Footsteps in the sand

Journey to Death Valley, walking around the Eureka Sand Dunes.

When looking at the photo I noticed that the corners are darker. I have read somewhere that's it's called something but why does it happen and how do I prevent it?

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Lovely photo, and I'm guessing the footprints are yours since they are coming towards you.

The darkening at the edges is called vignetting and can be caused when e.g. you are using a wide angle, such as your photo at 18mm, and any filters, or polarisers or lens hoods are not quite big enough and start to impinge on the photo.

I was using a polarising filter and using a hood. Thanks.

yeah, those were my foot prints.

this is a photo that my buddy took of me. You can get an idea of perspective.

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The valley sure is some size.

I also noticed that your first photo was wide open at f/2.8, and stopping down (e.g. to f/8 ) would have probably eliminated your vignetting. It would also be moving into the sweeter spot for your lens too.

I'm curious why you would shoot a sprawling landscape like this at f2.8, and what lense you are using that would have such depth of field at such a wide open aperature?

Congratulations for your composition of the shot!

Your footsteps provide a very effective anchor to the image!

Remember what Rudgej said: stop down the lens(use F 5.6~F 8.1) where there's enough light to get maximum sharpness and lose the vignetting.