Clifton Forge Twilight Controlling Exposure, Part I – Understanding Exposure Exposure in photography occurs when light from the scene comes through the lens and strikes the film or CCD. The greater the amount of light the greater the exposure and vice versa. However, the phrase “amount of light” needs to be explained. Amount refers to several components of the light in the scene, how much of the light is transmitted to the film/CCD, and finally to the sensitivity of the film/CCD. Brightness of Light – Refers to the intensity of the light that is shining on the subject, AKA, scene brightness. Light can be direct and bright like sunlight on a clear day, or it can be diffuse and soft like the light filtering through the atmosphere on a cloudy day. The light that shines on the subject is measured in two ways: intensity and color temperature. It’s the intensity of the light that affects the exposure. Duration of Light – Refers to the amount of time that light is allowed to strike the film/CCD. Although light may strike your subject for hours on end, you control the duration of the light that strikes the film/CCD with the shutter speed. Typical shutter speeds are measured in fractions of a second, but can also measure into full seconds or even minutes in extreme circumstances. Common shutter speeds start with 1 second and may go down to 1/1000 of a second in 50% increments. Running Stream, Douthat Park, VA, c2005 Controlling Exposure, Part II – Using the Shutter Speed Duration of Light – Refers to the amount of time that light is allowed to strike the film/CCD. Although light may strike your subject for hours on end, you control the duration of the light that strikes the film/CCD with the shutter speed. Typical shutter speeds are measured in fractions of a second, but can also measure into full seconds or even minutes in extreme circumstances. Common shutter speeds start with 1 second and may go down to 1/1000 of a second in 50% increments. Changing the shutter speed controls the duration of the exposure. Some digital cameras allow you to set shutter speeds from 15 seconds to 1/3000th of a second in manual mode. In Program or Auto modes the camera will select whatever shutter speed provides the best exposure. As an example of extremes in shutter speed, star trails and photos of the solar system may take hours of exposure while stop action images may require exposure times measured in the thousandths of a second. Tube Park, Winterplace at Sunset Controlling Exposure, Part III – Using the f/stop While scene brightness may be a given you have to live with, and shutter speed may be your first point of exposure control, another way to control exposure is the f/stop. The f/stop refers to the size of the aperture (lens opening) through which the light will travel to reach the film plane. The larger the f/stop the more light will reach the film and vice versa. F/stops are measured in fractions, so f/2 really means an f/stop of ½, while f/4 means an f/stop of ¼ which allows only half the light of f/2. Changing the f/stop controls the amount of light during exposure. Typical f/stops include: f/1.2, f/1.4, f.2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/64 Not all lenses include all these f/stops. Inexpensive digital cameras may include only 2 or three choices, and point and shoot cameras don’t offer a choice at all – just point & shoot. For greater control, most lenses and cameras allow you to manually vary f/stops in 1/3 stop increments. Your camera’s exposure scale probably shows the typical f/stops with two dots or marks in between indicating 1/3 and 2/3 of a stops.. On most modern 35mm film cameras, the f/stop is controlled by dials on the camera body; on digital cameras the f/stop is controlled by either a dial on the camera or from the Main Menu. Winter Leaves Controlling Exposure, Part IV – The ISO Setting The ISO setting adjusts the sensitivity of the CCD in a digital camera. The higher the sensitivity, the less light is needed for exposure. In a film camera, the ISO was referred to as the ASA or film speed. For the purposes of this discussion, ISO and ASA are identical and refer to the sensitivity of the film/CCD. Changing the ISO/ASA is another way to control the exposure of an image. If the ISO is doubled, for example from 100 to 200, then only one-half the amount of light would be required to make an equivalent exposure. If it is reduced from 100 to 50, then twice the amount of light would be required for an equivalent exposure. While changing the ISO is a convenient alternative in altering the image exposure, you should be aware that increasing the ISO also increases the digital noise (known as grain in film cameras) in the image. Shoot with the lowest possible ISO to get the best quality image, but increase the ISO whenever you have no other alternatives in light intensity, shutter speed, or f/stop in making a properly exposed image. This image was shot with an ISO of 400 and the noise was prominent because of a muted background. The noise was exaggerated for effect. Firefighter Backlit Controlling Exposure, Part V – Proper Exposure Proper exposure is when the detail is visible in the important shadow areas and in the important highlights of an image – with emphasis on the word important. Only you can determine what is important in your image. The camera will be easily fooled into giving you an average exposure for your scene and subject. You may need to override the camera settings to get the exposure that provide the image you want. While the primary and most important aspect of exposure occurs at the time the shutter is released, there is considerable enhancement that can be accomplished after the exposure using image editing software. Over Exposure – Over exposure occurs when detail is lost in the important highlights (the light tones) of an image. Under Exposure – Under exposure occurs when detail is lost in the important shadow areas (the dark tones) of an image. The tones in this example may be technically underexposed, however, the overall darkness adds to the drama highlighted by the backlighting. Some digital cameras provide a "historgram" to help you determine exposure. A historgram is a graph that charts all the tones captured in an image. It can show you if you have over or under-exposure. The display on your digital camera may also warn against overexposure by flashing overexposed areas during review/playback. Read the article on Histograms for more information.