Fokusverschiebung

Fokusverschiebung is the displacement of the image’s focal plane caused by changes in wavelength, object distance, or optical configuration, so that the plane in which objects appear sharp moves relative to the sensor or film. The term is most closely associated with axial or longitudinal chromatic aberration, where different colors focus at different positions along the optical axis due to dispersion of lens materials.

Causes include the wavelength dependence of the refractive index (dispersion), optical design choices, aperture setting, temperature effects, and, in microscopy, mismatches between the specimen, immersion medium, and objective. The result is that the sharpest focus for red light may lie at a different distance than for blue light, which can produce color fringing, reduced overall sharpness, or color-channel misregistration.

In practice, fokusverschiebung is most evident when high optical power or wide apertures are used, or when imaging across a broad spectrum. It can be accentuated in camera lenses during zooming or focusing and in high-NA microscope objectives.

Corrections and mitigations include optical design approaches (achromatic or apochromatic lenses, improved dispersion control, parfocal zooms), mechanical or electronic autofocus that seeks overall sharpness across color channels, and computational methods such as focus stacking or deconvolution. In microscopy, index matching and corrected objectives, or adjusting the immersion media, are common strategies. Understanding fokusverschiebung helps in selecting equipment and planning imaging workflows for accurate color and sharpness.