Jeremy Walker, a landscape photographer, Nikon ambassador and course leader, has noticed some recurring mistakes when it comes to filter use. Here's how to make sure you get it right, every time.
The same image, with a 0.9ND hard grad and Super Stopper attached, which increases the exposure by 15 stops
1 Wrong exposure mode
A lot of people try to shoot in aperture-priority mode. If you then put an ND grad in front of an aperture-priority meter reading, the camera just adjusts the exposure as it thinks the image has got too dark. To avoid your camera negating what the ND grad is trying to do, switch to manual exposure mode.
2 Keep the ND grad in
The Lee Filters Stopper range (Little, Big and Super Stoppers - long-exposure ND filters designed to reduce the amount of light entering the lens by 6, 10 and 15 stops respectively) is highly popular. If you have metered with an ND grad, though, and used it to get the exposure right, don't take the grad out before attaching the Big Stopper. You will need both filters - the grad to balance the sky, and the Stopper to affect the image as a whole.
3 Don't forget the viewfinder
Live View is a useful tool when starting out with filters, particularly if you also bring up the histogram so you can see the affect the filter is actually having on the exposure. But I prefer to look through the viewfinder when using filters. It concentrates the mind as you can't see anything else. With Live View, you get reflections and distractions, and can easily forget about the corners of the images.
4 Make sure everything is lined up properly
You need to ensure the ND grad filter is parallel to the lens, otherwise you start to get ghosting and reflections of what is behind you appearing in the final image. Filter holders usually have two slots, and it's easy to cross the filter from one slot into the other, so insert your filters carefully.
5 Use Big Stoppers appropriately
I see a lot of shots that didn't need 'stoppering' in the first place. There is no point using them to capture scenes where nothing is moving.
Hard or soft
After working out the density of filter required, the next decision is whether to use a hard or soft grad. Hard grads have a sharp transition from clear to dark, and allow the point of transition to be set on the horizon, where the sky is often at its brightest. Soft grads have a much more gradual change from clear to dark, and are suitable for landscapes where there are trees, mountains or buildings above the horizon. The use of a hard grad in these situations would produce a distinct line and ruin the shot.