Q I really don’t understand matte boxes. At first I thought they just were what you found on a big 35mm motion-picture camera because I see them in all the behind-the-scenes/on-the-set shots. But now with smaller cameras, I’m seeing them more and more. It has dawned on me that these must not be "built in" on the big cameras either, that they must be more like an accessory, since they certainly aren’t built in on smaller cameras. I guess what I’m asking is, what’s the point?
A It’s more than just matte boxes that you’re seeing on some camera setups. With the small form factors of some cameras, the matte box and other equipment have become more obvious. Besides your question of "What’s the point?" I’ve often been asked why someone would take a nice streamlined camera and put all that rigging on it. By rigging, they mean the rails, matte box and maybe a follow-focus control. All of that aluminum and carbon fiber does have a purpose, and it’s not just to make the camera look "Hollywood" (though I’ve heard from more than a few SLR shooters that sometimes they’re taken a bit more seriously when their small camera is all tricked-out; sad but true).
Let’s take a look at these features one at a time—matte box, rails and focus control.
The matte box has several uses. First, it controls flare or spill of unwanted light hitting the lens. Sometimes light bouncing into the lens is noticeable, such as with circular lens flares that appear in the image. (Of course, these have become so popular that people add them to generated scenes to add more "realism.")
Flare also can be more subtle. It can have a great impact on how much contrast your image has. Just a little bit of extra light mixed into the blacks in the scene can turn a great shot into an average one. That’s one of the reasons why lens manufacturers spend so much time and money developing high-tech coatings that are used on the surfaces of the lens elements.
Like many camcorder lens hoods or shades, the interior surface of the matte box is made of a nonreflective black surface that reduces the amount of light that bounces around. Let’s take the example of light coming in from a side angle and hitting the front element of the lens. Some of that light will enter the lens and some of it will bounce off. When the light bounces off the lens, it reflects into the side of the matte box or lens shade. If that surface is somewhat reflective, the light may bounce back into the lens again, so a highly unreflective surface is used on the interior.
But that doesn’t address matte box versus lens hood, since they both can have similar materials. The interior coating helps control light that’s bouncing around, but what about keeping light out in the first place? That’s where the big difference is and why people use matte boxes.
While some lenses come with lens hoods or lens shades, usually those hoods aren’t adjustable for different focal lengths. For example, if you have a zoom lens, the lens hood is optimized to work at the widest-angle focal length. If it wasn’t designed that way, you would get vignetting—darkening of the edges of the image—or you might even see the edge of the lens shade when the lens is zoomed all the way out. Since the hood is designed for wide-angle use, at other focal lengths it doesn’t provide as much light spill protection. If you’re just zooming in from the same angle, it may not matter. However, if you’re using a particular focal length on your zoom lens, you won’t have as fine a control of light spill.
The matte box is mounted on rails (more about them later), which allows you to slide it forward and back in relation to the front of the lens. This provides better control of the amount of light spill based on the focal length of the lens.
Unfortunately, this isn’t an adjustment that can be done during a take. When you’re "zooming," a matte box will have the same limitations as a lens hood. Fortunately, if you’re not zooming the lens’ full range of focal lengths, you can optimize the "light blocking" for that subset of focal lengths.