Why I Don't Like Sunset Filters
Sometimes when you go out to shoot a sunset, the sunset doesn't cooperate. I'm sure it's happened to you. It certainly has to me. Common advice for how to make the best of a bad situation is to place a "sunset filter" in front of your lens to simulate the colors you had hoped to see. But images created with such filters almost always have a certain telltale flaw that lets viewers know they aren't real.
Sunset filters get written about a lot in magazines and books, but for those unfamiliar with them, they are basically an orange graduated filter of the type made popular by Cokin. They are generally rectangular in shape and have a dark orange tint at one end, fading to a ligther orange at the other. When positioned over a scene as intended, they color the sky strongly orange, with the lighter colored end providing a more toned down orange tint to the foreground.
At first glance, an image shot with such a sunset filter gives the impression of a beautiful glowing sunset, belying the fact that at least most of the color was provided by the filter, not the setting sun. But there's a telltale give away. To understand what's wrong, we need to think about color temperature and white balance.
The human eye compensates automatically, but in fact the color of light varies based on the time of day and the conditions. Light early or late in the day is characteristically orange. In overcast conditions or in deep shadows, it can be downright blue. Photographers in the days of slide film learned this quickly since without compensating for color temperature images came out looking awful. I remember the first time I shot a roll indoors under incandescent light and was shocked at how strongly orange they came out. Print film was less problematic since the color was generally corrected to a degree during printing. You took your film in and got back reasonably decent prints since they fixed things for you. Digital shooters today can also remain unaware of color temperature if they want by letting their camera at least attempt to adjust white balance automatically. Most cameras will let you take control of white balance, but you generally don't have to.
To the degree that a sunset filter works, it does so by emulating the orange color temperature of sunset. The problem comes in that this orange light only exists for objects lit by the sun. Objects in shadow aren't supposed to be orange. In fact, any light that does reach into the shadows should be on the cool end of the spectrum: blue, not orange. So if you are shooting into the setting sun, the backside of any objects in the scene will be lit by the sunset, but not the side facing the camera. Any part of any object facing the camera is in fact in the shadows — these shadows having been created by the light from the setting sun itself. So while a sunset filter colors everything with orange to one degree or another, it should leave areas not lit by the sun unchanged. Or, if it kept with the theme of saturating color temperatures that should exist in such a scene, it should actually add a blue cast to such shadows. The very property of these filters that adds the missing orange colors to the setting sun also add orange to the shadow areas, the exact opposite color it should be adding. In so doing, they work against themselves in their efforts to simulate a real sunset scene.
Also, in a real sunset, the strongest orange color tends to occur when the sun is low in the sky, with the color transitioning to black or blue as you look further upwards. Yes, bands of clouds above the sun can pick up the color and reflect it back towards the camera to expand the area of intense color, but with a wide enough shot, there should be sky that tends more towards indigo above the orange. A real sunset can be a tricky thing to imitate.
Of course, if there are no objects in the scene to cast shadows, and if the image is framed tight enough around the sun, the filter can create a convincing effect, but images of just the setting sun with no other objects tend to less interesting that if they had a foreground. An image of just the sky lacks any real context or focal point apart from the sun and colored clouds. It could have been shot anywhere. Also, if the image is sufficiently underexposed to ensure that every object that is visible is seen only as a silhouette it can mostly hide the shadow color temperature problem. Shadows that are perfectly black don't have any color even with that darned orange filter doing its thing. This can be tricky though since any silhouette that isn't solid black show some color and the filter will result in that color being orange, not blue. Even light diffracting around the edges of foreground objects can look suspicious if it has too strong of an orange cast.
Some of these effects can be better done in software in this age of digital photography, but in my experience, the best way to create an image that looks like an amazing sunset is to be there, in the right place, when an amazing sunset happens. Call me old fashioned perhaps, but I like not only shooting images that look like sunset (or sunrise), I like being there to see such scenes with my own eyes. Beautiful sunsets can be amazing indeed. No filter can be as satisfying.