An achromatic lens or achromat is a lens that is corrected to reduce the effects of chromatic and spherical aberration.
Additive colour is based on the mixing of light of different colours. By mixing three light sources with the primary colours red, green and blue, any other colour can be created. The starting point is black (no light) and subsequently coloured light is added (hence the name “additive”) and mixed. A balanced mix of the… Continue reading Additive colour
Adjustable focus is a feature of a stereoscope that can focus the lenses for people with different eye sights.
See: Stereo ambrotype
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.
A Brewster-style stereoscope is based on the first design by David Brewster in 1849. It is a compact hand-held stereoscope consisting of a closed box with two lenses. A door on top can be opened to illuminate paper card stereoviews or daguerreotypes via reflective light. The bottom usually contains a frosted glass which is intended… Continue reading Brewster-style stereoscope
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.
See: stereo collodion negatives