The term "tube lens" is used informally to mean any lens that is used in conjunction with an infinity objective to complete the process of image formation. A more neutral descriptive term is "converging lens".
With an infinity objective and tube lens, it's convenient to think of the image as being formed in two steps. First, the objective takes a small object close up and makes it appear to be a large object far away (at "infinity"). Second, the tube lens refocuses the virtual large object "at infinity" to form a small real image on the camera sensor or at the eyepiece of a microscope.
The term "tube lens" originated in microscopy. There, it refers very specifically to a lens that sits at the bottom of the eyepiece tube assembly. Those particular lenses have other special characteristics, notably that 1) they are designed to give best performance even with significant separation (70 mm or more) between the tube lens and the objective, and 2) they are physically short and do not have internal apertures, so they do not contribute to vignetting until the distance from tube lens to objective becomes very large. The internal structure of a tube lens is not necessarily simple; for example the Thorlabs ITL200 is 4 elements in two groups (described as "two doublet lenses").
For use outside the microscope, it is now common practice to use other types of lenses for the same optical purpose. Examples that mount directly on the camera include 200 mm telephoto lenses and ~100 mm macro lenses focused at infinity. Examples used in conjunction with bellows or long extension tubes include Raynox "closeup" lenses, enlarging lenses, simple achromats, and view camera and process lenses.
None of these alternative converging lenses are specifically designed to work well in conjunction with a microscope objective. Many of them will degrade significantly if there is too much separation between the converging lens and the objective. The most common problem is vignetting, which is caused when off-center rays from the objective are blocked by an aperture within the converging lens. Vignetting is a particularly bad problem with zoom telephotos that are set much below their maximum length, with lenses that have relatively small apertures such as 200 mm f/5.6, and with lenses that have short focal lengths such as 100 mm.
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