Image resolution refers to the level of detail in a photographic or video image. It applies to image creation with film and digital cameras, image reproduction in print, and image projection on screens and monitors. In film photography, image resolution was determined by the size and quality of the film itself. Digital photography resolution depends on the density of individual image components, called pixels. In both media, high-quality lenses are also required for optimum image resolution.
In all visual media, the ideal is to create an image that contains as much detail as its original source. For much of the 20th century, the film format was far superior to television and video in this respect. This was because film reproduces the actual light patterns of an image, much the way the eye does, while early video produced only an approximation. By the 21st century, digital video formats had reached a level of resolution that the unaided eye could not distinguish from film images. This manifested in high-resolution still and video cameras, high-definition (HD) television screens, and digital effects and animation in big-screen movies.
Film cameras captured images by focusing light through a lens onto a still or moving recording surface, known as a frame or negative. The chemicals on this surface took on the exact shape of the light, resulting in very precise image resolution. The resulting images could often be enlarged greatly, using movie projectors or photo enlargers, without appreciable loss of quality. For large-scale reproduction, photographers preferred a large negative; some art photographers used frames that were 10 times larger than the standard 35-mm size. When small frames were enlarged, the chemical grains making up the image could be seen, resulting in what was known as a grainy image.
Digital images, including film images that have been scanned into a computer, are composed of tiny squares of color called pixels, short for “picture elements.” Image resolution is determined by the number of pixels in a given area, indicated by measurements such as pixels per inch (PPI or ppi). Televisions and video monitors create images by projecting lines of light onto the screen. High-definition images in all these media are created by increasing the density of pixels or lines. High resolution is also required for any images that will be published in a print medium.
Enlarging an image does not increase its resolution; in fact, this will actually make the pixels or grain more evident, reducing image quality. This process was a key plot point in the influential 1966 film Blow-Up, about a photographer who finds evidence of a crime in the background of a photo. Spy movies and TV crime dramas often gloss over this fact, allowing characters to enhance image resolution more than is possible with ordinary software. The science fiction cartoon Futurama once played on this by having a starship captain demand that an enlarged image show fine detail. When told this was not possible, he complained that it always worked on the TV cop shows.