Apodization

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Apodization (from ancient Greek - Latin apodisatio , " foot distance", from ancient Greek pod , from "πούς", "pús", foot ; in English Smooth Trans Focus (STF) ), is a method of optical filtering in which the outer rings of the Diffraction disks are suppressed in order to improve the contrast of an image at the expense of the resolving power . The image is thus changed by the fact that the brightening around the respective image points caused by diffraction is significantly weakened.

For this purpose z. B. a special graduated filter inserted into the aperture of the beam path, the transparency of which corresponds to the Gaussian bell function. A filtering in the plane of the aperture has an effect on the brightness distribution in the image plane as a Fourier transformation , so the imaging optics act as a Fourier lens . The special filter course now causes the disruptive diffraction rings around the image points to disappear. Expressed mathematically: the Gaussian function is invariant to the Fourier transform.

The effectiveness of the method increases with the diameter of the aperture.

Analogous to this optical process, the digitized data is electronically filtered with a comparable result in image processing .

In microscopy, apodization is a disadvantageous effect (loss of resolution) when using large numerical apertures , as extremely large openings (NA over 1.4) can be present here, which (even without a graduated filter ) lead to the suppression of the diffraction rings and widening of the point spread function .

Use in photography

The Minolta portrait lens STF 135 mm f / 2.8 [T4.5] presented in 1998 for particularly harmonious bokeh (or its optically identical successor from Sony introduced in 2006) was the first commercially available lens for creative imaging photography. that uses an apodization filter in the form of a concave gray glass element in the beam path. In this way, fuzzy disks with a practically ideal Gaussian distribution are generated, which achieve a particularly smooth transition from the high impression of sharpness in the focal plane to the area of ​​fuzziness in front of and behind the focal plane. The effect is controlled via the STF aperture. STF stands for Smooth Trans Focus .

In 2014 Fujifilm introduced the FUJINON XF 56mm F1.2 R APD lens, which is the first lens with an apodization filter and autofocus.

Web links

Individual evidence

  1. ↑ Fourier optics and holography, by Erich Menzel, Werner Mirandé, Ingolf Weingärtner, Technical University of Braunschweig, Springer-Verlag Vienna New York, 1973, ISBN 978-3-7091-8323-6 , p. 68 ( limited preview in Google Book search).
  2. ↑ M. Gu. Advanced optical imaging theory. Optical Sciences. Springer, 2000.
  3. ↑ Minolta patents on the use of the optical apodization effect to optimize the imaging properties of photographic lenses