Apodization

Apodization, "Removing the Feet " ( from Ancient Greek - Latin apodisatio, from ancient Greek: α, "a- " without, the root word pod, of πούς, " pus ", Foot, genitive: ποδός, " Podos ," and the Latinized ending " ISATIO [n ]") is a method of optical filtering, in which the outer rings of the diffraction disk can be suppressed to improve the contrast of an image at the expense of the resolution. The figure so becomes altered so that the resulting diffraction cloudy around the respective pixels are substantially attenuated.

Purpose, for example a special graduated filter is inserted into the aperture stop of the optical path, the transparency gradient corresponds to the Gaussian bell function. Filtering in the plane of the aperture stop affects the brightness distribution in the image plane as a Fourier transform, the imaging optical system thus acts as a Fourier lens. The special filter course now causes the disturbing diffraction rings disappear around the pixels. In mathematical terms: The Gaussian function is invariant with respect to the Fourier transform.

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

Analogous to this optical method to filter with equivalent effect in image processing the digitized data electronically.

In microscopy apodization is an adverse effect ( loss of resolution ) when using high numerical apertures, since extremely large apertures ( NA over 1.4 ) may be present, lead which (even without a graduated filter ) to suppress the diffraction rings and broadening of the point spread function.

Use in photography

In 1998, presented in a particularly harmonious bokeh that are optimized Minolta portrait lens STF 135 mm f / 2,8 [ T4, 5] (or its optically identical design Sony 's successor in 2006 imagined ) is currently (2011) only commercially available lens for creative - imaging photography that uses a Apodisationsfilter in the form of a concave-shaped gray glass element in the beam path to produce blur discs with practically ideal Gaussian distribution which derive a particularly smooth transition from high sharpness in the plane of focus in the area of the blur in front of and behind the focal plane. The effect is controlled on the STF - aperture. STF stands for Smooth Trans Focus.