Aberrations - Aberrations are image defects that result from limitations in the way lenses can be designed. Better lenses have smaller aberrations,but aberrations can never be completely eliminated, just reduced. The classic aberrations are: Spherical aberration. Light passing through the edge of the lens is focused at a different distance (closer in simple lenses) than light striking the lens near the center. Coma. Off axis points are rendered with tails, reminiscent of comets, hence the name. It can be shown that coma must occur if the image formed by rays passing near the edge of the lens has a different magnification than the image formed by rays passing near the center of the lens. Astigmatism. Off-axis points are blurred in their the radial or tangential direction, and focusing can reduce one at the expense of the other, but cannot bring both into focus at the same time. Think of it as the focal length as varying around the circumference of the lens. (Optometrists apply the word "astigmatism" to a defect in the human eye that causes *on-axis* points to be similarly blurred. That astigmatism is not quite the same as astigmatism in photographic lenses.) Curvature of field. Points in a plane get focused sharply on a curved surface, rather than a plane (the film). Or equivalently, the set of points in the object space that are brought to sharp focus on the film plane form a curved surface rather than a plane. With a plane subject or a subject at infinite distance the net effect is that when the center is in focus the edges are out of focus, and if the edges are in focus the center is out of focus.* Distortion (pincushion and barrel). The image of a square object has sides that curve in or out. (This should not be confused with the natural perspective effects that become particularly noticeable with wide angle lenses.) This happens because the magnification is not a constant, but rather varies with the angle from the axis. Chromatic aberration. The position (forward and back) of sharp focus varies with the wavelength. Lateral color. The magnification varies with wavelength. APO or Apochromatic - The distance behind the lens at which monochromatic light (light of a single wavelength) comes to focus varies as a smooth function of the wavelength. If this function has a zero derivative in the visible range, and hence if there are two wavelengths at which the light comes to focus in the same plane, the lens is called achromatic. If there is a higher order correction, usually with the result that 3 or more visible wavelengths come to focus at the same distance, the lens is called apochromatic. Some authorities add more conditions. Apochromatic lenses often contain special low-dispersion glasses. APO is an abbreviation for apochromatic. Asymmetrical - Front and rear lens groups of lens are not the same. Cells - Sets of lens groups designed to function as a unit Circle of good definition - Effective circumference of coverage that is usably sharp. Circle of illumination - Effective circumference of coverage that has useable/recordable light <LI>Coating - Before coating, each transmission surface resulted in about a 4% to 8% loss of light to reflection depending on the refractive index of the glass. So an uncoated Dagor or Protar with four transmission surfaces looses 15% to 29% of the light to flare. An uncoated Tessar looses 22% to 40% of the light to flare. An uncoated Planar with eight surfaces looses 28% to 49% of light to flare. The flare would exhibit itself on the film as unfocused non-image forming light which reduced the contrast of the picture. Single Coating - After single coating, this dropped to about 2% to 4% loss of light per transmission surface. Applying the coating at quarter wavelength thickness could greatly increase the effectiveness of the coat, but it could completely block some wavelengths of light and partially block others. Typically blue-green wavelengths were suppressed with an amber coat, or green wavelengths with a purple coat. Multi-coating - Multicoating was first done as two separate coats at different wavelength thickness on different transmission surfaces to balance the color of the light transmitted to the film. Later, multi-coating as we know it, one coat stacked on another (first used on a production lens by Leitz) reduced the light lost to diffraction further to about 1/2% to 1% per transmission surface. The classic second coat was bismuth oxide again applied at quarter wavelength thickness for a different wavelength, typically orange-yellow for the second coat and green-blue for the first coat giving a faint green reflection. A multi-coated Planar could now only loose about 4% to 8% of the light to flare, quite a difference. Coating and multicoating allowed designers to use more complex designs with more air spaces which allowed easier design for correction of spherical aberrations. The difference between uncoated lenses and coated lenses are great, the difference between single coating and multi-coating is visible, but not nearly as great as the first leap from uncoated to coated. Coating and multicoating opened the way for many otherwise unfeasible modern lens designs, such as complex wide-angle lenses, big multi-element zooms, and lots of marketing hype. Coating still won't save you from nasty flare in certain lighting conditions, such as shooting into the sun, so make sure to use those lens shades! Convertible Lens - A set cells that can be combined together in different pairs or singly to produce different effective focal lengths. Each cell must be able to correct and focus the image properly on the film alone. If a cell is used singly, it is mounted behind the shutter. Elements - individual lens Groups - single lenses or groups of lenses cemented together Symmetrical - Front section of lens is identical mirror image of rear section. Because of this, aberrations and astigmatisms are minimized. Inherently optimized for 1:1. Transmission surfaces - Lens element to air surface.