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Message: <blockquote data-quote="Surveyor" data-source="post: 3569308" data-attributes="member: 50720">Hi BinastroI do not have exact answers for your questions, only observations.Does the Nikon 8x20 XL have a distortion free eyepiece?I am not aware of any optics that are distortion/aberration free. The aberrations may not be measurable with the equipment at hand though. Eyepieces in general have far more distortions than the objective just because of the number of elements and short focal lengths. I am not sure if your photo shows slight barrel distortion or no distortion.The problem in real life is that I think people measure the field at much closer distances than 1000 yds or 1000 metres, where the field size differs slightly.The distortion does not matter as FOV only pertains to one tube, due to tolerances, each tube may have different FOV's. For you to measure FOV of two overlapping images, it would only be valid at the exact point of convergence of the two images for a given focus distance. The FOV accounts for all aberrations visible at the field stop.Also hand held I find that the field is slightly larger than tripod mounted because of slight movement and eye memory.And the IPD makes a difference if not set perfectly. Plus eye positioning.To me, what you are describing is more of an issue of eye placement in relation to the optical axis with differing locations at the field stop rather than a FOV issue.My problem is that I think in angles whereas birders may think linearly.Yet the AFOV even for birders is in degrees, which doesn't seem consistent.Can not argue the above two lines. Personally, I only think of FOV in angular terms since I can not see delineated distances in the image, only an apparent angular width.And how does one describe a 180 degree field in feet at 1000 yds or metres at 1000 metres? 3141.6 meters for a 1000 m radius, half a circle with a 1000 m radius. In a flat linear sense, probably infinite.Is it infinite, or maybe not, as space is curved.Valid point, you cannot make direct linear measurements of a curve, it has to be done by a remote method by establishing a radius for a simple curve or a delta radius for parabolic forms or chord measurements for nonstandard shapes, fractal geometry is best method for highly irregular shapes. Or the focal length of the lens, think fisheye lens here.My professional telescope making friend used a collimator and pinholes at about 15m representing close double star to test his telescopes. And also to examine the star images in and out of focus.Thanks for the thoughts, best</blockquote>

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