Why doesn't Aperture show black as black?

Question

Aperture seems to be displaying my images brighter than they actually are -- this makes colors seem washed out and makes it very difficult to accurately adjust the black point and other exposure parameters.

Here's a screenshot:

Notice that the loupe (set to the same magnification as the zoomed-in image) is more saturated, and blacks are blacker than the surrounding image.

Also compare the bottom edge of the image to the black strip just below it.

This happens even for totally black images, and is independent of any image adjustments:

Why is Aperture messing with my colors like this? How can I fix it?

Answer

You've accidentally turned on Onscreen Proofing, which is used to simulate the colors of a final output medium like CMYK ink on paper. Turn it off by going to the View menu and clicking Onscreen Proofing, or hitting Option-Shift-P

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