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I want to try out NIkon's new 2.8mm lens!
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<blockquote data-quote="Bob Blaylock" data-source="post: 216556" data-attributes="member: 16749"><p>It doesn't appear that I can read the Encyclopedia Britannica article without paying for a subscription. As I was pondering the question at the time of my previous posting, the word “retrofocus” popped into my head, leading me to look that up on Wikipedia.</p><p></p><p> As far as I can tell, no article I've yet read seems to suggest that what, lacking any other known term, I am going to call the “optical center” of a lens can be outside of the physical structure of that lens. The point of “retrofocus” was to move that as far back as possible, but the articles left me with the impression that it was still no further back that the rearmost element.</p><p></p><p> On the other hand, it does seem that I have one empirical, observable sample, in the form of my 18-55mm stick lens that came with my D3200. It claims to have a minimum focal length of 18mm, but it is fairly obvious to me that when mounted on the camera, there is not any part of it that is anywhere near that close to the film plane. I suppose, at some point, I'll have to try some experiments and measurements, to verify that when zoomed out to 18mm, it really does produce an angle of view that is consistent with an optical center that is 18mm from a DX-format sensor, but for now, I'll take it as a given that it does.</p><p></p><p> I guess, if you think about it, any fisheye lens must be similar in principle to what I am wondering about. Geometrically, to get a 180° field of view, you'd have to have an effective focal length of zero. Nikon's famous rare 220° fisheye would have to have an effective focal length that is negative. Obviously, there is some trick in optics to take in a wide field of view, and convert it into something that is then projected to focal plane that is farther away from the lens than the focal length that would geometrically produce that field of view; otherwise, fisheye lenses would be impossible. I guess any lens with an effective focal length that is shorter than the physical distance between that lens' rearmost element and the film plane must rely on such a trick, and is effectively a less-extreme version of a fisheye lens. Perhaps that's what “retrofocus”*really is about, and I've just not yet read any article that did a good job of explaining this to me.</p></blockquote><p></p>
[QUOTE="Bob Blaylock, post: 216556, member: 16749"] It doesn't appear that I can read the Encyclopedia Britannica article without paying for a subscription. As I was pondering the question at the time of my previous posting, the word “retrofocus” popped into my head, leading me to look that up on Wikipedia. As far as I can tell, no article I've yet read seems to suggest that what, lacking any other known term, I am going to call the “optical center” of a lens can be outside of the physical structure of that lens. The point of “retrofocus” was to move that as far back as possible, but the articles left me with the impression that it was still no further back that the rearmost element. On the other hand, it does seem that I have one empirical, observable sample, in the form of my 18-55mm stick lens that came with my D3200. It claims to have a minimum focal length of 18mm, but it is fairly obvious to me that when mounted on the camera, there is not any part of it that is anywhere near that close to the film plane. I suppose, at some point, I'll have to try some experiments and measurements, to verify that when zoomed out to 18mm, it really does produce an angle of view that is consistent with an optical center that is 18mm from a DX-format sensor, but for now, I'll take it as a given that it does. I guess, if you think about it, any fisheye lens must be similar in principle to what I am wondering about. Geometrically, to get a 180° field of view, you'd have to have an effective focal length of zero. Nikon's famous rare 220° fisheye would have to have an effective focal length that is negative. Obviously, there is some trick in optics to take in a wide field of view, and convert it into something that is then projected to focal plane that is farther away from the lens than the focal length that would geometrically produce that field of view; otherwise, fisheye lenses would be impossible. I guess any lens with an effective focal length that is shorter than the physical distance between that lens' rearmost element and the film plane must rely on such a trick, and is effectively a less-extreme version of a fisheye lens. Perhaps that's what “retrofocus”*really is about, and I've just not yet read any article that did a good job of explaining this to me. [/QUOTE]
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I want to try out NIkon's new 2.8mm lens!
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