Why is there a subtle shadow glow around the subject with large aperture?

Question

I've noticed that in a number of photos I've taken with a small depth of field due to wide aperture and a fairly close subject, I get a rather pleasing but not entirely explicable diffuse shadow around the subject, even when the camera is not between the subject and a light source. It's usually hard to see unless the background is uniform and the picture is small enough to make the gradient readily apparent. Could someone explain why this is happening?

Here's a shot with a Nikkor AF-S 35mm, f/2 at 1/250. There are two window to the side of the subject, and an awful yellow lamp overhead. I slightly adjusted the levels and took out the color, which makes the "glow" a little more visible, and in the next photo I have really blown it out to highlight the shadow glow.

Answer

That's your Nikon body doing its Active D-Lighting trying to show detail in the faintly lit wall on the background (remember, it's further away from windows and the lamp). Close to your well-lit subject, brightening of the back wall must be reduced to retain original local contrast.

In other words, your camera is creating an HDR image and this is a typical HDR halo.

I suspect the large aperture part is only coincidental because you (or the camera) naturally chose to use it in the dim room.

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