why does a pinhole camera invert an image

How did you look through the pinhole? If you put it far from your eye you have seen nothing. It is small hole and you have seen one "overexposed" point behind dark card. If you put the card as close to the eye as possible think of it as another lens in your eye. And the image was already inverted - it was projected on your retina and your brain automatically re-inverted it so up was up and down was down. There is no way how pinhole projection can not be inverted. There is no reason to make camera lenses that won't invert the image - the drawbacks of design are far more serious that dumping the chip in correct order, see, (digital cameras), turning the film correctly when developping (film cameras) and adding pentamirror ([D]SLRs' viewfinder). Note that
have inverted viewfinder. OK, there is one, but it is a solution: Build your camera with pinhole, translucent screen, another pinhole and the film/CMOS/CCD. This way you'll have inverted image on the screen and double-inverted image on the film.

If you want to see the pinhole effect, find a room, cover all windows except for one small hole. The room will be completely dark, except for the projection of the outside world on the wall opposite to the hole. You will be inside the camera obscura. Up to a certain point, the smaller the hole, the sharper the image, but the dimmer the projected image. Optimally, the size of the aperture should be 1/100 or less of the distance between it and the projected image. Within limits, a smaller pinhole (with a thinner surface that the hole goes through) will result in sharper image because the projected at the image plane is practically the same size as the pinhole. An extremely small hole, however, can produce significant effects and a less clear image due to the wave properties of light. Additionally, occurs as the diameter of the hole approaches the thickness of the material in which it is punched, because the sides of the hole obstruct the light entering at anything other than 90 degrees.

The best pinhole is perfectly round (since irregularities cause higher-order diffraction effects), and in an extremely thin piece of material. Industrially produced pinholes benefit from etching, but a hobbyist can still produce pinholes of sufficiently high quality for photographic work. One method is to start with a sheet of or metal reclaimed from an aluminium drinks can or /, use fine sand paper to reduce the thickness of the centre of the material to the minimum, before carefully creating a pinhole with a suitably sized needle. A method of calculating the optimal pinhole diameter was first attempted by. The crispest image is obtained using a pinhole size determined by the formula where d is pinhole diameter, f is focal length (distance from pinhole to image plane) and k is the of light. For standard black-and-white film, a wavelength of light corresponding to yellow-green (550 ) should yield optimum results.

For a pinhole-to-film distance of 1 inch (25Pmm), this works out to a pinhole 0. 236Pmm in diameter. For 5Pcm, the appropriate diameter is 0. 332Pmm. The is basically, but this does not mean that no optical blurring occurs. The infinite depth of field means that image blur depends not on object distance, but on other factors, such as the distance from the aperture to the, the aperture size, the wavelength(s) of the light source, and motion of the subject or canvas. In the 1970s, Young measured the resolution limit of the pinhole camera as a function of pinhole diameter and later published a tutorial in The Physics Teacher. Partly to enable a variety of diameters and focal lengths, he defined two normalized variables: pinhole radius divided by resolution limit, and focal length divided by the quantity s /k, where s is the radius of the pinhole and k is the wavelength of the light, typically about 550Pnm. His results are plotted in the figure. To the left, the pinhole is large, and geometric optics applies; the resolution limit is about 1. 5 times the radius of the pinhole. (Spurious resolution is also seen in the geometric-optics limit. ) To the right, the pinhole is small, and applies; the resolution limit is given by the far-field diffraction formula shown in the graph and now increases as the pinhole is made smaller.

In the region of near-field diffraction (or ), the pinhole focuses the light slightly, and the resolution limit is minimized when the focal length f (the distance between the pinhole and the film plane) is given by f = s /k. At this focal length, the pinhole focuses the light slightly, and the resolution limit is about 2/3 of the radius of the pinhole. The pinhole in this case is equivalent to a Fresnel zone plate with a single zone. The value s /k is in a sense the natural focal length of the pinhole. The relation f = s / k fk, so the experimental value differs slightly from the estimate of Petzval, above.

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