Graduated (GND) Filters or HDR?

With such powerful digital editing tools available, the use of color (e.g. warming) filters has dropped dramatically. Neutral density and circular polarizing filters are still very useful. But what about graduated neutral density filters? Many landscape photographers swear by them.

A graduated filter is dark on the top and clear on the bottom. In between is about a quarter inch where the dark area gradually blends away to the clear area. In the most common version of the filter, the dark half blocks two stops of light.

Often the sky will be much brighter than the subject. This leaves the photographer with a difficult choice: expose for the sky or expose for the landscape? The human eye can see both just fine, but our cameras have less dynamic range. Graduated filters are used to try to balance the exposure between the two. The dark part of the filter is positioned so it blocks the brighter light from the sky, while the clear part lets through the dimmer light from the mountain, waterfall, ocean etc. There are a variety of these filters with two different aspects to consider: the amount of light blocked and the width of the transition.

Problem 1 - Which filter to use?

Depending on the scene being captured, it may be desirable to have a very gradual transition--a good example is the ocean at sunset. In other cases, like at the edge of a mountain range, it's better to have a sharp transition. Does the filter need to block one stop of light, or two, or three? It's easy to choose the wrong filter and end up with less than perfect results.

Problem 2 - Where to position the transition?

In order to darken the sky but leave the landscape untouched, the filter has to be positioned correctly. Idealy all of the sky is fully covered by the dark part of the filter, and then the landscape is blended in by the transition between light and dark. In many instances, this is problematic. Look at the photo of the Watchman above and pay close attention to where the land meets the sky. The horizon is a jagged line that wanders up and down the frame! How should the filter be positioned in this case? If it's placed so that the sky is covered by the dark part of the filter, then the entire mountain will be underexposed by two stops. But if the filter is moved up, then we begin to overexpose the sky. There is no good way to position the filter.

This is a problem with most photos that were taken with a graduated filter. If you look closely, you notice that parts of the landscape are artificially darkened (often to black) by the placement of the filter.

Problem 3 - Shooting time is limited

The Sun never stays still in the sky, and this means that the angle, quality and amount of light falling on the landscape is always changing. This is especially true during sunrise and sunset--the best light only lasts a handful of seconds. In this brief window of time the photographer has to compose images, focus (perhaps do some focus stacking), set the correct exposure and take photos. Adding a graduated filter to the mix means choosing the right filter, mounting it on the lens, positioning it, and checking the balance between sky and landscape. All that fiddling with the filter takes valuable time that could be used for taking more photos.

Problem 4 - Filters are expensive and a hassle to mount

Landscape photographers are fond of using wide lenses, often the wider the better. But wide lenses are the hardest to mount filters on. The much loved Nikon 14-24mm lens requires a bulky aftermarket adapter to be mounted on the lens and the large graduated filters for it are expensive. For the Canon 11-24mm lens, there is no adapter for filters. That lens is so wide, it may not even be feasible. Filters can be placed on the bayonet end of the lens (between the lens and the camera body) but this doesn't allow for a graduated filter that must be adjusted up and down to match the horizon. Then if the photographer switches lenses, the filter must be moved from one lens to another.

Using HDR Techniques

The alternative is to take two or more exposures to capture the full dynamic range. In the photo above, one exposure was taken of the rocks and trees. In that shot, the sky was badly overexposed, so a second shorter exposure was taken of the sky. With the camera's exposure bracketing feature, this can be done with one press of the shutter button. Later, during editing the two exposures are combined into a natural looking image.

Conclusion

The argument for using graduated filters is that you can get it right in-camera. The problem is that with an uneven horizon, parts of the landscape are underexposed by as much as two stops. How can this be considered "getting it right?" It's better to take multiple images that correctly expose each part of the scene. Adding a graduated filter makes shooting slower and more complicated (as if it wasn't complicated enough!) Before the invention of the digital camera and editing software, these filters were useful if imperfect tools. But things have changed and better results can be obtained now with different methods.

It's likely that this discussion will change in the next couple of years. It's rumored that Sony is working on sensors with a much larger dynamic range (possibly as much as 15 stops). Researchers at MIT recently showed sensor technology where pixels could fill up, reach their maximum (white) value and then be reset, so they could continue gathering light. The sensor would keep track of how many times the pixel had been reset and save this value in the image. This would make it essentially impossible to over-expose an image.

If you would like to learn how to blend images to extend dynamic range, I can recommend an excellent tutorial series by Sean Bagshaw: Developing For Extended Dynamic Range