.
In this tutorial I estimated by using the IES light I use both a value of 20 and then later 10. So I was pretty close.
1. Create a point light
2. Set it’s falloff to Quadratic.
3. My estimate for intensity for physically correct candle lighting would be about 10. Feel free to experiment and do your own research.
A real Director of Photography (DOP) on a film set adjusts the exposure of their camera (a combination of ISO shutter and f-stop) and changes the intensities of lights to get the right mood/artistic effect of a shot.
We do the same, simply changing the “Intensity” of our lights and the “CM 2 Factor” (a type of exposure) of our lens shader.