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).<p>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.<p>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.<p>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.<p>Same with my mobile phone - if the front glass is greasy, well, of course the photo looks much, much worse.<p>I'm a professional photographer specializing in 360º imagery shot with fisheye lenses, so I know this stuff very well from experience.