Lens designs

An achromatic lens or achromat is a lens that is corrected to reduce the effects of chromatic and spherical aberration.

An anastigmatic lens or anastigmat is a lens that is completely corrected for the three main optical aberrations: spherical aberration, coma, and astigmatism. The anastigmatic lens was designed by Paul Rudolph for Carl Zeiss Jena in 1890.

The Aplanat or Rapid Rectilinear is a lens design, invented by John Henry Dallmeyer and simultaneously and independently by Hugo Adolph Steinheil in 1866. The design greatly reduces radial distortion and it was the most important lens design of the 1860-1880 period.

A lens with astigmatism has a different focal length for rays in a horizontal plane than for rays in a vertical plane. Suppose the object has the shape of a cross, then the horizontal line will be sharpened at a different distance than the vertical line.

Chromatic aberration is a failure of a lens to focus all colours to the same point. It manifests itself as “fringes” of color along boundaries that separate dark and bright parts of the image.

Comatic aberration or coma is an imperfection in the lens that results in off-axis point sources such as stars appearing distorted, appearing comet-shaped. Lenses with heavy coma can show sharp points of light in the center, but they become blurrier towards the edges of the frame.

The focus of a fixed-focus lens is not adjustable. It is usually set to the hyperfocal distance, so that the depth of field extends from a short distance to infinity. An example of a camera with fixed-focus lenses is the Vérascope.

The Petzval lens was developed by Joseph Petzval in 1840. It was the first portrait lens with a focal lenght of 160mm. It was much faster compared to previous lenses and had a maximum aperture of f/3.6. The lens allowed shorter exposure times and made it possible to portrait people by using the daguerreotype process.

Radial distortion is an optical aberration that causes a deviation from rectilinear projection where straight lines in a scene remain straight in an image. Radial distortions can be classified as either: Barrel distortion: lines that do not go through the centre of the image are bulging outwards the centre of the image, like a barrel.… Continue reading Radial distortion

Spherical aberration is caused by the spherical surface (curved shape) of a lens. Light rays that strike a spherical surface off-centre are refracted or reflected more or less than those that strike close to the centre. Spherical aberration manifests itself as blurriness at the edge of an image.