One of the delights of nature photography is that you’re never quite sure what you are going to see and photograph; and equally, one of the great frustrations of nature photography is……
Suppose you are in a field taking photographs of hares when a marsh harrier flies into view. You take a picture of the bird against a bright sky. If you are shooting in automatic mode (P) or semi-automatic mode (Av or Tv) you will be frustrated with the outcome because, with your camera set up appropriately to take photographs of the hares, the bird will appear very dark, if not black.
So, why does the marsh harrier appear very dark?
In order to set the exposure, DSLR cameras use an integrated light meter to measure the reflected light from the objects in the scene which you intend to photograph. The light meter can be seen when you look through the viewfinder. Also, there is often an option so that you can see it with other shooting data on the back screen of the camera. It is composed of a scale which usually extends from -3, through zero, to +3. It has an indicator which moves up and down the scale as the level of light increases or decreases. If you point your camera at a very bright scene the light meter will read about +3 and with a very dark scene it will read about -3. Each unit on the scale represents one stop of light.
A stop of light
A stop of light is a relative value: it has no specific units of measurements. It refers to a change in the amount of light. A change of one stop is equal to altering the amount of light by a factor of two. If you wish to reduce the exposure by one stop you would halve the amount of light striking your camera sensor. If you wish to increase the exposure by one stop you would double the amount of light striking the sensor. For example, to increase the amount of light by one stop, double your camera shutter speed (from say 1/1000 sec to 1/500sec) or double the area of your aperture by reducing the F number of your lens by one unit (from say F5.6 to F4). Alternatively, increase the sensitivity of your sensor by increasing the ISO value by one unit (from say 800 to 1600). Each of these changes would double the amount of light recorded by your camera sensor.
Auto mid grey
When you take a picture in P mode (automatic), Av mode (aperture priority) or Tv mode (shutter speed priority) the camera first makes an assessment of the amount of reflected light in the scene. Then, no matter how light or dark the scene, it adjusts the light levels reflected onto the camera sensor so that the average light level is a mid grey (ie: on the scale of 0 - 255, this gives a luminosity value of 128). This adjustment is usually valid; but not invariably. When you point the camera at a scene with little variation in the reflected light levels (eg all very bright or all very dark) the camera will still automatically set the average light level in the scene to a luminosity of 128. Of course, on a dark day any given scene will be relatively dark and on a bright day it will be relatively bright. But regardless of the actual reflected light levels, the camera will set the average light level of the scene to a mid grey. This ensures that the picture is well illuminated and sensor captures the full range of details in the scene with no areas too bright or too dark.
Black cats and snow
There are two classic examples which show that sometimes the camera gets it wrong. The first is a photograph of a black cat sat on a stack of coal. The whole picture is composed of shades of black. In P, Av or Tv modes both the coal and cat will appear grey. This is because the camera will allocate an average mid grey evaluation to the scene and because all elements of the scene have essentially the same luminosity level, the outcome is a mid grey cat and mid grey coal. The second example is of a winter snow scene. Again the light meter will average the light levels to mid grey and, because all the luminosity levels are about the same, the snow scene will appear in the photograph as mid grey, not snowy white.
The classic way of resolving the “black cat on coal” issue is to use exposure compensation. So, you use the light meter and darken the picture by about one or two stops of light so that the cat and the coal appeared suitably black; and with the snow scene the picture you would brighten the picture by about one or two stops of light so that it appeared appropriately white.
You can do the same with the hares and the harrier: so before you press the shutter to take the picture, use the light meter and turn the wheel on the back of the camera to add about two stops of light. The harrier will then be appropriately exposed.
An alternative approach is to set the camera shooting mode to M (manual exposure). You then choose both the aperture and the shutter speed, as well as the ISO, to determine the exposure. The camera is now suitably set for the amount of reflected light within the scene and the exposure will be set correctly whether you are shooting hares in the field or harriers in the sky.
But how do you determine both the aperture and the shutter speed, as well as the ISO?
You use the light meter.
Shooting mode set to manual, with spot metering and the light meter set to zero.
Using the light meter
First decide on the aperture. You will probably want the background to be out of focus so that the hares stand out in the picture. Consequently, choose the widest aperture (shortest depth of field) available on your lens, say F5.6. Then, point your camera at an object which you know within any given scene, should be mid grey and set the light meter to zero. In an appropriately exposed photograph, green grass has a luminosity of mid grey and I usually use the grass to take a meter reading. But you can use any object as long as you know its relative luminosity. For example, you can point the camera at a clear blue sky and set the light meter to +2. Or, you can point the camera at a dark shadowy object that you know should appear as a black in the photograph and then set the light meter to -2.5. If this meter reading does not give you a shutter speed fast enough to freeze hares chasing each other (say 1/2000sec) then increase the ISO until it does.
Now, when you take an opportunistic picture of a harrier flying overhead, it too will be appropriately exposed.
Of course, on a dark day the grass will be relatively dark and on a bright day it will be relatively bright. But compared to the luminosity of the other elements in the scene, the grass will always be mid range. By setting the luminosity of the grass to mid grey, with the light meter showing zero, the scene will be appropriately lit: none of the whites will be blown (ie so bright they are above the recording capacity of the camera sensor) and none of the blacks will be blocked out (ie so dark they are below the recording capacity of the camera sensor).
What’s the downside?
Downside of M mode
You need to keep a constant eye on the light levels. When the sun comes out from behind a cloud the light level suddenly changes and the scene is immediately brighter. The grass will no longer read zero in the light meter and you will need to take another reading and adjust the exposure so that the grass is again set to a meter reading of zero; and you need to keep on doing this as the light levels continually change.
Manual exposure puts you in charge of your camera and gives you an intimate understanding of the way the light is illuminating your scene. And with your DSLR, use the histogram on the back of the camera to ensure that you have not under exposed or over exposed your shot; and to fine tune your exposure.
Metering the light
You can choose the method your light meter uses to measure the light. With Canon cameras you can choose between evaluative, centre weighted, partial or spot metering.
evaluative - measures reflected light levels across the scene to determine an average value
centre weighted - gives priority to the light levels at the centre of the image
partial - uses only the central part of the image to measure the light levels
spot metering - measures the reflected light level at one defined small area of the image.
For other camera users
Nikon: matrix, centre weighted & spot metering
Sony: multi, centre & spot
Panasonic: multizone, centre & spot
With P, Av or Tv shooting modes spot metering is usually a bad idea because it measures the luminosity of about 1% of the scene at the centre of the viewfinder. This may be relatively very bright or very dark compared to the overall luminosity of the picture.
However, if you decide that Manual mode is for you then it is a good idea to use spot metering because other methods such as evaluative metering will give you an average meter reading from a large area of the scene. With spot metering you can point your camera at a very small part of the scene, take a meter reading and then make suitable adjustment. Many DSLRs have a specific option to select the spot metering mode when using manual exposure and some also provide an option to use either the central AF (autofocus) point or the selected AF point to take the reading.