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  2. 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 ...

  3. 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 ...

  4. 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 ...

  5. 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.

  6. Topcon RE Super - Wikipedia

    en.wikipedia.org/wiki/Topcon_RE_Super

    The interchangeable lenses for the RE Super. Camera with array of lenses and filters. The following lenses have their own focusing thread: RE. Auto-Topcor 1:4.0 f= 20mm 62 mm filter, introduced 1969; RE. Auto-Topcor 1:3.5 f= 25mm 62 mm filter, introduced 1965; RE. Auto-Topcor 1:2.8 f= 28mm 49 mm filter, introduced 1971; RE.

  7. 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...