Microcontrast. Another Look.

Microcontrast, or as it also called lens plasticity is property of an optical system describing the lens’ ability to create an illusion of 3-D on the flat medium. Yet, it has been one of the most confusing terms in photography, as most photographers have a very hard time explaining its physical sense and the difference between the overall contrast of the lens, which determines its resolving power, and the microcontrast, which is, in turn, related to 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 has been empirically established that generally lenses designed for larger formats have better plasticity than those for small frames. However, it is not that simple. There are lenses designed for the same frame size that sport the same resolving power, yet differ greatly in their ability to render 3-D-like image. How come?

In reality, it is easy to explain using a simple diagram below. Mathematically, acutance is a derivative of brightness with respect to space and can be explained as a gradient of density. Acutance is linear, and it is represented by the black 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 shorter is this gradient, the higher acutance is. Microcontrast is a non-linear function, which can be approximated by the line of acutance. While acutance shows the lens resolving power, and the Sin ϕ is its measure, it is the shape of the microcontrast curve that determines the delicacy of the focus transfer. If the microcontrast curve falls too steeply, the image looks too harsh, yet, if it is too close to the acutance line, the image feels soft.

The diagram represents how the grey scale shown is rendered by three different lenses.

All three lenses have the same acutance, i.e. edge sharpness.  It is shown by the fact that they equally represent full- and zero-luminance zones, hence the identical value of ϕ. However, they differ in their ability to resolve adjacent areas with similar tonal values:

  • Lens 1 has poor micro-contrast: luminance falls off sharply rendering zones 3 to 5 perceptually black, as they fall below the visibility threshold (Example: Sigma Art zoom)
  • Lens 2 has average micro-contrast (Example: Canon L prime)
  • Lens 3 has good micro-contrast, as it successfully resolves tones up to the zone 4 above the visibility threshold (Example: Leica M lens)

The numbers are arbitrary and given 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 what shape of the curve results in the most pronounced effect, however.

To be fair, not only Leica lenses are good at making a photo 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 get inside the image, the Zeiss picture is a kind of flat surface on which realistic three-dimensional objects lie.

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 deserves special attention. Incidentally, the photo featured in this article, was taken with this lens at full aperture.

 

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