Some people like to have two or three grads, and not just to vary the weight of the effect. Sometimes it is just to get the same effect with different lenses at different f/numbers (I'll explain
). I tend to go for a two stops reduction for a dull sky, and I try to modify the effect I want with the lens I'm using by varying the f/number. Some times you run out of f/numbers for whatever reason, so it's handy to have lighter/darker and harder/softer filters to hand.
Let me explain it like this; when you take pictures through the wire fence at a zoo with a long lens, if you move close to the fence and use a wide aperture low f/number, the wire fence is so out of focus that it actually disappears. You get the same effect, caused by depth of field, with a grad.
In other words, fit a grad on to a wide angle lens at f/16 and the cut off line will be hard, and the darkening effect maximised. Use the same filter on a long zoom at f/4, and the filter will be virtually invisible - it almost vanishes, like the wire fence.
A good way to see this in practise is to fit the grad and point the camera at a plain white surface. See how the effect changes as you zoom. Click the aperture stop down button and you'll see it change again, getting darker and sharper and you close down the aperture. Shoot a few pictures and watch the effect change as you scroll through the images on the LCD.
Having said all that, the grad I use by far the most is a 2 stops ND soft cut. For the things I shoot, ie landscapes, I can usually get what I want with that, one way or another.
