Making a Good Exposure
What is Exposure?
Exposure is the amount of light that contacts the imaging sensor (either film or digital chip) of your camera. There are two components of exposure: brightness and duration. In the camera, brightness is controlled with the lens diaphragm and duration is controlled with the shutter. Equivalent exposures can be made with many different combinations of diaphragm openings (also called aperture or f-number or f/stop) and shutter speeds. Although the exposure can be equivalent using different combination of aperture and shutter speed, the visual effects of these different settings will vary greatly. For this reason, it is good to have some understanding of how changes in f/stop and shutter speed will effect the final image.
Aperture and Depth of Field
The amount of light that passes through a camera lens is regulated by an iris diaphragm. The opening size, or aperture, of this diaphragm is referred to as its f-number or f/stop. The f/stop is derived by dividing the focal length of the lens by the aperture diameter. This leads to the counter-intuitive situation where a small number f/stop actually represents a large aperture, and a large number f/stop denotes a small aperture. More light will pass through a lens set to f/2.8 than one set to f/22.
Depth of field, which is the distance in front and behind the plane of focus that can be considered to be also in focus, is dependent on the f/stop. At a low f/stop (large aperture, say f/2.8) there is very little depth of field, while a high f/stop (small aperture, say f/22) exhibits much more depth of field. This can be visualized easily by photographing a ruler, tilted to a 45 degree angle, at these 2 f/stops. For this test a 60mm lens was focused on the number 6. At f/2.8, the depth of field extends from approximately 5.5 to 6.75 on the ruler. At f/22 the depth of field is greatly enlarged as the picture shows.
In real world terms this means that you can control the amount of your subject that will be in focus by choosing different f/stops. When photographing flat objects, the aperture setting is not critical, for if you have the camera back parallel to the subject it will all be in focus. When you photograph a 3D object though, you must decide which plane of the object you wish to be in focus and how you want to use depth of field. You can decide to use a shallow depth of field (low f/stop,2.8) to highlight one plane of the subject while blurring the rest. Conversely, you may decide to have as much of the subject as possible in focus by using a high f/stop like f/22. Remember though that if you have less light entering the camera, the exposure time must be longer in order to get a proper exposure on the image sensor.
Shutter Speed and Movement
The shutter on your camera controls the length of time that light exposes the image sensor. The numbers on a shutter speed dial refer to fractions of a second, thus the numbers 8, 15, 30, 60 really mean 1/8, 1/15, 1/30, and 1/60 of a second. It is not a good idea to hand hold a camera at exposure times longer than 1/60 second (unless blurs are what you’re after). For these longer times it is advisable to mount the camera on a tripod. If a tripod is unavailable, try bracing yourself against a wall or tree for added stability. Slight camera movement during exposure will result in poor quality pictures. If you notice directional streaks when viewing images at high magnification you should use a higher shutter speed or tripod.
When photographing botanical specimens it is also important to keep the subject from moving. This might require turning off fans, closing off drafty rooms, or blocking wind with sheets of cardboard.
The ISO setting refers to the sensitivity of film or digital chip to light. The ISO number of film is set in the factory when the film is manufactured. Film with a low ISO contains relatively less sensitizing material (usually silver), will require more exposure, and will exhibit finer grain than film with a high ISO. Some film cameras will automatically set the cameras light meter to the correct ISO by reading a bar code on the cassette while others require manual setting of the proper number.
The ISO setting of most digital cameras is variable and can be changed to fit the lighting conditions. On some cameras this is done through a menu system and on others it can be changed on a dial. It is best to set your digital camera to the lowest ISO that allows you to expose the subject with the settings (f/stop and shutter speed) you want. At higher ISO settings a digital image will get increasingly “noisy” and degrade the quality of the image.