Micro-contrast, or as it also called lens plasticity is property of an optical system describing the lens’ ability to create an illusion of depth on the flat medium. It has been one of the most confusing terms in photography, as most photographers have a very hard time explaining its physical sense and nature, and the difference between the overall contrast of the lens, which determines its resolving power, and the micro-contrast, which has more to do with the dynamic range. Incidentally, it is not dissimilar to the bit depth, which is a term known to anyone who is familiar with any type of digital-to-analog conversion, whether it would be in imaging, or sound. The higher is the bit depth, the greater is the number of “shades”, and the more adjacent tones the lens can resolve.

As we discussed earlier in this article, it is well known that photos taken with large-format and to a lesser extent with medium-format cameras look more three-dimensional than 35mm-format images. However, it is not that simple. There are lenses designed for the same frame size, have similar resolving power, yet differ greatly in their ability to render the realistic illusion of depth. How come?

Theory of it can be explained using a diagram below. The numbers are for illustration only. The graph describes a hypothetical area on the edge between black and white lit by a point light source and photographed with three different lenses under identical conditions. More experimental data needed to understand the curve shape that results in the most pronounced effect, however. Mathematically, acutance is a derivative of brightness with respect to space and can be explained as a width of the gradient of density. Acutance is linear, and it is represented by the light-blue line on the graph. The shorter gradient means more abrupt light falloff on the edge between black and white, which makes the edge look sharper. Therefore, the more compressed the gradient is, the higher is the acutance. However, in reality, the gradient is a non-linear function, which can only be approximated by the line of acutance. While acutance shows the lens resolving power, and the Sin ⍺ is its measure, it is the shape of the curve that determines the delicacy of the tonal transfer, or, in the terminology introduced by Carl Zeiss AG, micro-contrast.  There are three possible scenarios:

1. The curve is located below the acutance line and crosses it above zero (green area). The image looks soft. This is a typical curve for an inexpensive consumer-grade zoom lens.
2. The curve is almost horizontal and then it falls steeply to level below the detail visibility threshold (horizontal red line). The image looks very sharp, but has no depth, which is characteristic of Sigma Art primes and most professional zoom lenses made in Japan.
3. The curve convex (highlighted purple curve). It bends down smoothly and never crosses the acutance approximation line. Note the area shown by a red arrow: this is the area where no other lens is able to resolve details in the shadows. A lens behaving this was is said to have good micro-contrast, and its rendering is the most pleasing to the eye. In addition to medium format optics, Leica M lenses are especially well-known to act this way.

 

Lenses 2 and 3 have the same acutance.  It is shown by the fact that they equally represent full- and zero-luminance zones, hence the identical value of ⍺. However, they differ in their ability to resolve adjacent areas with similar tonal values, especially in the shadows. Information shown on the diagram can be represented visually and clearly show how lenses resolve the detail on the edge of an abrupt change of tone. The visible edge is shown by red boxes on each gradient.

• As you can see, the first lens (the consumer zoom) has no visible edge. There is only slight drop at the end of the curve, and it occurs outside of the acutance range, and below the visibility threshold.
• Lens 2 (Sigma Art prime) has an abrupt change close to the middle of the gradient, while both sizes have almost no visible change of tone.
• Lens 3 (a prime of German design) has the visible edge shifted towards the darker side of the gradient, yet the light falloff is visible through the whole gradient width.

 

To be fair, in the 24x35mm world, not only Leica lenses are capable of making photos look three-dimensional. Zeiss optics, especially ZM line, is exceptionally good at that too.  Zeiss picture differs from Leica’s, but it is no less interesting: while in Leica photos there is an illusion of depth, causing the desire to literally reach inside the image, the Zeiss picture is a kind of flat surface on which realistic objects rest.

Most of the old rangefinder lenses of Soviet manufacture also render beautifully. Especially good are Jupiter-3 and Jupiter-12. Recently, several interesting Chinese lenses have appeared. Among them, 7Artisans 50mm f / 1.1, 35mm f/2, and 28mm f/1.4 deserve special attention. Incidentally, the photo featured in this article, was taken with the 7Artisans Photoelectric 1.1/50 at full aperture.

Irakly Shanidze © 2019