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In situations involving low levels of light it can often be beneficial to obtain a longer exposure to allow the sensor/film more time to absorb color and glow. There can be a lot of color and light bouncing off clouds and hills that our eyes can’t pick up. Cameras are able to “see” into low light scenes by leaving the shutter open and collecting more light. If I really want to pull as much light and color from a scene as possible I’ll often use a neutral density filter (ND) along with my usual filter stack (polarizer and graduated neutral density when needed) and also set the ISO on my camera down to 50 in order to extend what was already going to be a long shutter speed.
A neutral density filter is a neutral gray piece of glass or plastic that is placed in front of the lens. Since it is neutral it doesn’t change the color of the image or do any other special effects. All it does is reduce the amount of light that can pass through the lens to the image sensor or film (kind of like wearing color neutral sunglasses). By reducing the amount of light coming in, the exposure time needs to be longer to get a properly exposed image. One effect of longer exposure times is the blurring of anything moving within the image (water, clouds, wind blown trees, etc.). Another affect is that if there is low level colored light washing over the scene it will saturate in the image over time. I use Singh-Ray neutral density filters because they are some of the most color neutral filters available and give excellent results. I also use Singh-Ray Graduated Neutral Density (GND) filters. GND filters are neutral gray at the top and fade to clear near the middle. They are used to hold back the light in one part of an image (like a bright sky) in order to balance the light across the scene.
The two photos above were taken one right after the other, the first at 6 sec @ f/20, ISO 100 with a 3 stop GND for the sky. The second was taken at 30 sec @ f/20, ISO 50 with a 3 stop GND and a 3 stop ND. Quite a difference 24 seconds of shutter time can make.
