How Everything Works
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Cameras
Page 2 of 3 (26 Questions and Answers)

851. Does blowing on or waving a developing Polaroid picture actually speed up its development process? — PS, Columbus, OH
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The speed of the development process is determined by the diffusion of molecules within the developing film and by the rates at which they react with one another. Both processes, diffusion and chemical reactions are temperature sensitive. If blowing on or waving the film manages to increase its temperature, then diffusion and reactions will both speed up and the development time will decrease.

928. When you are looking at something and there is an object partially blocking your view (e.g., a fence or a railing), why with one eye closed does the barrier block your vision but with both eyes open you seem to look through the barrier? — DS
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Your brain merges the images it obtains from your two eyes so that you "see" a composite image that is essentially a sum of what both eyes see. When you close one eye so that only the other eye is providing an image to your brain, any object that blocks your view chops a piece out of the distant scene. No light from that portion of the scene reaches your open eye, so you can't see that portion of the scene. But when you have both eyes open, the image observed by one eye can compensate for any missing pieces in the image observed by the other eye. Since the barrier you are looking through chops out a different piece of the distant scene for each of your two eyes, the composite image that your brain assembles from these two individual images will include the whole scene.

1027. How does a picture camera work? — HW, Ypsilanti, MI
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A picture camera uses a lens to form a real image of a distant scene on the surface of a sheet of film. The lens bends rays of light so that all the light from a certain spot on the scene that passes through the lens comes together to a single point on the film. You can see this real image formation process with a magnifying glass. Just go into a darkened room with one window on a sunny day and hold the magnifying glass a few inches away from the wall opposite the window. You should see an inverted image of the window and the scene outside it projected on the wall. If you don't move the lens toward or away from the wall until that image forms. Everything else about a camera is just helping that lens form its image on the film in a controlled fashion. The camera's shutter limits the amount of time that light has to form this image. The focus controls make sure that light from the object you are interested in forms a sharp image on the film and doesn't appear blurry.

1130. How does an overhead projector work? — SR, Hartford, CT
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An overhead projector uses a converging lens and a mirror to project a real image of your transparency onto a screen. A lamp brightly illuminates the transparency and a special surface under the transparency (actually a Fresnel lens) directs the light from the transparency through the projector's main lens. This lens bends the light rays in such a way that all of the rays spreading outward from one point on the transparency bend back together and merge to one point on the screen. For example, if you make a green dot on the transparency, light rays spread outward from that green dot and some of them pass through the main lens. The lens bends these rays back together so that they form a single green dot on the screen. There is a single point on the screen for the light rays from each point on the transparency.

The pattern of light that forms on the screen is called a real image because it looks just like the original object—in this case the transparency—and it's real, meaning that you can touch it with your hand. Real images are usually upside-down and backward, but the overhead projector uses its mirror to flip the image over so that it appears right side up. Because of this vertical flip, the side-to-side reversal is a good thing—the right side of the transparency becomes the left side of the screen image (as viewed by the same person) and the screen image is readable.


1131. How would I go about making a camera that's more than just a pinhole camera? — JL, Longview, WA
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While a pinhole will project the image of a scene on a piece of film, it doesn't collect very much light. That's why a pinhole camera requires very long exposures. A better camera makes use of a converging lens. If you hold a magnifying glass several inches away from a white sheet of paper, you will see that it forms a real image of anything on the other side of it—particularly bright things such as light bulbs or well-lighted windows. A typical camera uses a converging lens that's not unlike a magnifying glass to form an image of this sort. You could use a magnifying glass to build a camera, but I'd suggest that you start with a camera and rebuild it yourself. Go to a company that processes film and see if they will give you any used disposable cameras. These cameras are of essentially no value to them and they either discard them or recycle them. If you ask around, you should find a photo shop that will give you a couple. You can then disassemble them. You'll find a very nice lens, a shutter system, a film advance mechanism, and so on. You can use a toothpick or small screwdriver to turn the exposure dial backward so that the camera behaves as though it still has film left. You can then "advance the (non-existent) film" by turning the film sensing gears in the back of the camera with your fingers until the shutter cocks. Finally, you can press the shutter release and watch the shutter open the lens to light. Disposable cameras are great because if you break something in your experimenting, you can just throw away your mistake.

1174. How does a single lens reflex camera work?
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When rays of light from a distant object reach the camera's lens, those rays are spreading apart or "diverging." You can understand this by following the rays of light from one spot on the object, say the tip of a person's nose. The rays of light reflected from the nose spread outward in all directions and only a small portion of them passes into the camera's lens. These light rays are diverging from one another as they travel.

The camera's lens is a converging lens, meaning that it bends the paths of these light rays so that they diverge less after passing through it. In fact, the lens bends the rays so much that they begin to come together or "converge" after the lens and all the rays of light from the person's nose merge to a single point in space somewhere beyond the lens. Exactly how far from the lens the rays come together depends on the structure of the lens and on the distance between it and the person's nose. When you focus the lens, you're moving the lens so that the rays come together at just the right place to illuminate a single spot on a piece of photographic film. When the distance between the lens and film is just right, all the light from each point on the person comes together at a corresponding point on the film. The lens is then forming a real image of the person on the film and the film records this pattern of light to make a photograph.

