Frequently Asked Questions
Is the camera obscura a new discovery, and who invented it?
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It seems awkward to view an unsharp and inverted image. Can these problems be solved?
Both difficulties were solved soon after the invention of optics in the early 1600s. When a lens replaced the hole in the wall it produced across the room an image that was both brighter and sharper. However, the scene was still upside down. That problem was solved by arranging the lens to look vertically upward into a flat mirror held at about 45° to the optical axis. Now the image is projected down onto a horizontal white table where the scene will appear right side up if the viewer stands with his back to the outside area of interest.
Early lenses were single pieces of glass that produced a greatly improved image but one that still suffered from color fringes around bright objects with image sharpness decreasing toward the edge of the viewing table. A camera obscura today uses a lens with two or more glass elements that reduce these problems.
Four hundred years ago, the flat mirror was simply a polished metal plate. About 1850, opticians learned how to apply a shiny silver film to a polished flat piece of glass, thereby producing a flatter mirror of much higher reflectivity. Today, most flat mirrors are made by evaporating a film of aluminum onto a polished glass plate. This technique makes a much more durable reflecting surface.
How can I make a simple camera obscura?
Such a basic device is often called a pinhole camera. It is made by cutting a ½ inch hole in one end of a light-tight cardboard box and placing a white paper viewing screen on the opposite side of the box. The imaging pinhole is made in a small piece of aluminum foil that is taped in place over the ½ inch hole.
The pinhole is made in the foil with a needle to produce a clean, sharp hole with a diameter of about 1/100th of the distance from the hole to the screen. For example, if that distance is 10 inches, the pinhole should be about 1/10th of an inch in diameter. Larger pinholes make a brighter but fuzzier image, while because of optical effects, a smaller pinhole also yields a less sharp image.
The image on the white screen may be viewed by mounting the white screen over a hole cut in the back of the box. In this case you view the inverted image through the backside of the white screen. Another arrangement uses a small hole cut in the side of the box that is carefully shielded to keep out stray light yet allow the viewer to see the screen.
How can I make a brighter and sharper image than I get with a pinhole camera?
A better image is made by replacing the pinhole with a lens whose focal length is equal to the distance from the lens to the viewing screen. The diameter of the lens might be ½ to 1 inch for a focal length of 10 inches. In a basic instrument this can be a simple lens made of one lens element. Such lenses are available from the Edmund Optics Company.
Remember, both the pinhole and the lens produce an inverted image.
Can I expect to view astronomical objects through the camera obscura?
Most camera obscuras are arranged to view the surrounding landscape of buildings, mountains, a coastline or a harbor scene. These views occasionally will include a sunset or moonrise and indeed, such a scene can be especially dramatic. Observers must use great care in looking toward the sun, as even a reddened sun near the horizon can easily cause serious eye damage or blindness if viewed directly. The solar image on the view table may be dazzling but it will not cause permanent eye damage.
The instrument can be specially designed to see the moon and bright planets even when 30° or 40° above the horizon. Such a camera obscura must include a flat mirror that is significantly larger than normal. A conventional camera obscura uses a flat mirror that is an inch or two wider than the lens diameter and also about 1.5 times longer than its width. The difference between length and width is cause by the need to place the mirror at 45° to the lens optical axis. For example, a lens of 6-inch diameter would require a flat mirror of about 7×10 inches.
If the camera obscura is to view objects higher in the sky it must use a flat mirror of the standard width but with up to twice the usual length. Such a flat will be much more expensive than a normal mirror.
Is it possible to build a non-motorized instrument?
It is possible to build a camera obscura that is operated manually. Such an instrument might appeal to users in remote locations or observers who desire a portable or movable instrument. The cost of a manually operated unit is not necessarily less than a motorized version, and using the manual unit could be cumbersome. Typically, one or two rods hang freely below the lens/mirror assembly – one connected via universal joints to move the flat mirror and the other arranged to rotate a pinion/gear system that turns the top turret. Focus is usually adjusted by raising and lowering the view table through a very sturdy rack and pinion.
In some designs the viewing head is placed at the top end of a tube that slides smoothly down into a second tube for storage or transport. The cost of motors and power supplies must be weighed against the convenience and great flexibility of electric controls.