[Canada_Rockies]

OK, why does it affect the right hand side of the image and not both sides?

Betcha the right side was closer to the camera than the left.

[RobG]

Betcha the right side was closer to the camera than the left.

I'm pretty sure that the Brindabella ranges were at infinity with respect to the lens. If anything, the section of the range on the right was closer than the left, but in either case the horizon was probably in the order of 20+ kilometres away.




Please follow the horizon in this image at 100% and you'll see that the ridge is sharp except for the right hand edge.

[Na Horuk]

Decentering can make a photo unsharp on one side only. This means that the optical elements are not perfectly aligned. Either its because they were put together poorly, because they were rattled/damaged during transport or use.. or the lens bends. It might be that final option, if you only notice problems at one focal length/focus range. You see, some lenses extend quite a bit when they are focused and zoomed in certain range, and if this extension is long, the lens barrel might bend a little due to gravity. Just imagine, its a long tube with heavy glass inside. It doesnt take much for there to be a slight decentering. Most viewers wont notice it, though.
Of course, it could also just be that one part of the lens needs to be wiped clean. It could even be a smudge on the back of the lens - these are often more visible than smudges on the front. Filters can also be the cause of such things.