"Spyglass" effect in photoshop...?

[KongKurs]

Hello you all

Does anyone know of a good way to create a "spyglass" effect in Photoshop?
I'm talking about leaving most of the picture unaltered, but giving it a circular ring of shade around it - just like looking through a spyglass...

The nearest effect I've found is lighting effects, but these tend to overexpose the middle of the photo, which wasn't the intention..

Alternatively I could use a light black "brush" tool in the picture corners, but this will never result in a perfect circle..

Can anyone help?

 

[Peano]

Use the elliptical marquee tool to make a round mask on a fill layer. If you want a soft edge on the circle, just blur the mask. If you want to see the image through the black, decrease layer opacity.