When a photograph is converted to mono and the colour removed, what remains is tone. Tone is in fact one of the reasons black and white photography has continued to be popular after the advent of colour. It was the tonal quality of Paul Strand’s work that inspired landscape legend Ansel Adams to make a career change from classical pianist/bobby photographer as a hobby to full time professional. It was the“full, luminous shadows and strong high values, in which subtle passages of tone were preserved” in Strand’s photography that Adam’s found so compelling.
Tonal range refers to the range and distribution of tones in an image from the darkest to the lightest values. An image with a wide tonal range will include very dark (black) and very light (white) elements, whereas as one with a narrow tonal range will be limited to a more restricted area and will have less contrast. Lighting conditions have a great impact on tonal range. On a misty day in Oxford, UK the difference in brightness between the lightest part and the darkest parts of the scene will be relatively insignificant, but under the harsh light of the mid day sun in Arizona the range will be much greater. Extremes such as that aside, images with a wide tonal range tend have both greater contrast and greater appearance of drama and depth. A narrower tonal range makes the image look flatter, though this may be by design. Tonal range is influenced by the amount of and quality of light (harsh or soft) and the reflectance of the subject. The addition of a polarising filter, something I use extensively, improves contrast and can thus help expand the tonal range. Tonal range is different from dynamic range which refers to the ability of the camera’s sensor to capture shadow detail from the darkest values and highlight detail from the brightest values in the same shot. Dynamic range is maximised in RAW format which records all the image data the sensor is capable of recording, and is one of the main reasons this format is used. Cameras.about.com describes the the tonal range of a digital image as “the number of tones it has to describe the dynamic range”, which helpfully describes the difference between the two.
Tonal contrast is created when light tones and dark tones are adjacent to one another – it has the effect of directing the viewers attention to the contrasting area and is the basis of the success of many black and white images. It is an essential element of composition, but is not as well known as other compositional rules such as leading lines, the rule of thirds, framing, and diagonals.
A tonal curve is typically applied to the image by the camera by default as the image data from the photo sites is linear (the values are directly proportional to the light received), whereas human vision is non-linear. The process is known as (tone curve correction aka gamma correction) and makes the pictures look right to the human eye on a screen display. This process generally lightens the midtones. This default tonal curve can be adjusted in the curves tool of RAW conversion programs (such as the ageing but excellent Aperture, which I still use). The Aperture user manual describes the use of the Curves tool quite well:
“You use the Curves adjustment controls when you want to manually set the tonal values of the shadows, midtones, and highlights in an image using a tonal curve. Unlike the Levels adjustment controls, the Curves controls do not reapportion the luminance values in the image by constraining the white and black points. Instead, you use the Curves controls to precisely remap the position of the midtones relative to the white and black points. Because the human eye’s perception of light is logarithmic rather than incremental, a curve is necessary to distribute the luminance values across all tonal ranges in an image in a way that matches how the eye perceives light in nature.”