Transcript
Controlling Exposure Are you ready to leave the safety of Auto mode and gain more control of your camera? Here are the three main variables we can control. Aperture The aperture refers to the lens opening of a camera, which is traditionally identified as a set of blades that form a circular hole, letting light onto the film, or sensor. A larger aperture opening allows more light to fall onto the sensor while a smaller opening restricts the amount of light. However, there is something else that happens when you alter the aperture. When the aperture is wide open the depth of field (i.e. the amount of the picture in focus) is shallow, and the camera can only focus on a portion of the image. When the aperture is closed down to its smallest setting much more of the image will be in focus, but less light falls on the sensor because the hole, or opening, is smaller.
The numbers given to the aperture setting can be confusing. The smaller the number e.g. f2 the larger the aperture. The bigger the number e.g. f16 the smaller the aperture. To make it even more confusing f4 lets in twice the amount of light as f5.6, and f5.6 lets in twice the light of f8. The number (called the f number) is the amount of times the diameter of the aperture can be divided into the focal length of the lens. E.g. an aperture of f4 on a 100mm lens has a diameter of 25mm. On a 200 mm lens f4 has a diameter of 50 mm. However an aperture of f4 has the same effect on exposure regardless of the focal length of the lens. I know it does not seem to make sense but trust me it happens. So, how do we get more light at small apertures and less light at larger apertures? Shutter Speed We alter shutter speed. Shutter speed regulates the amount of time that the aperture stays open. So, if we are shooting at that small f/16 aperture, we’ll need a slower shutter to allow in more light. This means the camera will have to remain steady for longer.
In low light conditions a tripod or some other support may be needed to avoid the picture being blurred through camera shake. If you want to capture fast sports action, a fast shutter speed is necessary. Usually 1/500 to 1/1000 second range will stop anything, but may require a wide aperture and therefore give us a narrow depth of field. Is there a way we can have both a fast shutter speed and a small aperture? How much light do we need? ISO Back in the film days, you would buy different ASA (ISO) types of film. 100 – 200 was good for bright light while 400 – 800 was suitable for low light. In the digital world, this translates to the sensitivity of the camera’s sensor and we have the advantage that we can alter the ISO for every shot, rather than wait until we have finished the film. When the ISO is raised to 800 or 1600, the low light sensitivity is greater, but at the sacrifice of noise, or graininess. If you shoot in Auto mode a lot and wonder why your images are always noisy, it’s because the camera is most likely setting a high ISO in order to ensure a well-focused shot with a fast shutter speed in lower lighting conditions. Camera performance at high ISO levels (i.e. low light levels) is improving all the time, but if you want to avoid grainy pictures, manually set the ISO speed lower and see what happens. Just be sure to use the correct shutter speed/aperture ratio as well. Each time we double the ISO we double the sensitivity. If we close the aperture by one place e.g. from f8 to f11, or open one place e.g. from f16 to f11 we have to double or half the shutter speed, or double or half the ISO to compensate, we call this a “stop”. The light meter in the camera will measure the available light and tell us how much we need to correctly expose the picture. It’s up to us to determine what combination of shutter speed, aperture and ISO we wish to use for the effect we want. The following exposures all let in the same amount of light and give the same exposure, but with different effects. F2.8 1/1000
F4 1/500
F5.6 1/250
Keith Leatham December 2011
F8 1/125
F11 1/60
F16 1/30
F22 1/15
