Exposure is the heartbeat of photography.and put simply, it is the process of light striking – and exposing – a material, be it a piece of film or a digital sensor. An exposure is controlled by three variables – shutter speed, lens aperture and ISO Understanding the role and relationship of these three variables is essential if you wish to master photography.
APERTURE Aperture is the common term used for the iris diaphragm in a lens.,this is an adjustable hole through which light can pass to expose the sensor (or ﬁ lm). The numbers indicating the size of the aperture are fractions, referred to as f/numbers or f/stops. Aperture settings can be slightly confusing at first, as a large aperture is represented by a small number (such as f/2.8), while a small aperture is indicated by a large number (such as f/22). When a large aperture is used, more light can pass through; when the aperture is smaller, less light passes through, however, the resulting depth of ﬁield is more extensive. Changes in aperture and shutter speed are referred to in ‘stops’ (hence f/stop), with one stop equal to a halving or doubling of the amount of light reaching the sensor.
SHUTTER SPEED The shutter speed is the length of time that the camera’s shutter remains open during an exposure. If the shutter speed is too short, the image will be too dark; if it is too long, too much light will strike the sensor and the resulting image will be too light. A camera’s metering system will help determine the correct shutter length depending on the aperture, ISO sensitivity and available light. While its primary function is to ensure the right amount of light is allowed to reach the sensor, the shutter length also dictates how motion is recorded. This is a powerful aesthetic tool; faster shutter speeds can be used to freeze motion, while slower ones will blur it.
ISO (International Standards Organization) refers to a sensor’s sensitivity to light. A low number, such as ISO 100, indicates lower sensitivity, so the sensor requires a greater level of light to achieve the correct exposure. Conversely, a high ISO, such as ISO 6400, indicates high sensitivity to light, so less light is required to make an exposure. Doubling the ISO speed halves the amount of light required to produce the correct exposure, and vice versa. With digital cameras it is possible to alter the sensor’s ISO sensitivity from one frame to the next. Increasing ISO is an effective way of generating a faster shutter speed, which is useful if you wish to freeze subject movement or eliminate camera shake when shooting hand-held. However, when practical, it is best to employ low ISO settings, as image-degrading noise is more prevalent at higher ISOs.
DEPTH OF FIELD Depth of ﬁ eld is the zone of acceptable sharpness within an image.both focal length and camera-to-subject distance also determine depth of ﬁ eld, aperture size is its overriding control. By adjusting the aperture you alter the amount of light passing through the lens, and you also determine the level of frontto-back sharpness. Only by understanding depth of filed – and the signiﬁcant role it plays in photography, will you be able to capture the images as you see them. Aperture choice is a key consideration when setting up your landscape images, so deﬁnitely not a decision you can leave to your camera. Insufﬁcient depth of ﬁ eld will result in parts of the image being recorded out of focus, which – unless intentional is likely to ruin the photograph. At large apertures f/2.8 or f/4 – depth of ﬁ eld is shallow; at small apertures – f/16 or f/22 –front-to-back sharpness is extensive
WHAT IS THE BEST APERTURE?
There is no simple or correct answer to this – it greatly depends on the shooting situation, the result you desire and, to some extent, the camera’s format.