Category: cameras

How does a single lens reflex camera work?

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.

How does an overhead projector work? — SR, Hartford, CT

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.

How would I go about making a camera that’s more than just a pinhole camera? — JL, Longview, WA

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.

How does a picture camera work? — HW, Ypsilanti, MI

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.

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

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.

Does blowing on or waving a developing Polaroid picture actually speed up its development process? — PS, Columbus, OH

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.

How does the auto-focusing system on a camera work? — RM, Lititz, PA

There are several different systems for autofocusing. I think that the three most popular systems are optical contrast, rangefinder overlap, and acoustic distancing. The optical contrast scheme places a sophisticated light sensitive surface in the focal plane of the camera’s lens. This sensor recognizes when sharp focus is achieved by looking for the moment of maximum contrast in the image. When the lens is out of focus, the image is fuzzy and has little contrast. But when the lens is focused properly, the image is sharp and the sensor detects the strong spatial variations in darkness and brightness. The camera automatically scans the focus of its lens until it detects maximum image contrast.

The rangefinder overlap system observes the scene in front of the camera through two auxiliary lenses that are separated by a few inches. It uses mirrors to overlap the images from these two lenses and can determine the distance to the objects in the picture by the angles of the mirrors. The camera uses this distance measurement to set the focus of its main lens.

The acoustic distancing system bounces sound waves from the objects in front of the camera to determine how far away they are. The camera then adjusts its main lens for that distance. While this acoustic scheme has the advantage of working even in complete darkness, it’s confused by clear surfaces—if you take a picture through a window, it will focus on the window. The optical schemes will focus on the objects rather than the window, but they will only work when there is light coming from the objects. That’s why many autofocus cameras that use optical autofocus schemes have built in lights to illuminate the objects during the autofocusing process.

How do you calculate the path light takes after going through a lens and how do you measure the curvature of the lens? — AS, Champaign, IL

The surfaces of most lenses are shaped like the surfaces of spheres. Such “spherical” lenses can be characterized by a single distance: the focal length. For converging lenses, those with convex or outward-bulging surfaces, light from a distant object such as the sun will converge together after passing through the lens and will form an image of the object at a distance of the focal length from the center of the lens. You can find this “real” image by holding a sheet of white paper beyond the lens and looking for a clear pattern of light corresponding to the object. If the object is closer to the lens, the image will form a bit farther from the lens. The relationship between the distance to the object (the object distance or OD), the focal length of the lens (F), and the distance to the image (the image distance or OD) is given by a simple formula: 1/F = 1/OD + 1/ID.

This lens formula works for diverging lenses, too, but those lenses have negative focal lengths and produce their images on the object side of the lens. You can only view these “virtual” images by looking at them through the lens itself.

The easiest way of determining a lens’s focal length is by measuring the distance between the lens and the real image it forms of a distant object. However, you can measure the curvatures of the lens’s surfaces and calculate its focal length. Special gauges exist that touch the lens at several points, usually a circle and a central point, and determine how curved its surface is.

Why are there various types of film (speed, purposes, etc.)?

The different speeds of film have to do with how light sensitive the film emulsion is. A portion of the surface of a high-speed film will register exposure to light when only a few particles of light (photons) reach it. In contrast, a low speed film requires more photons per square millimeter to undergo the chemical changes of exposure.

While high speed film can take pictures with less light than low speed film, there is a trade-off. High-speed films are grainier and have less resolution than low speed films. Thus photographs that you would like to enlarge should be taken with relatively slow film.

Why do camera flashes make eyes red and why do two flashes correct this problem?

The retinas of your eyes appear reddish when you look at them with white light. The red eye problem occurs because light from the flash passes through the lens of your eye, strikes the retina (which allows you to see the flash), and reflects back toward the camera. This reflection is mostly red light and it is directed very strongly back toward the camera. The camera captures this red reflection very effectively and so eyes appear red. The double flash is meant to get the irises of your eyes to contract (as they do whenever your eyes are exposed to bright light or you are startled or excited). The first flash causes your irises to contract so that less light from the second flash can pass into and out of your eyes. Unfortunately, this trick doesn’t work all that well.