The GenIBL-Lens and Probe are the devices that are sampling your environment. The camera is 42 inches "south" of those, but because of the way the lens works, it's as if the camera is 42,000,000 inches "south".

So all that really matters is where the lens is, because it is the image on the lens that is being rendered into your IBL probe.

The data collected by the lens is the light reaching that point in space, from all directions. So you should position the Dolly Camera so that the light being sampled by that lens is the light you want to use to light your main subject - usually a figure.

Depending on the scale of the environment, this can matter more or matter less.

For example, if sampling a football stadium, anywhere near the center, even 50 feet off, is about the same as anywhere else.

On the other hand, sampling inside a closet, you'd better be within a few inches of the actual position of your subject.

Regarding human figure lighting - you have to choose whether you want  to sample from just above the floor (like a few inches) or chest height, or head height. Again, the issue is it depends on what is nearby.

The impact of a wall or floor that is only an inch from the lens is going to be huge compared to the same wall or floor that is 60 inches from the lens, unless that wall or floor is already enormous, in which case it won't matter much.

Look, just pretend you're an IBL probe. Stand somewhere in a room and pick an object such as a door. Imagine the door is lighting you. Move closer to do the door. Now imagine the door is lighting you in this situation. Which way does the door make a bigger influence on how you are lit? Obviously when you're closer to it, so that it subtends a larger angle, therefore is contributing a greater percentage of the total ambient light reaching you.

When you use my EnvSphere, and measure lighting from it, every surface of the sphere is 750 feet from the origin. Moving the probe a few dozen feet won't make any noticeable difference. It will in a room that is only 20 feet long.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)