I’ll keep adding to this as I learn new terms!
aperture: the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are, which is of great importance for the appearance at the image plane. If the admitted rays also pass through a lens, highly collimated rays (narrow aperture) will result in sharpness at the image plane, while uncollimated rays (wide aperture) will result in sharpness for rays with the right focal length only. This means that a wide aperture results in an image that is sharp around what the lens is focusing on and blurred otherwise. The aperture also determines how many of the incoming rays are actually admitted and thus how much light that reaches the image plane (the narrower the aperture, the darker the image). (From Wikipedia)
barlow lens: named after its inventor, Peter Barlow, a barlow lens increases focal length and magnification (you put it in the telescope first, then your lens goes into the barlow)
focal length: the distance from a lens to its focus
focal point: the point at which light rays from a mirror or lens intersect
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