1. Set your camera to any of the auto or semi-auto modes. Photograph a dark tone (such as a black jacket), a mid-tone (the inside of a cereal packet traditionally makes a useful ‘grey card’) and a light tone (such as a sheet of white paper), making sure that the tone fills the viewfinder frame (it’s not necessary to focus). Add the shots to your learning log with quick sketches of the histograms and your observations.
You might be surprised to see that the histograms for each of the frames – black, grey and white – are the same. If there’s not much tonal variation within the frame you’ll see a narrow spike at the mid-tone; if there is tonal variation (such as detail) you’ll see a more gentle curve. If you find the tone curve isn’t centred on the mid-tone, make sure that you have your exposure compensation set to zero. You may see an unpleasant colour cast if you’re shooting under artificial light, in which case you can repeat the exercise using your monochrome setting (a light meter is sensitive to brightness, not to colour).
This simple exercise exposes the obvious flaw in calibrating the camera’s light meter to the mid-tone. The meter can’t know that a night scene is dark or a snow scene is light so it averages each exposure around the mid-tone and hopes for the best. But why can’t the camera just measure the light as it is? The reason is that a camera measures reflected light – the light reflected from the subject, not incident light – the light falling on the subject. To measure the incident light you’d have to walk over to the subject and hold an incident light meter (a hand-held meter) pointing back towards the camera, which isn’t always practical. If you did that each of the tones would be exposed correctly because the auto or semi-auto modes wouldn’t try to compensate for the specific brightness of the subject.
2. Set your camera to manual mode. Now you can see your light meter! The midtone exposure is indicated by the ‘0’ on the meter scale with darker or lighter exposures as – or + on either side. Repeat the exercise in manual mode, this time adjusting either your aperture or shutter to place the dark, mid and light tones at their correct positions on the histogram. The light and dark tones shouldn’t fall off either the left or right side of the graph. Add the shots to your learning log with sketches of their histograms and your observations. Switching to manual mode disconnects the aperture, shutter and ISO so they’re no longer linked. Because they’re no longer reciprocal, you can make adjustments to any one of them without affecting the others.
- I performed this exercise in my workplace that has daylight equivalent lighting using grey card from the back of a note pad, white paper and black card used for binding documents.The results are as follows:
As the brief predicted the histograms for the three images are all close to the mid tone values. Also as can be seen the images for the white and the black are more grey than their true colours.
- The exercise is repeated with my Fuji X30 in manual mode. Below are the same three images with adjustments made to shutter speed (whilst maintaining constant aperture of f4 and ISO 800) to adjust exposure to correct for camera metering:
As can be seen the manual adjustments to exposure produce results with truer colours. Therefore one has to be aware of the limitations of the cameras light metre in these type of situations, and to use the histogram as a tool to guide you towards correcting the exposure to produce a correct image.