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T
115 objective focal length and f-number
Some tolerances are indisputedly important, but I'm not sure that spacing is one of them. A focussing lens is an integral part of the objective design and at its nominal focal length of 500 mm a Kowa 88 is f/5,7. At 5 m close focus the focal length would be a mere 445 mm (f/5,2). Interpolating... -
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115 objective focal length and f-number
Example:- Assume a birding scope objective had a focal length of 500 mm when focussed at infinity. To achieve close focus at 5 m (not uncommon) the focusing lens would have to reduce the focal length to 455 mm. John PS:- The above assumes a viewer with normal vision, who would bring the image... -
T
115 objective focal length and f-number
Just a few comments:- Firstly, for all waterproof binoculars and scopes where the distance between objective and eyepiece is fixed, the focal length of the objective is variable. Close focus is achieved by shortening the focal length of the objective, either by shifting a positive focussing lens... -
T
115 objective focal length and f-number
Could one not take the coincident point of an incoming ray parallel to the optical axis and the refracted ray as being in the effective plane of the objective? With a telephoto design this plane would of course be in front of all objective elements. Regards, John