In the experiment below, I have set the camera to ‘Aperture Priority’ which is a semi automatic setting, and taken three photograph. One of a sheet of black paper, one of grey cardboard and one of a sheet of white paper. For the experiment to work it was important that the viewfinder was completely filled with the subject matter, i.e. there was no variation in tone. As can be seen, there is only a slight variation in what has been recorded, and this is because the camera’s sensor records tonal differences, and NOT colour variations.
Below are the corresponding histograms (top right). Generally speaking the bulk of the recording is registered around the central mid tones. This is because the camera’s light meter will always take the average/mid tone reading from an image. So here I have demonstrated that regardless of colour, if an image is recorded where every part of it is reflecting equally onto the light meter, what is produced is interpreted as the mid tone. You can see on the histogram that there is a spike with a slightly broader base recorded. This is due to the natural light falling onto the material unevenly (The light source was a window off to one side).
The experiment was carried out again, this time with the camera set to ‘Fully Manual’ mode. As you can clearly see the recorded difference is much more noticeable. With the camera set to manual, you have total control of tonality. This is because the camera’s link between aperture, speed and ISO is disabled i.e. increasing the shutter speed is no longer compensated (to calibrate to the mid tones) by an automatic widening of the aperture or an adjustment to the sensitivity of the sensor (In real terms, a digital camera only has one true ISO rating, and that is 100, but for now we will call it a sensitivity adjustment). So focusing on the black card, I was able to control the tonality accordingly (by increasing the shutter speed or decreasing the aperture). By making an adjustment I was able to move the meter reading to the left (as can be seen in the corresponding histogram below) and in doing so produce an image that better represents what I was seeing with the naked eye.
Now the histograms show a corresponding difference. The histogram of the black card image has been manipulated across to the left, The grey card image was set to centre and the white sheet of paper image was moved to the right.
The reason that the camera has to be manually corrected to create a ‘true’ image is because the sensor in the camera is actually registering the light that is hitting the sensor via the image (aka reflected light). To obtain the correct image you need to use a hand-held incident light meter next to the object, but pointing towards the camera. This measures the ‘incoming’ light i.e. before it has hit the object.
I see! 