The speed of a camera lens measures its ability to...

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)
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

Key Concepts

Stopping Down

Stopping down is the process of reducing the effective aperture of a lens, typically by adjusting the iris. This technique is used to control light intake, depth of field, and to manage various optical aberrations. Understanding how stopping down affects the f-number is crucial for photographers who wish to manually control their exposure settings in varying lighting conditions.

Inverse-Square Law in Imaging

The inverse-square law in the context of photography explains that the light intensity on the sensor is proportional to the aperture area divided by the square of the focal length. This means that both the aperture size and the focal length collaboratively determine the effective illumination of the image, influencing how bright or dim the captured photo will be.

Aperture and Light Gathering Ability

Aperture refers to the opening in a lens through which light passes. The size of this opening, determined by the diameter, directly influences the amount of light that is admitted. Since the area is proportional to the square of the diameter, even small changes in the aperture size can have a significant impact on the light intensity, which in turn affects the brightness and exposure of the captured image.

f-number (f-ratio)

The f-number, or f-ratio, is the ratio of a lens's focal length to the diameter of its aperture. It quantifies the ‘speed’ of the lens; a lower f-number means a larger aperture, allowing more light to hit the sensor, which is particularly important in low-light photography. This concept is foundational in understanding how different lenses capture light and affect exposure levels.

Transcript

Please give Ace some feedback