On a range of weapon or the magnifying lens?
not move the lens more or father to share what you see more?
focal length is fixed, not dependent on anything other than the glasses themselves. I do not quite understand what you mean by moving closer / further away, from where? scope of a gun works very differently from a glass optical lens. The human eye, in passing, is part of the optical system (many persons not trained in optical engineering usually forget). Also, many people seem to think that while moving a lens before back is all you need to enlarge a picture, just because that's the impression that playing with a magnifying glass gives when we just put in front of an object to create virtual images, such images are poorly resolved in the eye. This may be a surprise for you, but very few people in this world know how to properly use a simple magnifying glass. In the world of professional viewpoint, much larger picture, more than time is not as simple as moving the lens around. In 99% of cases, you must purchase another objective. A range of weapon is more like a Galilean telescope, the object is assumed to be infinite, and the view through the human eye, you need a eye. In this case, you must save the eye closest to the eye opposite the scope does not work correctly. Since the object is assumed to be infinity, the scope is closer to the object made no sense of perspective and is not intended to be like that. A magnifying glass is supposed to be used as an eye and (your eyes close as possible) and the object is supposed to be placed right the focal length of the lens. In this case, movement distance does not make sense either as optical you perceive defocused if you move the object off and on the length of the lens. The answer to your question is not trivial, optics do not work that way, even if we think it does.