Large Copy Lens by Minolta. These beautiful copy lenses measure 3.5" (90mm) diameter and 4" (120mm) long. There are four holes in the front flange for mounting and a groove in the rear collar for a clamp to hold the lens in place. Minolta is the mfr and the part number 4108122-001A/19970226MN is on the lens. When we had someone measure the focal length they did a 'nodal test' this means that they projected the images of distant objects (greater than 50 feet like the sun) onto a white card and produce a focused image. One can measure the distance of the last element to the image and get a measurement of back focal length. This number is 2 inch and not even close to the real focal length. The nodal test has one hold the lens as if a shaft were running down though the lens (thumb on top finger on the bottom) and point the lens from left to right. If the image moves the fingers should be moved up or down the barrel of the lens until a place is found where the image does not wander when lens is rotated, and this position is marked. The distance from the point of rotation to the image is the focal length. The distance from the pivot point to the image is the true focal length. A point just behind the wide front flange to the image is a distance of 6.25? and that is the focal length. This is an unusual lens. It is not a copy lens designed for 1:1, it is best a 20:1 or greater magnification; it is designed to project an image of an object at greater than 20 x 6.25 distance. This means that the image is formed at 6.25 and an image is formed at 120 inch or greater for best performance. The original use of this lens was to take a point source and project it as collimated ( read as parallel rays ) light projected to some distant point like infinity. Since one side of the lens forms an image at 6.25 inches and the other produces an image at infinity the lens is called an 'infinite conjugate' having a 1:infinity magnification. This means that the lens would make a nice telescope or a laser collimator. It is a 150mm fl f2.5 infinite conjugate as nearly as we can tell with current testing.
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