dnas wrote in post #7786277
There are both internal and external focusing in SLR lenses. More modern lenses, particularly zooms, will often focus internally, in which case some lens elements will move.
However, most prime lenses focus by moving all of the elements together in relation to the focal plane(film or sensor), so none of the internal elements move.
In a perfect world, we would only need one glass element for a perfect lens. However, because different colors of light have different dispersion characteristics through glass(they bend by different amounts), the same colors don't focus on exactly the same point. In order to compensate for this, two different types of glass are used: "Crown glass" for convex lens elements, and "Flint glass" for concave elements. These two different types of glass have a different refractive index and different dispersion. When used in combination in a lens, depending on the design, the red and blue light will focus more or less on the same point. This is called an achromatic doublet. In order to correct for green as well, one or more extra low dispersion lens elements can be used, which along with the other two different types of glass, are called apochromatic lenses. The aim is this case, is to focus red, blue AND green on the same point. However, because of the cost, most lenses don't have the low dispersion lenses.
Lens elements are also mostly ground to a spherical shape(the ideal is parabolic), so additional compensation must be designed to reduce this problem. In some cases, some aspherical lens elements are used.
The lens elements must also compensate for optical design. For example, in a EF mount Canon SLR, the distance from mount ot focal plane is more than 40mm. So how do you use a lens with a 20mm focal length, without hitting the mirror?? The lens must be designed to give the same field of view as a 20mm lens, while keeping the rear lens element around 40mm from the focal plane. This type of design is called "Retrofocal". And if you want a long focal length lens to be shorter than its required focal length (e.g. a 400mm lens shorter than 400mm), the you need to use an optical design called "telephoto".
Of course, it's even harder to design a zoom lens to compenstate for the different colors, plus the different optical designs. Telephoto zooms adjust the degree of telephoto optical design within the lens.
There are both internal and external focusing in SLR lenses. More modern lenses, particularly zooms, will often focus internally, in which case some lens elements will move.
However, most prime lenses focus by moving all of the elements together in relation to the focal plane(film or sensor), so none of the internal elements move.
In a perfect world, we would only need one glass element for a perfect lens. However, because different colors of light have different dispersion characteristics through glass(they bend by different amounts), the same colors don't focus on exactly the same point. In order to compensate for this, two different types of glass are used: "Crown glass" for convex lens elements, and "Flint glass" for concave elements. These two different types of glass have a different refractive index and different dispersion. When used in combination in a lens, depending on the design, the red and blue light will focus more or less on the same point. This is called an achromatic doublet. In order to correct for green as well, one or more extra low dispersion lens elements can be used, which along with the other two different types of glass, are called apochromatic lenses. The aim is this case, is to focus red, blue AND green on the same point. However, because of the cost, most lenses don't have the low dispersion lenses.
Lens elements are also mostly ground to a spherical shape(the ideal is parabolic), so additional compensation must be designed to reduce this problem. In some cases, some aspherical lens elements are used.
The lens elements must also compensate for optical design. For example, in a EF mount Canon SLR, the distance from mount ot focal plane is more than 40mm. So how do you use a lens with a 20mm focal length, without hitting the mirror?? The lens must be designed to give the same field of view as a 20mm lens, while keeping the rear lens element around 40mm from the focal plane. This type of design is called "Retrofocal". And if you want a long focal length lens to be shorter than its required focal length (e.g. a 400mm lens shorter than 400mm), the you need to use an optical design called "telephoto".
Of course, it's even harder to design a zoom lens to compenstate for the different colors, plus the different optical designs. Telephoto zooms adjust the degree of telephoto optical design within the lens.
Nice! Thanks for that. Makes all those focus / directerix problems from Algebra much more relevant!
