Modern Photography/Format

The format of an image is said to be the size of the recording medium or its digital output.

Format as aspect ratio
Essentially all cameras have a rectangular (or square) frame, the shape of which is expressed in terms of the 'aspect ratio', or width divided by height.


 * $$\textrm{Aspect Ratio} = \textrm{Width} \div \textrm{Height}$$

Typically aspect ratios vary from 1:1 for square-format cameras to 16:9 for some mobile phones, and as much as 4:1 for panoramic cameras. For most cameras, this is a set quantity, although many modern cameras allow the photographer to set the aspect ratio by cropping the image in camera.

Common aspect ratios
For example, the most common still image photography format in the world is 35mm, which was the dominant film size at the close of the analog film era. It is still produced today, but more importantly its aspect ratio has been inherited by digital camera image sensor formats. The 35mm format is actually, somewhat confusingly, 36mm in width by 24mm in height. The aspect ratio of this format (dividing by twelve to obtain the simplest whole-number ratio) is therefore said to be 3:2. Many common formats (APS family of formats, for example) are equal to or closely approximate this aspect ratio.

A notable exception to the 3:2 still image format aspect ratio is the Four Thirds System, with an aspect ratio of 4:3 as seen in some compact digital cameras.

From analog to digital
In the analog era, it was common to express formats in terms of the physical size of the recording medium, for example 4×3 inches. These days it is more common to express the format in terms of the accepted short form for the standardized sensor size. For example '35mm' or 'APS-C'.

In terms of composition, these format names are arguably more usefully expressed in terms of aspect ratio, ie. 'the shape of the frame', such as 3:2 or 4:3.

In terms of the resolution available for post-processing digital images, the format alone cannot tell us, we must look at the sensor. Sensor sizes are expressed in terms of pixels, eg. 8688×5792 pixels. It is not possible to determine the pixel count from the sensor format, and vice versa, though there is some relation: larger sensor sizes tend to have larger pixel counts. Because large modern pixel numbers can be hard to remember, the industry has standardized on megapixels, or the number of total pixels divided by 1,000,000 (one million). For example, the Canon 5DS has a resolution of 8688×5792 pixels, which is equal to 50,320,896 pixels in total, or roughly 50.3 megapixels.