Does spot/partial metering always assume 18%?

Question

I'm beginning to work on understanding and hopefully effectively using different metering modes in my photography. I have done some reading and understand the benefit of using spot and partial metering in portrait photography. I am using a Canon 500D for reference, but I imagine it would be the same for most DSLR cameras.

My question is this: When I set the camera to spot metering, find the location in the frame which is the "most important" and let the camera meter that spot, will the camera always set the exposure with the assumption that the location I metered has 18% luminance?

To this effect, if I'm shooting a portrait and focus on the forehead for example, I can assume the camera set the exposure as if it were neutral gray, and then increase the compensation for lighter skinned people, and decrease compensation for darker skinned people?

Answer

Yes, that's how it works.

This can be helpful when you want to ensure that some subjects are to be exposed correctly. For example, meter off a white shirt and open up 1.5-2 stops to ensure it's not blown out. Another classic example is a black leather jacket. Metering off it and then stopping down 1,5-2 stops will ensure it looks black without losing detail.

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