In a single lens reflex camera, light passing through the lens doesn't always fall on the film. Most of the time, this light is redirected by a mirror that follows the lens so that the real image forms on a special glass sheet near the top of the camera. When you look through the viewfinder of the camera, you are actually using a magnifying glass to inspecting this real image, making the camera effectively a telescope. You (or the camera, if it is automatic) then focus the lens to form a sharp real image on the glass sheet before taking the picture. Since this glass sheet is the same optical distance from the lens as the film is, focusing on the glass is equivalent to focusing on the film. When you take the picture, the redirecting mirror quickly flips out of the way and a shutter opens to allow light from the lens to fall directly onto the camera's photographic film. For a brief moment, light from the person passes through the lens and onto the film, forming a real image that is permanently recorded on the film. Then the shutter closes and the mirror swings back to its normal position.


1227. How does a projector work?
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A projector is essentially a camera that's operating backward. When you take a picture of a tree, all of the light striking the camera lens from a particular leaf is bent together to one small spot on the film. Overall, light from each leaf is bent together to a corresponding spot on the film and a pattern of light that looks just like the tree—a real image of the tree—forms on the surface of the film. The film records this pattern of light through photochemical processes, and subsequent development causes the film to display this captured light pattern forever. Because of the nature of the bending process, the real image that forms on the film is upside-down and backward. Because it forms so near the camera lens, it's also much smaller than the tree itself.

A projector just reverses this process. Now light starts out from an illuminated piece of developed film—such as a slide containing an image of a tree. Now the projector lens bends all of the light striking it from a particular leaf spot on the slide together to one small spot on a distant projection screen. Again, light from each leaf on the slide is bent together to a corresponding spot on the screen and a pattern of light that looks just like the slide—a real image of the slide—forms on the surface of the projection screen. As before, this image is upside-down and backwards, which is why you must be careful how you orient a slide in a projector, lest you produce an inverted image on the screen.


1446. I will be teaching first graders how to use simple magnifiers. What are the basic safety rules for magnifiers that I should share with them with regard to sunlight, heat, etc. — JR
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The only source of common light source that presents any real danger to a child with a magnifying glass is the sun. If you let sunlight pass through an ordinary magnifying glass, the convex lens of the magnifier will cause the rays of sunlight to converge and they will form a real image of the sun a short distance after the magnifying glass. This focused image will appear as a small, circular light spot of enormous brilliance when you let it fall onto a sheet of white paper. It's truly an image—it's round because the sun is round and it has all the spatial features that the sun does. If the image weren't so bright and the sun had visible marks on its surface, you'd see those marks nicely in the real image.

The problem with this real image of the sun is simply that it's dazzlingly bright and that it delivers lots of thermal power in a small area. The real image is there in space, whether or not you put any object into that space. If you put paper or some other flammable substance in this focused region, it may catch on fire. Putting your skin in the focus would also be a bad idea. And if you put your eye there, you're in serious trouble.

So my suggestion with first graders is to stay in the shade when you're working with magnifying glasses. As soon as you go out in direct sunlight, that brilliant real image will begin hovering in space just beyond the magnifying glass, waiting for someone to put something into it. And many first graders just can't resist the opportunity to do just that.


1447. When you open your eyes underwater everything is blurry, but when you wear a mask, you can see clearly. Why can't the eye focus underwater unless it has an air space, provided by the mask, in front of it? — DW, Cork City, Ireland
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Just as most good camera lenses have more than one optical element inside them, so your eye has more than one optical element inside it. The outside surface of your eye is curved and actually acts as a lens itself. Without this surface lens, your eye can't bring the light passing through it to a focus on your retina. The component in your eye that is called "the lens" is actually the fine adjustment rather than the whole optical system.

When you put your eye in water, the eye's curved outer surface stops acting as a lens. That's because light travels at roughly the same speed in water as it does in your eye and that light no longer bends as it enters your eye. Everything looks blurry because the light doesn't focus on your retina anymore. But by inserting an air space between your eye and a flat plate of glass or plastic, you recover the bending at your eye's surface and everything appears sharp again.


1470. I have a digital camera and when I put an IR remote control in front of the lens and press a button, a bluish white light is visible on the camera's monitor. Why is that? — MC
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What a neat observation! Digital cameras based on CCD imaging chips are sensitive to infrared light. Even though you can't see the infrared light streaming out of the remote control when you push its buttons, the camera's chip can. This behavior is typical of semiconductor light sensors such as photodiodes and phototransistors: they often detect near infrared light even better than visible light. In fact, a semiconductor infrared sensor is exactly what your television set uses to collect instructions from the remote control.

The color filters that the camera employs to obtain color information misbehave when they're dealing with infrared light and so the camera is fooled into thinking that it's viewing white light. That's why your camera shows a white spot where the remote's infrared source is located.

I just tried taking some pictures through infrared filters, glass plates that block visible light completely, and my digital camera worked just fine. The images were as sharp and clear as usual, although the colors were odd. I had to use incandescent illumination because fluorescent light doesn't contain enough infrared. It would be easy to take pictures in complete darkness if you just illuminated a scene with bright infrared sources. No doubt there are "spy" cameras that do exactly that.


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