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What is micro contrast, and why is it important? How is it different than regular contrast?

Matt Grum mentioned it in his answer about larger-format cameras:

There are other advantages to medium format other than image resolution... better micro contrast on account of the format size... the genuine advantages of format size in terms of sharpness and micro contrast... will always hold out.

This is the first I'd heard of the term.

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Micro-contrast refers to contrast as measured between adjacent or nearly adjacent pixels. It is often perceived as sharpness.

Contrast usually refers to the contrast of the entire image which is an indicator of the captured dynamic-range.

It is possible for an image to show high micro-contrast (being very sharp) and low image contrast (being a subject with very uniform tonalities).

The converse is also possible, as any scene which exceeds the dynamic-range of a sensor will have high contrast but if shot with a poor lens or at an aperture beyond the diffraction limit it will have low micro-contrast.

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  • \$\begingroup\$ That's another question I've wondered about, but it's probably not worth a whole SE post. Is (macro) contrast effectively the same as the dynamic range of a particular photo? \$\endgroup\$
    – Craig Walker
    Commented Apr 27, 2011 at 23:20
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    \$\begingroup\$ Actually, that is a great question! Dynamic-range is the range of luminance that was captured but contrast refers to the range of luminance of the image. If the mapping was linear, they would be the same, but it is rarely so, most images are produced using a non-linear tone-curve from RAW data to a specific color-space such as sRGB. \$\endgroup\$
    – Itai
    Commented Apr 27, 2011 at 23:28
  • \$\begingroup\$ Hmm, interesting. If I ask a full-blown question, would you like to expand on the details in your answer? \$\endgroup\$
    – Craig Walker
    Commented Apr 28, 2011 at 14:38
  • \$\begingroup\$ There could be a little more to say that would be of general interest. Not sure there would be much more that would be that important for photography though. \$\endgroup\$
    – Itai
    Commented Apr 28, 2011 at 20:00
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To add to Itai's great answer, Micro-contrast is an aspect of digital photography that is extremely vulnerable to anti-alias filters, or low-pass filters. A digital image sensor is only capable of processing information down to a certain limit (the Nyquist Limit), after which any additional information will be captured as undefined or ill-defined "noise". This undefined information can be a great detriment to image quality for high density sensors such as those found in your average DSLR. A low pass filter is often placed in front of a digital sensor to eliminate image frequencies below the limit of a sensor. Poorly or cheaply designed filters can have a visible and detrimental effect on micro-contrast in the other direction...softening an image and reducing its sharpness.

Larger format sensors have the benefit of high resolution and lower density on their side. With larger pixels, the need for a low pass filter is greatly lowered, and they are usually eliminated entirely. This allows larger format sensors to achieve much greater micro-contrast...contrast differentiation between individual pixels, than is usually possible with smaller format sensors such as APS-C and Full Frame.

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  • \$\begingroup\$ Is that low-pass filter optical, electronic, or software? I've often wondered about that. \$\endgroup\$
    – Craig Walker
    Commented Apr 27, 2011 at 23:23
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    \$\begingroup\$ Its an optical filter. Its part of a stack of filters that usually sit in front of the CMOS or CCD sensor itself. These days, the filters themselves are usually rather complex...often consisting of several layers. In the case of Canon: IR filter, dichromic mirror (reflects IR rather than absorbing it), low pass horizontal filter, quarter wave plate (converts linear polarized light to circular), low pass vertical filter, often with multicoating on each element of the whole filter setup. \$\endgroup\$
    – jrista
    Commented Apr 28, 2011 at 1:11

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