The lens on a camera has only two tasks. First, it must gather in as much light as possible in order to activate the sensitive chemicals on the film. Second, it must organize the light rays so that they form a sharp image on the film. These may sound like simple tasks, but they are not.
One of the sharpest lenses is merely a pinhole in a sheet of cardboard, metal, plastic, or a similar material. If the pinhole is tiny enough, the image can be quite sharp, but then very little light is admitted. For most purposes, even the most sensitive film would take too long to record an image.
A glass lens is much better because it lets in much more light and focuses it on the film. Yet simple glass lenses are sharpest only in their centers. As more of the lens is used, the image suffers in sharpness.
One reason a simple lens can cause problems is that it is shaped like a section of a sphere. Spherical lenses do not focus perfectly on flat film, so the image is slightly distorted, especially at the edges. Another reason is that the lens can act partly like a prism. This means that some of the colors in the image will not focus properly, and the image will be fuzzy.
One solution is to block off all but the sharp-focusing center of the lens. If you block off the edges of the lens, however, less light will get to the film. Early lenses had to compromise between sharpness and light-gathering power.
Very sharp lenses that admit as much light as possible can be built by making them with several separate lenses, or elements. A multiple-element lens has from two to nine separate lenses. Some elements are cemented together, and some have a gap between them. Furthermore, the elements are often made of different kinds of glass, each with a different ability to bend light rays. Some of the elements are there just for correcting problems caused by the other elements! The results are worth it, though: pictures can be taken in many different light conditions, and they have a sharpness you can almost feel.
1. The word it in the first paragraph refers to:
- a. Camera
- b. Lens
- c. Film
- d. Chemicals
2. The word distorted in the fourth paragraph means:
- a. Out of a proper or natural relation
- b. Clean and in shape
- c. Purified, as one
- d. Proper
3. In what ways does an image suffer if too large an area of the spherical lens is used?
- a. The edges of the image become fuzzy
- b. Too much light is admitted
- c. Too little light is admitted
- d. Colors change
4. What is the main disadvantage of a simple lens that is made sharp by using just the center?
- a. With less light-gathering power, the lens is utterly useless.
- b. With less light-gathering power, the lens is useful only in bright light.
- c. With more light-gathering power, the lens is useful only in dim light.
- d. With more light-gathering power, the lens is utterly useless.
5. The word sharpest in the third paragraph is closest in meaning to:
- a. Having clear form and detail
- b. Terminating in an edge or a point
- c. Intellectually penetrating; astute
- d. Having a thin edge or a fine point suitable for or capable of cutting or piercing
6. The word it in the fourth paragraph refers to:
- a. Glass lens
- b. Prism
- c. Simple lens
- d. Flat film
7. The word sensitive in the second paragraph could best be replaced by the word:
- a. Delicate
- b. Irritable
- c. Reactive
8. The word ability in the sixth paragraph could best be replaced by the word:
- a. Ignorance
- b. Weakness
- c. Ineptness
- d. Capacity
9. The meaning of the word solution as used in the fifth paragraph is closest in meaning to:
- a. A homogeneous mixture of two or more substances, which may be solids, liquids, gases, or a combination of these
- b. The answer to or disposition of a problem
- c. The state of being dissolved
- d. Release; deliverance; discharge
10. What is the minimum number of lenses in multiple-element lens?
- a. Nine
- b. Two
- c. Ninety-two
- d. Twenty-nine
11. The word fuzzy in the fourth paragraph means:
- a. Clear
- b. Unclear
- c. Exact
- d. Precise
12. The word admitted in the sixth paragraph is closest in meaning to:
- a. To permit to enter
- b. To have room for; accommodate
- c. To grant to be real, valid, or true; acknowledge
- d. To make acknowledgment