Modern cameras have great built-in light meters, but sometimes taking photographs using available light (i.e. without flash or other photographic lighting) can be a challenge, especially with long exposures in low light levels or very high intensity lighting. Fortunately in extreme conditions you can usually use an exposure guide like this one. Simply enter your film speed setting, choose the lighting conditions from the list and then see a table of shutter speed and aperture combinations. This table allows you to use a camera with manual control in conditions outside the range of the built-in light meter. The app will also compute the Exposure Value and light level in lux. It's a simple tool to help you take good photographs in difficult conditions.