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Ghostly Images

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Ghostly Images

  1. Task<br />When a photo is taken with a flash, bright “disks” may appear as shown in the picture.<br />Investigate and explain the phenomenon.<br />
  2. The Ghost<br />The ghost is the image of a little flashed object, that was located out ofthefield depth.<br />
  3. Principle of the phenomenon<br />The light from a flash reflectsfrom a small surface in the unfocused area – it acts as a small source of light.<br />Focused object<br />Object out of field depth<br />Photosensitive<br />layer<br />lens<br />
  4. Camera<br />Can always be approximated to a single convex lens with one aperture.<br />Photosensitive layer<br />A system of lenses<br />Photosensitive layer<br />Convex lens<br />Aperture<br />5<br />
  5. Focus<br />Object located further than focused objects<br />Focused object<br />Object located closer than focused object<br />photosensitive layer<br />focus depth<br />lens<br />iris<br />
  6. Aperture<br />Smallaperture, big focusdepth<br />Big aperture, smallfocusdepth<br />
  7. Focusingtheobjects<br />
  8. Lensmaker’s equation<br />L0– focused object distance<br />L0’ – distance of the object’s <br /> projection – distance of the ph. layer<br />L1– distance of the ghost-generating object<br />L1’– distance of it’s projection<br />
  9. F-stop number (aperture)<br />f – focus distance<br />d – the size of the aperture<br />
  10. d – diameter of the aperture<br />s – diameter of the ghostly image<br />L0´ – distance between the ghostly image and the aperture = f<br />L1´ – distance of the particle’s projection<br />Size of the ghost<br />for infinite focused object distance <br />
  11. Size of the ghost<br />The particle is not a perfect point source of light, it has a certain diameter p (we have to count it in).<br />There is an increase in the size of the ghostly image described by our final formula:<br />
  12. We wanted to measure the exact size of the ghostly image on the chip, thus we introduced a size coefficient.<br />One pixels size:<br />wm –width of the ph. layer<br />wpx –width of the image in pixels<br />Actual size of the ghostly image in pixels – Dpx<br />Experiments<br />Used in our experiments to determine the size of a ghostly image.<br />
  13. Experimental setup<br />Independent variables<br />Aperture<br />Distance from the lighted object to the lens<br />Dependent variables<br />Size of a ghost<br />Small particle<br />Focusedobject<br />
  14. Method of measurement<br />f1.8<br />f19<br />
  15. Varying distance<br />Constant parameters<br />Focal length f = 50 mm<br />Aperture numberA = 1,8 (d = 27,8 mm)<br />Focused object distance L0 = 6 m<br />Particle size p: negligible<br />Varied parameter<br />–L1(the distance of the particles from the lens)<br />
  16. Varying distance [m]<br />5,5 5 4,5 4 3,5<br /> 3 2,5 2 1,5 1<br />
  17. Varying distance<br />Projected ghostly image size [mm]<br />Particle distance [mm]<br />
  18. Constant parameters<br />L1 = 1500 mm<br />f = 50 mm<br />L0 = 6 m<br />p = negligible<br />Varied parameter<br />A(by altering the diameter of the aperture)<br />Varying aperture size<br />
  19. Varying aperture size<br />f1.8f2.8f3.3f5.6<br />f8f13f19f22<br />
  20. Varying aperture size<br />Projected ghostly image [mm]<br />F-stop number (Aperture)<br />
  21. Constant parameters<br />L1 = 1500 mm<br />f = 50 mm<br />L0 = 6 m<br />p = 6mm<br />Varied parameter<br />A(by altering the diameter of the aperture)<br />Varying aperture size<br />
  22. f1.8f2.8f3.3f5.6<br />f8f13f19f22<br />Varying aperture size<br />
  23. The shape is dependent <br /><ul><li>on the location in the image
  24. on the distortions induced by the lens
  25. on the structure of the aperture</li></ul> Different color of the edges of the ghostly image<br /> The texture of the ghostly image<br /> Particles with non-typical light reflection<br />Other observed effects<br />
  26. <ul><li>Given by aperture shape
  27. Increasing theF-stop number increases the visibility of thiseffect</li></ul>Shapeoftheghost<br />
  28. Shape dependence<br />Flaws of the lens<br /><ul><li>The optical enlargement isn’t the same in all parts of the lens</li></li></ul><li>Colored <br />edges<br />
  29. Colored borders between light and shadow – caused by chromatic aberration<br /> Dependency of the refraction index of light on it’s wavelength<br />Colored edges<br />
  30. Texture of the ghostly image<br />Small particles (with their size comparable with the wavelength of light) (1&lt;λ&lt;100) have an easily observable texture<br />This texture consists of concentric circles<br />Diffraction occurs on the small particles<br />Other, larger particles do not create ghostlyimages with these circles<br />
  31. Conclusion<br />Ghostly images are created when a particle outside the depth of field is lit<br />The diameter of the ghostly image is mainly dependent on the particle distance and aperture size<br />The experiments confirm our theory with remarkable precision<br />Other effects can be observed in the ghostly image and we have described and explained their influence<br />Thank you for your attention<br />
  32. Objective<br /><ul><li> Nikon f1,8/50mm
  33. Double Gauss Lens</li></ul>Equipment<br />Chip<br /><ul><li>Nikon D80 RGB CCD
  34. Dimension 23.6 x 15.8 mm</li></li></ul><li>Large enough drops of water produce visibly separated multiple reflections of the flash and also refract light from other sources<br />Non-typical reflection<br />
  35. Task<br />Task<br />When a photo is taken with a flash, bright “disks” may appear as shown in the picture.<br />Investigate and explain the phenomenon.<br />
  36. Ghostly images<br />
  37. <ul><li>Different particles with different size were used
  38. A lot of interesting effects were explained
  39. Type of photosensitive layer (film or pixels) has no influence
  40. Opponent did not accept my approach through geometric optics to a task</li></ul>Overview<br />
  41. Conclusion<br />Ghostly images are created when a particle outside the depth of field is lit<br />The diameter of the ghostly image is mainly dependent on the particle distance and aperture size, which were investigated<br />The experiments confirm our theory with remarkable precision<br />Other effects can be observed in the ghostly image and we have described and explained their influence<br />Thank you for your attention<br />

Editor's Notes

  • Appendix - vysvetlenie
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