In landscape photography, it is helpful to have three specific lens filters in your camera bag. These are the circular polarizer (CPL) filter, the neutral density (ND) filter, and the graduated neutral density filter (GND).
Can you use a flip-flop -- or any solid item, for that matter -- as a graduated neutral density (AKA grad ND) filter? Sure. But, it doesn’t work in all situations.
With the advent of better camera technology and clever processing software, do we really need neutral density filters any more? Can’t we just replicate the effect of filters in Photoshop or even in camera? Isn’t the dynamic range sufficient in my shiny new Sony camera? Well, yes and no…