Before we dive into the complexities of correct exposure of a digital photograph it is perhaps appropriate to discuss how the images are saved in the camera. Most digital cameras offer a choice on the quality and file format to be used to save the images on the memory card of the camera. The file formats offered are JPEG or RAW (the RAW format is specific to each manufacturer). The basic difference between these two formats is size the JPEG file is a compressed lossy file (typically 10:1 compression) that is much smaller than the corresponding RAW file. Although the loss of image quality can be quite small information from the original file is irretrievably lost by the lossy compression. In contrast the RAW file retains all the digital information from each pixel and is therefore the format of choice if careful subsequent image processing is contemplated. After the RAW file is processed a smaller JPEG copy can then be saved. In the past it was worthwhile to save both RAW and JPEG versions in the camera so that the JPEG versions could be used for initial image selection, however, with greater computing power now available this is no longer necessary. The penalty for storing RAW images is their greater size, for a full frame 35 million pixel camera they can be more than 20 mb each and this requires storage cards in the camera of 64 gb to cover a few days intensive photography.
The exposure ie. the quantity of light reaching the sensor in a digital camera is a function of the aperture and the shutter speed. Modern digital cameras offer a number of different basic approaches to achieving the ideal exposure for an image and these can be accessed through some form of selector on the camera with settings labelled P, A, S or M. The P setting, which stands for Programmed Auto and not Professional Setting as I have heard it called, is really the point and shoot option. The camera adjusts aperture and shutter speed according to the ISO value (the equivalent of film speed) selected and tries to achieve a satisfactory exposure. In many situations this will produce a perfectly acceptable outcome, but for more testing situations the photographer will require more control. The A and S settings are aperture and shutter priority settings ie. on the A setting a specific aperture can be selected manually and the camera will adjust the shutter speed to achieve a suitable exposure. Similarly with the S setting a specific shutter speed can be selected. One of the best options for wildlife photography is to combine the aperture priority setting with an auto ISO facility (accessed through the menu system). The auto ISO facility allows the photographer to select a maximum and minimum value for ISO and the camera will automatically vary the value in response to the conditions. A suitable range to use for this is 640-1600, but a wider range would be perfectly acceptable. The mode of working is then to select an appropriate aperture value, depending on depth of field required and the available light and then to focus on the subject. The camera will display the appropriate shutter speed and if this too slow the aperture can be progressively opened until a balance is achieved between shutter speed and aperture. Using the auto ISO facility allows this parameter to be largely ignored while taking the photograph. The M setting on the camera is the Manual setting where aperture and shutter speed are under the control of the photographer and can be used to achieve results under special circumstances.
HOW CAN WE TELL WHEN THE EXPOSURE IS OPTIMUM?
When the photograph has been taken it can be immediately viewed on the camera screen and some judgement made, however, in the field it is often difficult to clearly assess the quality of the image exposure. Cameras have another selectable tool which displays the tonal histogram of the image, the tones range from 0 (pure black) to 255 (pure white) and individual colours (hues) are composed of Red, Green and Blue combined in varying ratios. If the peaks of the histogram are clipped at the top on the right hand end of the histogram this indicates that the bright highlights will have lost detail (blown) while a similar clipping at the left hand side indicates loss of detail in shadow. In the same way that we can select on the camera an area of the viewfinder image to be used for focussing eg. centre weighted the exposure metering area can also be selected ie spot, centre weighted or matrix . This selection can be used to obtain a satisfactory exposure for that part of the image of interest. If the results are still not perfect exposure compensation can be used. This technique affects the entire image and compensation is applied as steps either negative, to reduce exposure or positive, to increase exposure. The individual steps are usually 0.3, 0.7, 1.0, 1.3, 1.7 etc up to 5.0, although not all cameras are identical. The exposure compensation tool is particularly useful for example when trying to photograph a bird outlined against the sky or in flight (see illustrations). Setting an exposure compensation value of + 0.7 to +2.0 ev can often bring out the detail in the bird, but the correct value for the exposure compensation can only be determined by trial and error. Similarly if the subject is in sunlight and has very pale coloured areas, white or yellow, it will often be necessary to underexpose the image to retain any detail in the pale areas. As a rule of thumb more can be done to process an underexposed image than can be done with an overexposed one and once a highlight area is blown there is nothing there to retrieve.
CAN ANYTHING BE DONE IN THE FIELD TO HELP?
Great Sparrowhawk – Accipiter melanoleucus photographed in Kibali National Park, Uganda.
Photographs showing the effect of exposure compensation to bring out the detail of a subject set against a bright background. The images are as taken from the camera without further processing.
Although birds and animals are seldom obliging in where they perch, fly or stand it is possible for the photographer to think carefully about the direction of the sun and try to ensure that the subject is not between the camera and the sun. Diffuse light, thin cloud, is by far the best condition for photography, where there is sufficient light, but no hard shadows. Birds or animals in dappled light, where there is a mixture of bright sunlight and shade, makes for the most difficult photographic conditions. Photography of plants has many advantages since the plant is not moving, unless it is very windy, and the photographer can often take time to move to the most advantageous position to get the best illumination of the subject. Here again diffuse light is the best, particularly with white or yellow flowers, and it is often useful to have an assistant to stand and create a shadow or to hold a diffuser.
Much is often made of the photographic opportunities that are created by the warm sunlight early in the morning and at sunset and in the next article we will consider the topic of colour temperature, white balance etc.
Roger Marchant - ABS Member