Re: Aperture nodus geometry
Stenopiac image just means pinhole image. While pinhole images can look fairly sharp, they can't match the sharpness of a lens. The optimal pinhole for an 8x10 inch camera can resolve about 5 lines per millimeter, which will look sharp as a contact print. A good lens can resolve 100 lines per millimeter. The optimal pinhole for a given projection distance is a tradeoff between the size of the geometrical beam passing through the hole and the size of the diffraction disk that any hole produces. The diameter of the optimal pinhole only increases as the square root of the projection distance. For a 10 meter projection distance, the optimal pinhole is only 3.6 mm in diameter. --- On 2020-04-08 12:04, Dan-George Uza wrote: Hello, I'm a big fan of meridian lines inside churches and I know these are sort of camera obscura sundials. While I understand the geometry behind pinhole camera projections I can't seem to find any help on how the solar image forms after the rays pass a sizeable aperture nodus (for example a vertical 25cm nodus projected onto a wall 10 meters away) and how the ratio of hole size vs. projection distance affects the size and fuzzyness of the final projected image. So what's the geometry behind that? PS: Some sources refer to the projected image as "stenopaic image". Is this universally acceptable? -- Dan-George Uza --- https://lists.uni-koeln.de/mailman/listinfo/sundial--- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Denizli sundial
Dan-George, thank you for the link! That is a beautiful ancient scaphe dial. The article says: "The sundial features ... Greek names of seasons". I can read a number of characters, which at all three date lines (equinox and solstices) seem to include MEPINH. What season names are these? Keep healthy! Frans Maes On Tue, Apr 7, 2020 at 12:33 PM Roser Raluy wrote: > Thank you, it looks great! > Roser Raluy > > Missatge de Dan-George Uza del dia dt., 7 > d’abr. 2020 a les 10:12: > >> Hello, I've just read about the discovery of an antique sundial in Turkey. >> >> >> https://www.dailysabah.com/life/history/2000-year-old-sundial-unearthed-in-southern-turkeys-denizli >> >> Best regards, >> >> -- >> Dan-George Uza >> --- >> https://lists.uni-koeln.de/mailman/listinfo/sundial >> >> --- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Aperture nodus geometry
Dan-- . If the hole is very small compared to the projection-distance, then the image of the Sun projected on the wall would be sharp and clear-edged, nearly free of fuzziness. Its size will be about 1/100 of the projection-distance. . The un-fuzziness of a small-aperture projection is the reason why they're used to get precise Solar noon from a noon-mark. . Englarging the aperture enlarges the image by the same amount, and of course makes it fuzzier, because each little element of the previous image is now duplicated over a region the size of the aperture. . In your example, the aperture is about twice the size of the tiny-aperture image. . Michael Ossipoff Aprilis 8th, 2020 Aries 20th 16 W . On Wed, Apr 8, 2020 at 12:05 PM Dan-George Uza wrote: > Hello, > > I'm a big fan of meridian lines inside churches and I know these are sort > of camera obscura sundials. > > While I understand the geometry behind pinhole camera projections I can't > seem to find any help on how the solar image forms after the rays pass a > sizeable aperture nodus (for example a vertical 25cm nodus projected onto a > wall 10 meters away) and how the ratio of hole size vs. projection distance > affects the size and fuzzyness of the final projected image. So what's the > geometry behind that? > > > PS: Some sources refer to the projected image as "stenopaic image". Is > this universally acceptable? > > -- > Dan-George Uza > --- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Aperture nodus geometry
Hello, I'm a big fan of meridian lines inside churches and I know these are sort of camera obscura sundials. While I understand the geometry behind pinhole camera projections I can't seem to find any help on how the solar image forms after the rays pass a sizeable aperture nodus (for example a vertical 25cm nodus projected onto a wall 10 meters away) and how the ratio of hole size vs. projection distance affects the size and fuzzyness of the final projected image. So what's the geometry behind that? PS: Some sources refer to the projected image as "stenopaic image". Is this universally acceptable? -- Dan-George Uza --- https://lists.uni-koeln.de/mailman/listinfo/sundial
