In this youtube video (High-Tech Photography of Nature in Japan), at this point, what is that device attached to that photographer's camera?
It is a dot sight that is intended for use with a rifle, and has been adapted for use with a camera. They are useful for finding (and tracking) elusive subjects without having to look through a telephoto lens. Dot sights designed for use with cameras are available, as the following YouTube video shows:
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1I've never used one. In fact, I didn't even know that they existed. The idea is that they give you a reasonably wide field of view, so that you can track a moving object -- something that is very difficult to do with a telephoto lens. I don't see any need for one to be coupled to a zoom lens. – Mick Jan 2 '17 at 15:05
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1If you watch the video, you will see that it needs to be adjusted for each lens that you use. If you are using a zoom lens, it recommends that you adjust it for the longest focal length. If I had one (and getting one is tempting), I would make sure that it was correctly adjusted every time that I used it. – Mick Jan 2 '17 at 15:36
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2If it is accurately aligned with the axis of the lens mount, it should be aligned with any (straight-through) lens that you fit. You will achieve the most accurate alignment if you first set it up with a long-focal length lens. However, there is always the risk of knocking it when changing a lens, so I would check the alignment with every lens change. – Mick Jan 2 '17 at 15:47
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1It is the same concept as a Telescope. They have a low resolution telescope included, to make a general search, then you look details on the large one. – Rafael Jan 2 '17 at 16:29
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2You'd get parallax error, the amount depending on focal length and distance to subject. At any reasonable distance (for wildlife, sports, etc) it usually won't matter. Don't use it for macro work though :-). – BobT Jan 2 '17 at 18:40