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
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.
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.
As directed in the course exercise about photographing black, grey and white objects, I used a 1.5x crop sensor camera with a 60mm (90mm full frame equivalent) lens and focused on black, white and neutral grey cards. In actual fact the grey card was more brown but still neutral. The portrait lens used allowed me to fill the frame with the cards colour. The images are set out below:
There are some variations in the tones and some strange vignetting (probably a result of the lens struggling to autofocus on the neutral subject and the uneven light from my desk lamp) but broadly they are very similar. I sketched these into my notebook and then photographed the page in the notebook.
I then repeated the exercise but this time with the camera set to manual. The images again are set out below:
As can be seen these images print an all together different view of the three cards and begin to illustrate the point being made in this exercise. As in the first part of the exercise I did a drawing of the histograms in my notebook and these can be seen below:
This time the histograms are very different. with the camera in manual mode it is seeing the light reflecting back and no compensation is being applied as would occur in auto mode.
I was aware of this principle and knew in advance that in auto mode the camera would set the bulk of the seen to a neutral mid tone. So it did not matter, grey, black or white, the auto meter would think grey. The camera does not know what it is seeing and in auto mode assumptions are applied. As this practical activity highlights the camera is easily tricked.
Although I knew this in advance and had read many times of this effect it was valuable to actually prove the point through experiment!