Checking the Enable realtime lights option will make lights sources update instantly as you work, letting you preview how they will change when you try out different settings.

Enabling realtime lights when using the photograph setup Product lighting will show a very dark, practically black, view in Photo Lab and the 3D View until lights are placed in the drawing. Although this makes it hard to make out details, it does not have a negative impact on the actual rendering, which will look just fine.

Because this is a very resource-intense operation, it is turned off by default. It is recommended that the checkbox remains unchecked when working on larger drawings with many light sources, as this can slow down the 3D graphics.

Enable Realtime Lights off	Enable Realtime Lights on

You will notice that with Enable Realtime Lights on, the lighting is quite extreme. The contrast between light and dark is exaggerated so that it is easy to see light sources, where they are pointed, and how they will blend with other sources. Once the image is rendered, the lighting will have a much more realistic appearance: