Working with Light: Modifiers and Filters
Light travels from its source to the subject and then to your sensor. Along the way, it can be modified in ways than help or hinder your composition. We’ve looked at several pictures that show how the angle of the sun changes the color of its light, and how fog and haze can scatter it. The strength of sunlight changes throughout the day. All of those conditions affect the amount of light reaching the sensor, requiring a change in exposure and often a shift in color balance.
The river scene was taken in late afternoon, and the light was coming in at an angle that caused it to rim the crew’s shoulders. The sun was about to go behind the hills, and was starting to cast a shadow on the slope across from the camera position, and on the water on the other side of the boat. The day was humid, and the haze scattered the light, making the distant trees look soft. The vertical position of the sun determined both the location of the highlights on the rowers and the placement and intensity of the shadows. The sun’s position and the haze were both light modifiers.
The picture of the large fish was taken on a bright sunny day, but 50 feet underwater!
Air and water are both light modifiers. Water is denser, so some effects are easier to see in underwater shots. I used a strobe unit on a long arm attached to the camera as the light source. The side of the fish toward the flash shows its true color. The side in shadow has a noticeable green cast. And the rest of the picture is bathed in a blue cast. That’s due to the way water absorbs light, starting with the red portion of the spectrum. The particles in water also scatter light and reduce sharpness.
Optical Filters
Once upon a time, when we used film, serious photographers carried a pouch full of glass filters they could screw onto the front a lens to modify the light as it entered the camera. Some were used to change the color balance. Color films were designed to be used under daylight or tungsten conditions, and these corrected the existing light to match the film’s preferences.
A light yellow filter improved skin tones with black and white film. With a DSLR, many of the tasks handed by filters can be done in an editing program like Photoshop. As a result, few photographers carry filters, and many don’t even know about them. There are some types that still come in handy.
Polarizers
If you have ever used polarized sunglasses, you know how they can make the sky look bluer and reduce glare on water and glass. A polarizing filter does just that with a camera, both digital and film. This image shows just the kind of scene that makes me reach for the filter kit.
Polarizers work sort of like a gate that only lets in light rays in that line up a certain way. The ones that don’t are blocked. You rotate the filter until you get the desired effect (or the best available). You can also use one to add glare or reduce the blue in the sky. The angle of the sun and the reflective surface make a difference in just how much effect you see. Under the right conditions, the change can be dramatic. Most DSLRs require a circular polarizer. That’s so the filter won’t confuse the auto-focus system. There will be a change in exposure when you use a polarizer, because part of the light is being blocked from reaching the sensor. The exact amount varies with the strength of the effect.
Neutral Density and Graduated Filters
Sometimes you want to be able to use a slower shutter speed to blur the motion, or a wide f/stop to reduce depth of field, but the light is too bright. A neutral density (ND) filter is a dark sheet of glass designed to reduce exposure without adding any color to the light reaching the sensor. They come in different strengths, and are labeled in stops. A 2x ND filter cuts the light in half, or one stop; 4x by two stops; etc. Graduated filters come in different flavors. A graduated neutral density filter is lighter at one edge and slowly gets darker as it moves toward the other. There are graduated color filters that let you blend a color to scene. Some are combined with a graduated neutral density layer.
These can be very handy for intensifying the warm orange of a sunset or increasing the dark blue in the sky, without changing the overall color balance or exposure of the main point of interest. This photo shows this effect with a darkened sky in the upper-left portion of the picture.
Infrared Filters
There are portions of the light spectrum that we can’t see. Ultraviolet lies beyond the blue portion of the visible spectrum, while infrared extends beyond the red, and both are invisible to the human eye. Some DSLRs can be used to take pictures using ultraviolet (UV) or infrared light (IR)with the right modifications and filters. Both UV and IR images are labeled false color, because the shift in spectrum alters the way colors are recorded, in both black-and-white and color. There are several programs, like Alien Skin Exposure 2, that can mimic false color photography (with varying degrees of success). The next photo shows what an infrared image looks like.
Notice how dark the sky is and the way green is rendered very light to white.