2. 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. 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.
For this exercise I happened to get the correct ‘black’ shot when I adjusted from auto to manual so I had a good starting point, I only had to adjust the aperture to get the correct balance on the grey and this also worked for getting the correct white tone although for the white I did also have to adjust the exposure compensation to +3.
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.
For this exercise I used the cover of my black sketchbook, a grey card and the white pages of my sketchbook. I set my camera to auto mode, turned off the flash and set the focus to manual. I also made sure that exposure compensation was set to 0. I tried this exercise twice and interestingly both sets of images yielded slight variations to the final outcome due to a change in the ISO, apart from the white pages, but they were still all centred around the mid-tone.
Light is measured by the camera’s light meter – but how does the camera translate this measurement into a usable exposure? In auto and semi-auto modes the light meter is calibrated to the mid-tone; this means that, after capture, the tone curve of the histogram will be centered around the middle of the graph. This works reasonably well if there’s an even spread of tones throughout the subject, but it doesn’t work so well if the subject is lighter or darker overall than the mid-tone