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  2. Sigma 4.5mm f/2.8 EX DC Circular Fisheye HSM lens - Wikipedia

    en.wikipedia.org/wiki/Sigma_4.5mm_f/2.8_EX_DC...

    It is a circular fisheye lens, designed to project a 180-degree field of view in all directions onto a circular image. It is the first such lens to provide the complete circular fisheye effect on crop factor digital SLR cameras, which do not have a full 36x24mm sensor. [1] This lens is available in Canon, Nikon, Sigma, Pentax and Sony mounts ...

  3. Bull's eye level - Wikipedia

    en.wikipedia.org/wiki/Bull's_eye_level

    The circular bubble is used to roughly level the instrument in two dimensions. The plate level is then used, rotating it 90 degrees between readings to level the instrument more precisely. The Fell All-Way precision level, one of the first successful American made bull's eye levels for machine tool use, was invented by William B. Fell, Rockford ...

  4. Spirit level - Wikipedia

    en.wikipedia.org/wiki/Spirit_level

    If the bubble housing has graduated divisions, then the sensitivity is the angle or gradient change that moves the bubble by one of these divisions. 2 mm (0.079 in) is the usual spacing for graduations; on a surveyor's level, the bubble will move 2 mm (0.079 in) when the vial is tilted about 0.005 degree.

  5. Sigma 8mm f/3.5 EX DG lens - Wikipedia

    en.wikipedia.org/wiki/Sigma_8mm_f/3.5_EX_DG_lens

    The Sigma 8mm f/3.5 EX DG is a photographic lens introduced on 9 August 2006. It is a circular fisheye lens, designed to project a 180-degree field of view in all directions onto a circular image when used on a full-frame camera. The lens is available in Canon, Nikon, and Sigma mounts.

  6. Level (optical instrument) - Wikipedia

    en.wikipedia.org/wiki/Level_(optical_instrument)

    The Y level or wye level is the oldest and bulkiest of the older style optical instruments. A low-powered telescope is placed in a pair of clamp mounts, and the instrument then leveled using a spirit level, which is mounted parallel to the main telescope. The term dumpy level (also builder's level) endures despite the evolution in design. They ...

  7. Circle of confusion - Wikipedia

    en.wikipedia.org/wiki/Circle_of_confusion

    Such a blur spot has the same shape as the lens aperture, but for simplicity, is usually treated as if it were circular. In practice, objects at considerably different distances from the camera can still appear sharp; [1] the range of object distances over which objects appear sharp is the depth of field (DoF). The common criterion for ...

  8. Angular resolution - Wikipedia

    en.wikipedia.org/wiki/Angular_resolution

    where θ is the angular resolution , λ is the wavelength of light, and D is the diameter of the lens' aperture. The factor 1.22 is derived from a calculation of the position of the first dark circular ring surrounding the central Airy disc of the diffraction pattern. This number is more precisely 1.21966989...

  9. Photographic lens design - Wikipedia

    en.wikipedia.org/wiki/Photographic_lens_design

    Prime lens - a photographic lens whose focal length is fixed, as opposed to a zoom lens, or that is the primary lens in a combination lens system. Zoom lenses - variable focal length lenses. Zoom lenses cover a range of focal lengths by utilising movable elements within the barrel of the lens assembly.