剑桥雅思套7听力test1section4原文答案解析+文本精讲

[00:30.15]Last week, we focused on the creative side of photography, composition etc, but this week it's time to get a bit more technical.
[00:38.63]Today, our focus is going to be on exposure. 
[00:41.99]The term exposure simply refers to the amount of light your film is exposed to.
[00:46.51]Or put another way, the amount of light you allow to strike your film.
[00:51.38]A good photographer has got to know two things.
[00:54.62]One, how much light is required to capture a particular image.
[00:58.90]And two, how to control the light reaching the film.
[01:02.71]The former is usually determined by the camera's inbuilt light meter and the latter is taken care of by means of the aperture and shutter settings. 
[01:12.22]Essentially, exposure time is controlled by opening the aperture and allowing light to pass through it for a fixed duration.
[01:20.00]Aperture sounds like a fancy word, but in simple terms, it is basically a hole whose size can be varied to allow more or less light to pass through it.
[01:29.40]Aperture sizes described in F numbers, with each F number being half as bright as the previous one, the difference in value between one full f number and the next is known as a stop.
[01:41.44]The smaller F number, the larger the aperture and the greater the amount of light being let pass through it.
[01:51.30]Shutter controls also play a crucial role in determining the exposure for a shot.
[01:56.54]The shutter prevents light from reaching the film until the instant of exposure when a picture was being taken.
[02:02.82]Then it opens for a predetermined amount of time, allowing light to pass through the aperture and onto the film.
[02:09.94]Shutter speed is expressed in seconds or fractions of a second.
[02:14.14]A one unit change to the shutter speed is also known as a stop, and the change of one stop to the shutter speed has a similar outcome to a one stop aperture size adjustment.
[02:25.29]Overexposure, as the name suggests, occurs when you give your film more exposure to light than is necessary to capture a clear image.
[02:34.10]Telltale signs of overexposure include pictures dominated by pale or light shades and poor washed out colours.
[02:42.50]Underexposure then occurs when there is not enough light and produces the opposite result.
[02:48.06]A dark image with poor detail and shadows.
[02:51.38]Before we go on to look at how to ascertain the correct exposure settings for a particular shot, let's take a short break.