Exposure really boils down to whether or not the image handled the level of brightness that was available. This is to say if it was too bright, not bright enough, or somewhere in the middle where it was acceptable.

Over Exposed Image

Controlling that exposure, and the exposure range in which you work, is an important part of actually being a videographer capable of creating images that are acceptable for you and the medium you are producing in. To really craft these images correctly you have to initiate exposure control, which means simply a way of controlling your video equipment and situation so that the overall image exposure will fit within the realm of usability.

Under Exposed Image

The first thing you have to look toward is your video equipment specifically. Look at your digital video camera over all to see what kind of images it is capable of producing. Many low end digital video cameras do not have the faculties to register images in very low or very high levels of light. Look to your lens specifically to see if you can handle the situation you are in.

Motion Blur

Since your iris diaphragm is going to be the most important place you take control over your exposure you have to think about this. Try to see if you can apply different filters to make your exposure more manageable, especially neutral density filters. Overall shutter speed is also an important aspect of this, so you should keep the possible increase or decrease of shutter speed in mind. At all times you are going to have to remember that shutter speed has a direct correlation to the frame rate of your final digital video image.  In general, the lower the shutter speed is the more motion blur will be allowed into the image.  This is because the lower the shutter speed, the longer the shutter stays open, and the longer the sensor is exposed to the image.  Do not take your shutter speed much below 1/60, which is actually a commonality between video and DSLR still photography.

Shallow Depth of Field

The aperture is going to remain your primary area for altering the exposure of the video image, but you need to keep in mind that this will also change the depth of field.  Opening up the aperture to let more light in will also shrink the depth of field, so remember how the image will end up looking and what the range of acceptable sharpness is you are going to need.

If you are working in film you are going to have to consider the film stock you are using. Remember that temperature balancing is central to all film stock and they are usually balanced for specific types of light. Film speed is also altered with push or pull methods, which will relate to over or under development. Understand the types of film stock you are dealing with ahead of time and make sure to do film stock tests before a major shoot.

Take a look at your situation and all the light sources and surfaces you will be dealing with. Until you really look closely at a give environment you will probably not see how many different reflective sources of light there are. All of these are going to eventually play a serious role. Find out if there are lights you can control and try to plan for any ambient light or natural light that may cause issues.