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I've noticed that if I have an existing scene setup for Blender Internal and I switch to cycles, the lighting is completely off. I sometimes have had to start a new scene and import the models I want from the previous scene to get the lighting to work. What is different in the way they work?

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    $\begingroup$ What are the main differences between Blender Internal and Cycles? $\endgroup$ – iKlsR May 24 '13 at 5:02
  • $\begingroup$ In Cycles, part of the way that lights work is through an emission shader; unlike BI which uses lighting primarily from points, the way Cycles works is friendly to the concept of physical objects emitting light; this leads to incompatibilities because traditional "lights" don't physically occupy space, and sometimes seem to confuse Cycles. $\endgroup$ – Kyle Willey May 24 '13 at 5:10
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Cycles uses a physically correct light falloff by default, Blender Internal uses a smoothed falloff with a Distance parameter. Cycles can give a similar falloff by using the Light Falloff node with Smooth parameter.

Cycles lamp strength for point, spot and area lamps is specified in Watts, while Blender Internal has no clear physical unit that I know of. For Cycles this means you typically need higher values, as you couldn't use a 1W lamp to light a room, you need something stronger like a 100W lamp. For Blender Internal the default Distance of 30 means you can use lower strength values.

Sun lamps in Cycles are specified in $\frac{\text{Watts}}{\text{m}^2}$, which requires much smaller values like 1 $\frac{\text{Watt}}{\text{m}^2}$. This can be confusing, but specifying strength in Watts wouldn't have been convenient, the real sun for example has strength 384600000000000000000000000W. Emission shaders on meshes are also in $\frac{\text{Watts}}{\text{m}^2}$.

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    $\begingroup$ +1 for units; that's cool that it actually uses watts. Is one Blender unit = 1 meter? $\endgroup$ – wchargin Jun 9 '13 at 23:15
  • $\begingroup$ Yes, one blender unit is one meter. $\endgroup$ – Andrew Price Apr 25 at 6:15
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In Cycles, part of the way that lights work is through an emission shader; unlike BI which uses lighting primarily from points, the way Cycles works is friendly to the concept of physical objects emitting light; this leads to incompatibilities because traditional "lights" don't physically occupy space, and sometimes seem to confuse Cycles.

In addition, Cycles is a more complex render; light bounces around off of even diffuse surfaces, meaning that a scene that is not enclosed or that is enclosed with objects that are likely to bounce back unexpected light (for instance, a normal outdoors scene within a large magenta cube), may wind up causing different results than were originally anticipated.

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    $\begingroup$ Little Stack Exchange tip; you really don't want to award answers until at least 24 hours after asking the question-I've heard this sort of thing explained much better than I did, and people tend not to answer questions that have already been accepted; this is particularly mediocre answer, in my opinion. $\endgroup$ – Kyle Willey May 24 '13 at 5:53
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    $\begingroup$ Looks like light strength in Cycles is 100 for converted lights (give the default light, which has strength 1.0 in Blender Internal, after switching to Cycles nodes, you'll see what I mean). A Sun light will blow everything away easily if you don't take care about tweaking settings. $\endgroup$ – jesterKing May 24 '13 at 10:25
  • $\begingroup$ Yes, definitely. It's part of the compensation for the changes in how things light the scene. $\endgroup$ – Kyle Willey May 24 '13 at 15:26

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