Originally Posted by CAVX
For clarity and the benifits of not only myself, but the other forum members, please define an 'element' Vs a lens. When I got to see your work known as HORIZON
in 2008, you described it as a 4 lens system even though it appeared to have 6 elements (2 doublets and 2 singlets). The prisms based units have always referred to doublets as dual elements because even though a doublet is (lens or prism) a single unit, it is originally made from from 2 elements.
Happy to oblige.
First let me note that in the industry there is no small amount of confusion over this topic.
A "Lens" can be an entire opto-mechincal device, like a camera lens, or an anamorphic lens (with or without stand).
A "lens" may be two "lenses" (pieces of glass that share a common radius of curvature on the inside, so that they can be cemented together perfectly - a "doublet"), or a single "lens" or piece of glass (sometimes called a "singlet").
The way I (and most
others) use the term is as follows:"Element"
A ground and polished piece of glass, usually employing curvature (circular, parabolic, or aspherical), that posseses positive or negative refractive properties in a consistent manner across the face of the element."Singlet"
A single, standalone element, employed in an optical system for a particular refractive purpose."Doublet"
Two elements made of different types of glass, sharing a common inside radius of curvature, usually cemented together with clear optical cement.
be used to correct color or other aberrations as standalone components, or may be used to provide pre or post aberrations that affect or are affected by other elements respectively."Triplet"
A 3-element version of a doublet. Very rare, and not usually required in a system unless it has extremely high performance specifications (i.e. much higher than projection or SLR-type camera systems)."Group"
A fairly vague term... Usually it denotes an arrangement of singlets and/or doublets that function as a sub-system within an optical design (e.g. zoom lenses consist of "focus" orientated groups and "zoom" orientated groups)."Cylindrical Curvature"
Curved in only one direction... a section of a cylinder, not a sphere (which is curved equally in all
sense, cylindrical lenses are aspherical, as their curved surfaces do not represent a section of a sphere.
Usually, however, "aspherical" means a curvature that is not based on a circle, a sphere or a plane. It could be a simple parabolic surface (y = x-squared + K), or one based on a complex multi-term mathematical function of any degree of complexity.
Aspherical surfaces are usually employed sparingly in an optical system when there is no other option, because they are so expensive to manufacture. You can't use a template, or simple rotation (as with a spherical lens) to grind and especially
polish an aspheric. A diamond-tipped CNC cutter is used that must not only cut out the irregular, aspheric shape, but also
polish it. Aspheric surfaces refract light differently in different positions on the "sag" (or curve). They tend to correct the inherent aberrations of cylindrical or spherical surfaces and can save glass in an optical system, but at huge effort and expense. There is a lot of controversy as to whether they are really needed in a simple projection or camera system (although some higher end camera lenses do use them, also simpler systems with plastic - highly repeatable - lens elements, like CD players). They tend to be more required in areas like modern microlithography systems (e.g. microchip manufacture, nanotechnology engineering) where large basic artwork is reduced to micron and sub-micron size, requiring as close to perfection as possible in the imagery."Lens"
Can be an element, a singlet, a doublet, a triplet or an entire optical device, depending on context... which is why I tend to use the more precise terms unless that context is clear.Examples
1. An anamorphic lens with only 2 cylindrical elements would consist of two singlets - the one nearest the projector being a converging (positive focal length) element, and the one nearest the screen being a diverging (negative focal length) element, separted by an air gap calculated by a function of their combined focal lengths.
This would be the simplest possible cylindrical anamorphic lens.
It could be designed to eliminate color aberration at one particular throw distance, but would be unlikely to work properly in correcting color errors at any other throw distance.
This lens will have a maximum possible 4 air-glass surfaces that can contain curvature.
These designs are rarely built, and are used mostly in textbooks to illustrate the basic principle of anamorphism, or (more generally) a lens system that has infinite focal length (parallel rays that pass through it stay parallel, they never focus. Conversely, a focusing beam will still focus, at roughly
the same focal point as it would without the lens in the beam... "roughly" is the key word here... it is where "astigmatism" comes into play, because the new focus point is close but not quite the same in the horizontal direction as it is in the vertical direction). The air gap is varied to reduce astigmatism.
2. An anamorphic lens with 4 elements usually consists of 2 doublets... two elements per doublet. The function of the doublets is mostly to provide color aberration correction, when used in conjunction with each other. Refractively speaking, anything regarding "bending" the projector beam that the simple "Type 1" lens can do, can be done with this "Type 2" lens, with the added function of more universal color correction, over a wider range of throws.
3. My lens has 5 elements: 2 doublets and one singlet, arranged as three lenses (or groups, or stages). The two end lenses are fixed in place. The middle element moves back and forth along a precision (tolerance measured in microns) rail. As the air gap between the first and middle lenses decreases, the air gap between the last and middle lens increases by the same amount. The extra glass serves to both color correct and to bend the beam in a more controllable way that lenses with lesser numbers of elements cannot do, as they have less degrees of variation available to them. The extra 2 air-glass surfaces mean that each individual surface has to do "less work" to achieve the final result. Therefore, the extra surfaces allow my lens to be "gentler" on the beam, correcting or reducing some aberrations as it goes, in order to achieve different results, particularly as regards image geometry.In Summary
To put this all into perspective, you can use a simple magnifying glass as a camera or projector lens (but NOT as an anamorphic lens - for this you need a minimum of two elements see "Type 1" lenses above
). The resulting image from a simple magnifying glass projection or camera "lens" will be blurred at the edges (astigmatism) and will have severe color problems ("Newton's rainbow effect" or Color Aberration - "CA"), as well as other less well-known aberrations such as spherical aberration and coma, sometimes extending through several algebraic orders (1st order... 5th order... etc.).
Lenses can be combined as groups, doublets etc. to reduce aberrations, although they are unlikely to be completely eliminated. Sometimes correcting one aberration promotes another. Generally (and I mean generally
) speaking, the more glass you use, the more you can use one refractive property in one element (or group) to correct aberrations resulting from another element, without adding too much by way of "collateral" aberration by the very act of correction. Eventually, the designer has to balance performance against cost, weight and ultimately expense, especially with such big pieces of glass as anamorphic add-on adapters employ.
There are no "1s" and "0s" in optical design, like there are in digital sound or CNC machining. Optics is all "analog" and fraught with challenges. Optical manufacture is very difficult and time consuming. The aim is to achieve performance with the minimum number of elements that deliver your specifications within an acceptable practical environment - be that cost, size, image quality, or ease of use.