Dirty lens article
The wider the lens, the more dust on the front element affects the picture. Especially specular light (bright light sources, especially the sun).If you have a very wide-angle or fisheye lens (e.g. gopro) then you will most certainly have lots of nastiness in your photo from having a dirty lens.You'll notice that in this person's test shots, he has no specular light or sunlight in the images. If he did, the test would have had completely different results.If you have a gopro, try it yourself. Have a dusty or greasy lens and take a picture with/without the sun in it and the photo will certainly be worse.Same with my mobile phone - if the front glass is greasy, well, of course the photo looks much, much worse.I'm a professional photographer specializing in 360º imagery shot with fisheye lenses, so I know this stuff very well from experience.