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2. Telephoto lens - Wikipedia

   en.wikipedia.org/wiki/Telephoto_lens

   A telephoto lens, also known as telelens, is a specific type of a long-focus lens used in photography and cinematography, in which the physical length of the lens is shorter than the focal length. [ 1 ] : 93 This is achieved by incorporating a special lens group known as a telephoto group that extends the light path to create a long-focus lens ...

3. f-number - Wikipedia

   en.wikipedia.org/wiki/F-number

   The f-number N is given by: = where f is the focal length, and D is the diameter of the entrance pupil (effective aperture).It is customary to write f-numbers preceded by "f /", which forms a mathematical expression of the entrance pupil's diameter in terms of f and N. [1]

4. T-mount - Wikipedia

   en.wikipedia.org/wiki/T-mount

   The T-mount is a standard lens mount for cameras and other optical assemblies. The usual T-mount is a screw mount using a male 42×0.75 (42 mm diameter, 0.75 mm thread pitch) metric thread on the lens with a flange focal distance of 55 mm and a mating female 42mm thread on a camera adapter or other optical component.

5. List of telescope parts and construction - Wikipedia

   en.wikipedia.org/wiki/List_of_telescope_parts...

   Field lens: A correcting lens placed just before the image plane of a telescope. [citation needed] Telecompressor or focal reducer: Optical element to decrease the telescope's focal length and magnification (usually by a fixed percentage) and widen the field of view, providing opposite effects of a Barlow lens.

6. Refracting telescope - Wikipedia

   en.wikipedia.org/wiki/Refracting_telescope

   Since a lens can only be held in place by its edge, the center of a large lens sags due to gravity, distorting the images it produces. The largest practical lens size in a refracting telescope is around 1 meter (39 in). [35] There is a further problem of glass defects, striae or small air bubbles trapped within the glass.

7. 35 mm equivalent focal length - Wikipedia

   en.wikipedia.org/wiki/35_mm_equivalent_focal_length

   The resulting images from 50 mm and 70 mm lenses for different sensor sizes; 36x24 mm (red) and 24x18 mm (blue) In photography, the 35 mm equivalent focal length is a measure of the angle of view for a particular combination of a camera lens and film or image sensor size.