A bit of a mystery! Actually its one of those images that look one way or the other depending on how one perceives it. Wittgenstein would have been proud of this!
Its something in the style of the famous duck/rabbit, the Neckinger cube and other similarities that change according to one’s perspective, yet more akin to the rotating hollow concave/convex face effect (see this example.)
This picture is the bottom of a tree on the edge of Winchmore Hill wood, Grovelands Park, North London. Its a park I have known since I was a kid. Had been walking past this tree when some unusual topology to one of the tree’s trunks caught my eye. A face present (there are others too I hadn’t spotted at the time – look at the small leaf – its a fantastic FACE) is that of a bearded guy which ironically looks better in the concave version!
The face, seen at right in this crop from the main image.
I shot this picture quite low looking over the top of the tree trunk, the background is the earth surrounding the trunk and assumed it would show the top of the trunk as a clear line against the background. A quick look at the camera’s monitor showed the dynamic range and clarity was okay.
When I got home and put the image on screen it was a surprise. It wasn’t the picture I took, so asked myself where is the one I took? It was indeed this one but it seemed so very different, just couldn’t tell it was a tree trunk.
On closer inspection the image looks convex rather than concave. If it is rotated from the horizontal to vertical it immediately becomes obvious it is concave. So weird and never saw it that way.
It must have been the lighting at that particular time although once again it wasn’t noticeable at the time of composition. Clearly the image has some effect from the light that causes it to look like this. See this You Tube discussion on illusions that present themselves differently according to perception.