 Taking better photos made simple. These are Mary and Paul. They're looking forward to their vacation. But what cameras should they take on their trip? Mary knows that many modern cameras are so good that they usually take great pictures in automatic mode. But some of her photos still don't turn out well. What can Mary do better? Photography literally means painting with light. Why? Because everything around us reflects the rays of light in the environment. A camera lens captures those rays. That's what forms a picture in the camera. The light, and therefore the photo's quantity, is controlled by several determining factors. The most important ones are aperture, exposure time, and light sensitivity. Imagine the aperture as a pair of curtains. It controls the amount of light entering the lens. When it's dark, Mary wants to capture as much light as possible. To do that, she needs to open the aperture wider. If it's very bright, Mary draws the aperture curtains closer together. Otherwise, her picture will be overexposed. The F number, or focal ratio, specifies up to how far the curtains can be opened. At F2, only halfway. At F4, only a quarter of the way. So, the lower the number, the more light can pass through a particular lens. But those aperture curtains don't always stay open. They only open for a split second. That length of time is called the exposure time, or shutter speed. The longer the exposure time, the more light enters the photo. That's fantastic if Mary wants to take a photo at twilight, for example. But if a car drives through the photo, it will reflect the light from different positions, so it just appears blurry in the picture. The same thing will happen if Mary moves slightly while taking the photo. That's why when using long exposure times, the camera should always be resting on a steady surface. But when it's bright out, Mary only needs to briefly expose her photo. So the car comes out nice and sharp. No tripod needed. Mary still isn't quite satisfied. Despite using a wide open aperture and a long exposure time, the result is still too dark. That's where light sensitivity, or ISO, comes into play. It's more or less the photo's skin type, and determines how sensitively the camera reacts to the light. High sensitivity can quickly lead to a sunburn. Then the photo has something called noise. Mary is overwhelmed. Fortunately, she doesn't have to do everything manually to take pretty pictures. In semi-automatic mode, Mary specifies just one value. The camera figures out the rest. Even so, the perfect setting for all situations just doesn't exist. A good photographer reacts to prevailing light conditions, uses average values, and accepts a compromise now and then. It's almost like a healthy relationship.