What is 'Microcontrast' please?

Started Sep 21, 2016 | Discussions thread
Joel Halbert Contributing Member • Posts: 599
Microcontrast vs. Resolution and Sharpness
27

adamgrin wrote:

Hi All,

Can someone please explain the term micro contrast to me?

I hear some lenses (I own some of them apparently ;)) having good micro contrast but to be honest, I don't really understand what that means.

Described in simple terms if possible please

Thanks team!

I'll try - simple terms, but sorry not a short answer, and address sharpness vs. contrast:

Light dots or points:
All images can be thought of as a huge collection of minuscule dots of light (I don't mean "pixels" of a digital camera - people used the light-dot concept long before digital photography came along). By considering the imperfections of various single light-dots one by one, we can gain insight into how they will combine to form the final image.

Spreading of the dots:
When a lens captures a picture of a pin-sharp dot of light, the result is never perfect even when the dot is in best-possible focus. The reproduced image will always have a somewhat spread-out dot, and the nature of this dot-spreading is important to the quality and "feel" of the image.

Note that here I am not talking about the familiar Out-of-Focus blobs you can see with the naked eye (popularly called "bokeh" characteristics); I refer to pin-point, much smaller dots that must be highly magnified to see their hidden detail.

With most good lenses, the spreading is a fairly evenly-lit round disk with a fading-away soft edge. This is generally a good characteristic for a nice image, but even a nicely spread-out dot becomes undesirable if it spreads out too far. Wide spreading kills the "resolving power" or "resolution" of the lens, so that if you take a picture of very finely-spaced lines (a common test), they blend together and you cannot tell them apart (i.e. you cannot resolve them). In practical terms, the image detail will be lost, smeared away and people will say the lens is not "sharp".

High resolution but with poor contrast:
Since lens makers want to score well in a resolution test and have people say the lens is "sharp", they want to reduce the diameter of the spreading. The best way is to reduce the dot-spreading diameter and still keep the nice fading-edge dot look - typically difficult and expensive to achieve.

But another way is to accept large-diameter dots that do not fade away nicely at the edge, i.e. spread-out dots with a smaller, bright central region surrounded by a halo region that is somewhat dimmer. (The dot may display a noticeable ring artifact at the edge, or even several concentric rings surrounding the bright center). Because this halo region is, as I said, dimmer than the center, you can still recognize (resolve) distinct closely-spaced dots (or lines in test chart image). So the resolution spec is indeed good - the lens is still "sharp" but the space between the resolved lines is not empty - it is filled with less-bright artifacts from the halos surrounding dots. This light "pollution" between the lines is not enough to kill the resolution, but it does reduce the contrast. of the microscopic dots or lines - in short, the microcontrast is poor. The lines become dark grey instead of black, and the space between becomes light grey instead of white. Furthermore, the way the halos combine with one another can make edge detail look strange, and can also change the visual effect of complex textures in the image.

The microcontrast issue is not the only characteristic of a good photographic lens, but it is a basic and important one. People who talk about it will say that it makes photos more life-like and pleasant, and they may be challenged by other people who rely on published resolution specs. There are some expensive lenses with relatively poor microcontrast, a deliberate trade-off for resolution. I believe you would find these lenses discussed proudly but not fondly in gear forums - I hope that makes sense.

Another note regarding sharpness - if something causes edges and lines to be enhanced with artificial local contrast at the edges, this may give an initial impression of sharpness but is actually distorting fine details, very destructive of complex textures. Try turning up the sharpness setting on your TV - it seems to crisp-up initially, but you probably won't like it after a while. This effect can also happen without electronic image manipulation, particularly in lenses with certain kinds of dot-structure and/or specific color aberrations.

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JoelH

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