A camera forms a real, inverted image of the object being photographed on a light-sensitive surface. The amount of light striking this surface is controlled by the shutter speed and the aperture. The intensity of this light is inversely proportional to the square of the f-number of the lens.
\[ \text{f-number} = \frac{\text{Focal length}}{\text{Aperture diameter}} = \frac{f}{D} \]
In the eye, refraction at the surface of the cornea forms a real image on the retina. Adjustment for various object distances is made by squeezing the lens, thereby making it bulge and decreasing its focal length. A nearsighted eye is too long for its lens; a farsighted eye is too short. The power of a corrective lens, in diopters, is the reciprocal of the focal length in meters.
The simple magnifier creates a virtual image whose angular size \(\theta'\) is larger than the angular size \(\theta\) of the object itself at a distance of 25 cm, the nominal closest distance for comfortable viewing. The angular magnification \(M\) of a simple magnifier is the ratio of the angular size of the virtual image to that of the object at this distance.
\[ M = \frac{\theta'}{\theta} = \frac{25\ \text{cm}}{f}\]
In a compound microscope, the objective lens forms a first image in the barrel of the instrument, and the eyepiece forms a final virtual image, often at infinity, of the first image. The telescope operates on the same principle, but the object is far away. In a reflecting telescope, the objective lens is replaced by a concave mirror, which eliminates chromatic aberrations.
Written by Albert Marin