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A 1951 USAF resolution test chart is a microscopic optical resolution test device originally defined by the U.S. Air Force MIL-STD-150A standard of 1951. The design provides numerous small target shapes exhibiting a stepped assortment of precise spatial frequency specimens.
The magnification m is the ratio of image height y v to object height y u : =; y u and y v are of opposite sense, so the magnification is negative, indicating an inverted image. From similar triangles in Figure 6, the magnification also relates the image and object distances, so that
which is the ratio of the output beam width to the input beam width. Note the sign convention: a telescope with two convex lenses (f 1 > 0, f 2 > 0) produces a negative magnification, indicating an inverted image. A convex plus a concave lens (f 1 > 0 > f 2) produces a positive magnification and the
The focal point F and focal length f of a positive (convex) lens, a negative (concave) lens, a concave mirror, and a convex mirror.. The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power.
Likewise, the magnification of a lens is given by = = where the negative sign is given, by convention, to indicate an upright object for positive values and an inverted object for negative values. Similar to mirrors, upright images produced by single lenses are virtual while inverted images are real.
When measuring the height of an inverted image using the cartesian sign convention (where the x-axis is the optical axis) the value for h i will be negative, and as a result M will also be negative. However, the traditional sign convention used in photography is " real is positive, virtual is negative". [ 1 ]
The inverted telephoto configuration does the reverse, employing one or more negative lens groups at the front to increase the back focal distance of the lens – possibly to a figure greater than the focal length – in order to allow for additional optical or mechanical parts to fit behind the lens.