I'll be in the minority to say below the lens. I have two sets; above and below the lens. I used both, and I can say with confidence that there is absolutely no difference between the two. I prefer the below the lens set as it's much easier to work with. Besides, I have to use under the lens for my 4x5 enlarger. Scratches and dust aren't a problem - just keep the filters in the case when you're not using them.