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 centered 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.
I took two pieces of mount card and my 18% Grey Card; placed them into daylight just outside my front door against a wall and used the auto setting (P Mode) and added the results of each histogram to my learning log.
Set your camera to manual mode. Now you can see your light meter! The mid- tone 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.
I placed the same mount board in the same location as the previous exercise and using manual setting keeping the aperture at f8 and ISO 400 I was able to control the light meter by adjusting the shutter speed. The screenshots of the histograms have been added to the learning log.
I noticed far more control using manual mode.