In the last two postings I discussed the concept of the exposure triangle and how the interaction between ISO, shutter speed, and aperture create the best exposure for a digital image. In the last post I touched on the sunny f/16 rule and how you can use the concept of stops of light to adjust exposure for desired creative effects. Now we will look at the details and practicalities of setting the exposure in a dSLR camera.
Advanced photographers rarely use the Automatic and Programmed modes of exposure control because they want to be able to control the artistic qualities of the image by manually adjusting elements of the exposure triangle. In the Automatic mode for exposure control, the camera controls the balance between the three exposure elements. In low light it may increase the ISO, open the aperture to a larger setting and slow the shutter speed - all increasing the exposure of the image. This mode may create a grainy, noisy image if the ISO is too high; it may create a shallow depth of field if the f-stop is too large, and, it may create blur if the shutter speed is too slow. Each of these can ruin a photo. The Program mode allows the photographer to set the ISO to a known and constant value but the camera still adjusts the other two elements in a manner that is not always optimal. So, how to we achieve control over our dSLR cameras?
All dSLRs have (at least) three other exposure control modes. In the Manual mode the photographer controls all three element of exposure - usually by adjusting them according to readings from the internal light meter of the camera. Usually the ISO is set first and then either the aperture (to control depth of field) or the shutter speed (to freeze motion or control blur). If the right combination of aperture and shutter speed cannot give an adequate exposure , ISO is adjusted and the process starts again. This sounds complicated but it is not and with experience, manual exposure control can be properly set in seconds.
Two other modes help the photographer achieve both proper exposure and creative control of the image. In Shutter Priority (Tv on a Canon) the photographer sets the ISO and adjusts the shutter speed for the best creative effect. The camera then changes the aperture to achieve proper exposure. If the camera cannot get proper exposure with the settings, a beep and/or flashing alert is seen in the view finder and the shutter release may not activate the shutter. In Aperture Priority (Av on a Canon) exactly the opposite occurs - the photographer sets the ISO and aperture and the camera optimizes the shutter speed. This give creative control for depth of field and a huge range of shutter speeds - usually from many seconds to a fast as 1/8,000th of a second. I find that I personally use the Av mode about 80-90% of the time. I can get large depth of field for scenic landscapes using a small aperture (f/16 to f/40) or a large aperture (f/2.8 to f/4.5) to create a shallow depth of field to blur the background and focus the viewer on the subject.
Each of these exposure control modes is dependent upon the through-the-lens light meter of the camera. The light meter is capable of viewing different parts of the image depending upon the need of the photographer. One of the most sophisticated metering settings is commonly called evaluative metering. With evaluative metering the camera assesses the entire frame of the image to adjust exposure for the best combination of light and dark areas. The camera meter senses the entire image and "recommends" an exposure that is the best average for the entire image. It particularly attempts to avoid over-exposure that will destroy details and cannot be recovered by post-processing software. The exposure can be adjusted manually using the viewfinder light meter or the value from the light meter can be used to adjust Automatic, Programmed, Shutter Priority, or Aperture Priority settings.
Similarly, most dSLRs have the ability to select metering from only a portion of the viewfinder. These metering modes are called partial (field), center-weighted, or spot metering. Each of these metering modes weights a smaller and smaller portion of the image so that only the lighting of the subject will be assessed in the determination of exposure. While each of these metering and exposure modes is very good in determining the exposure for a perfectly lighted subject made of mid-range color tones, judgment is still needed to properly expose subjects that are very white or very black and in background that are very bright (snow or bright sand) or very dark (shadows.) Here, the experienced photographer manually tweaks the exposure up or down using the Exposure Compensation setting. Typically a pure white subject such as an Egret with a neutral or dark background will yield a blown-out, over-expose bird so the exposure should be turned down 1/3 - 1 full stop. On the other hand photographing a dark brown bison in the snow will need a positive exposure compensation of 1/3 - 2/3 stops to avoid severe under-exposure of the bison.
Understanding exposure and its control is fundamental to good photography. It require a solid understanding of your camera function and controls and experience to select the proper exposure and metering modes and manually compensate for difficult lighting conditions.
In the next posting we will move on to selection of lenses for nature photography.