Understanding exposure

Even though today's DV cameras are more capable than ever of shooting decent images in low light, the simple rule still applies that the more light you have to work with, the better the image you can capture on video. More light gives you more control over exposure, focus, depth of field, and the flexibility to shoot with more filters. A properly exposed image should clearly show all the visual details in your scene. Pay particular attention to the pattern on the pavement, the fountain, the subject's face and the light and dark areas of the frame. Now here's the same scene overexposed. Notice how the details of the building in the background, the pavement, our subject's face, and fountain are all lost to overexposure in this scene. Here is the same scene, yet again, this time it's underexposed. In this shot we lose the details in the shadows and the darker area to the scene, particularly the trees, the subject's hair and the bottom half of the fountain. And here it is one last time properly exposed so you can see the difference. Now as you can tell, the big issue with the bad exposure--whether it's over or under-exposed-- is that crucial visual information is lost when we don't have a proper exposure. So a good exposure is absolutely essential to controlling your video images. Now let's talk about the whole chain of events that leads to getting a good exposure in your camera. Factor #1: Light. The very first thing we need to get an exposure is light, whether it's sunlight, fluorescents, incandescent bulbs, or candlelight, we need a healthy source of light to illuminate our scene. Remember, we can always easily deal with excess light, but not having enough light is an issue that's much more problematic. We'll talk about both of these scenarios a little later, but for now let's continuing examining how light gets into a camera and is captured to help create an exposure. Factor #2: Is the Focal length of your lens, generally, the longer and more telephoto the lens, the more light it takes to pass all the way through and create an acceptable exposure. The more shallow or wider the lens, the less light it takes to pass through and create an acceptable exposure. In the case of a zoom lens, you can adjust it to be wide or telephoto, by zooming in and out. When you zoom all the way in, your lens is longer, when you zoom out, the lenses shorter. You can't see this change of lens length because it happens internally right in here. Factor #3: Aperture settings. When light enters a glass at the front of the lens, it must go through the aperture before it gets into the camera. The Aperture, also known as Iris, is the adjustable opening on the lens that controls the amount of light allowed into the camera. The more the aperture is open, the more light is allowed to pass through the lens, the brighter the image will be. And conversely, the more the aperture or Iris is closed, the less light will be allowed to pass through the lens, the darker the image will be. So when you hear the terms fast or slow used to describe a lens, it's really a description of how easily light passes through the lens. So-called Slow lenses require more light to get a good exposure, while Fast lenses are more desirable and more expensive, because they don't require as much light. So, Fast lenses are friendlier in low light situations and more versatile. Not all lenses are created equally, but generally speaking, telephoto and zoom lenses are slower and wide and short prime lenses are faster. Factor #4: Is the Image Chip. Now you have heard me talk a lot about imaging chips before, that's because it's really the heart of the camera. Once the light passes through the barrel and Aperture of the lens, it lands on the all important imaging chip, which is where the image is actually electronically captured in the camera. Remember, as I said many times before, the imaging chip is to video what the negative is to the film. It's the electronic surface where light is captured and translated into a digital image. And just like with the negatives, the bigger the imaging chip, the more light it captures, the brighter the image. So smaller imaging chips like those found in consumer cameras aren't nearly as good in low light situations as much bigger imaging chips, like those found in the DSLR cameras, which can capture decent image with much less light, because they have a much larger surface to gather light. So those are the four primary factors that affect how bright or dark your image will be. Now we'll take a look at how we can control and manipulate some of these settings to get the best exposure possible.