A low f-number allows a smaller depth of field. allows using a faster shutter speed. causes less spherical aberration. is not related to any of the above options.
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The $f$ -number of a photographic system determines not just its speed but also its "depth of field," the range of distances over which objects remain in acceptable focus. Low $f$ -numbers correspond to small depth of field, high $f$ -numbers to large depth of field. Explain this.
The speed of a camera lens measures its ability to photograph in dim light. Speed is characterized by $f$ -ratio, also called the $f$ -number, defined as the ratio of focal length $f$ to lens diameter $d$. Thus an $f / 2.8$ lens, for example, has diameter $d=f / 2.8 .$ The actual amount of light a lens admits depends on its area $A$, but the inverse-square law shows that the light intensity at the camera's imaging sensor is proportional to $A / f^{2} .$ Most cameras have an adjustable iris that obscures part of the lens to change the f-ratio in response to available light. Point-and-shoot cameras adjust the f-ratio automatically, but serious photographers use their camera's manual f-ratio adjustment (Fig. 31.37 ). Stopping down is the photographer's term for reducing the lens area using the adjustable iris. A 35 -mm camera lens. The numbers from 22 to 2.8 at the bottom are values for the f-ratio, $f / d .$ Turning the ring with these numbers adjusts the iris that covers the outer part of the lens, thus changing the f-ratio. (FIGURE CAN'T COPY) You're given two lenses with different diameters. Knowing nothing else, you can conclude that a. the larger lens is faster. b. the smaller lens has the shorter focal length. c. the smaller lens suffers less spherical aberration. d. none of the above
The speed of a camera lens measures its ability to photograph in dim light. Speed is characterized by $f$ -ratio, also called the $f$ -number, defined as the ratio of focal length $f$ to lens diameter $d$. Thus an $f / 2.8$ lens, for example, has diameter $d=f / 2.8 .$ The actual amount of light a lens admits depends on its area $A$, but the inverse-square law shows that the light intensity at the camera's imaging sensor is proportional to $A / f^{2} .$ Most cameras have an adjustable iris that obscures part of the lens to change the f-ratio in response to available light. Point-and-shoot cameras adjust the f-ratio automatically, but serious photographers use their camera's manual f-ratio adjustment (Fig. 31.37 ). Stopping down is the photographer's term for reducing the lens area using the adjustable iris. A 35 -mm camera lens. The numbers from 22 to 2.8 at the bottom are values for the f-ratio, $f / d .$ Turning the ring with these numbers adjusts the iris that covers the outer part of the lens, thus changing the f-ratio. (FIGURE CAN'T COPY) If a lens suffers from spherical aberration, stopping down will a. worsen the focus. b. improve the focus. c. not affect the focus.
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