Basics of black-and-white tone reproduction
During the period that black-and-white photography was the benchmark for fine artistic imagery, the basic facts about the tonal reproduction were required knowledge for every photographer. The importance of studying tone values was recognized by every one. This knowledge was relegated to oblivion since the technique of digital capture dominated the photographic scene.
Nik’s Silver Efex Pro software is a very fine tool, but a blind utilization of the software will not bring the envisaged results.
A scene to be captured by photographic means is simply a pattern of patches of different size and shape and different intensity. Our eyes scan this pattern and our brain constructs from this information a meaningful image: a landscape or a cat or a skyscraper. The camera dos not have the ability to reconstruct the illuminance pattern, but the capture medium (emulsion or solid state) must reproduce this original pattern such that when we see it our brain should construct the same image as it would do when looking at the scene itself. The brain is very good in building meaning from any pattern and is also very good in discriminating between two adjacent stimuli. We can decide without problem between a darker and a lighter patch, but we are unable to state how much darker or lighter these patches are.
A tone is defined as as any patch of any subject which emits a uniform light intensity over its whole area.
Any scene is composed of patches with high and low intensity. The range of tones (illuminances) depends on several factors: the quality and quantity of the light source illuminating the scene and the reflection factor of the material of the objects in the scene.
A uniformly overcast sky has an average lighting contrast of zero and the low direct sun in early morning has a contrast of 4 - 5 stops. The maximum reflection can be found with magnesium carbonate and the minimum reflection with black velvet. The maximum tone range between these extremes is 49:1 or 5.6 stops. The average luminance range of a scene is a combination of these two parameters and studies have found that the brightness scale of scenes ranges from 27:1 to 760:1 with a modal value of 160:1. This last value represents a luminance range of 7.3 stops. The distribution of the tonal values in the scene can make a big difference in exposure however. The tonal values may be biased to the low illuminance values (dark parts of the scene) or to the high values (light parts of the scene).
The Zone System is a method that pegs the various tonal values to specified areas of the characteristic curve. Any recording material (emulsion or solid state) has a minimum and maximum sensitivity, beyond which no tone differences can be detected. The Zone System ensures that all important tonal values of the scene are recorded within the dynamic range of the capture medium. If the contrast range of the scene is smaller than the dynamic range of the medium, the tonal range can be stretched and as a consequence the steps between detectible tonal differences will be increased (expanded). If the contrast range of the scene is larger than the dynamic range of the medium, the tonal range can be compressed and as a consequence the steps between detectible tonal differences will be decreased. The just detectible tonal differences between adjacent grey values can be calculated with Fechner’s law that states that to be detectible two values of grey should be differ in brightness by 1% or a factor of 0.010. This implies that the maximum tonal scale from 0 to 100% reflectance can be divided in 214 parts that are visually detectible under normal conditions. Or in other words we can detect about 200 different shades of grey in a printed picture and possibly slightly more on a computer screen. This number should not be confused with the 256 values (sometimes referred to as ‘tones’ that are available when using a bit-depth of 8. The bit-depth of 12 and 14 have ranges from 0 - 4096 and 0 - 16384, but it is not correct to assume that these ranges produce a tonal scale with 4096 and 16384 visual tonal steps.
The Zone System and in general every rule that matches the dynamic range of the capture medium and the maximum contrast of the scene tries to get two results: correlating the minimum and maximum points of subject contrast with the dynamic range/contrast index of the recording material and to record the maximum number of tones between these extremes.
When working with black-and-white materials (Leica Monochrom and black-and-white silver emulsions) a third parameter is required: the spectral sensitivity of the material must equal the spectral sensitivity of the eye. This introduces a subjective factor, because every human will have a different perception of the brightness of colors.
The brightness of colors as perceived by the normal human eye has been studied and quantified. The four main colors (blue, green, yellow, red) have relative brightness values of 1:7:12:5. This means that blue has only 1/7 of the brightness of green and compared to yellow is very dark. Red is 0.7 times as bright as green or one sees red as slightly darker than green. Yellow is a mix of red and green and should have a relative brightness of 7 + 5 = 12.
Colors in nature are almost never pure and consist of more colors: a certain hue of red for instance will be composed of 90% red, 5% blue and 5% green. For the sake of the discussion we will neglect this fact and assume pure colors. A traditional pan-chromatically sensitized black-and-white emulsion has relative spectral sensitivities of 1:0.5:2:1.5 for the blue:green:yellow:red colors. It is obvious that blue will be reproduced in a much lighter tone that is ‘unnatural’. A yellow filter will correct these tones and produce this range: 1:3:12:9.
Now we have blue and yellow in the correct relationship, but green and red have not yet the required brightness value. To get these values a yellow-green filter is the best option, assuming the picture is taken during the period from 11 AM to 03 PM: light in early morning and early afternoon have a higher red content!
A test pattern for the Leica Monochrom then will concentrate on these values and see how the LMM will perform.