The magnification ratio is the relationship of the size of an object on film to the size of the object in real life. The magnification ratio is expressed as two numbers: the image size on film and the image size in real life, where the image size on film is always listed first. Most standard lenses are capable of magnification ratios of 1:8 to 1:10. If you need more magnification, you have to use macro lenses, diopters, extension tubes or bellows.

Examples:
The Normal 35 Academy camera aperture measures 16 mm high by 22 mm wide. If you want to fill the frame with a kumquat that is 22 mm wide, you would need a magnification ratio of 1:1.
If you fill the frame with a close up of a cell phone that is 88 mm wide, the magnification ratio would be 22 mm/88 mm, or 1:4. The image of the cell phone on film is therefore 1/4 life-size.

Freely adapted from page 239 of Jon Fauer's "The ARRI 35 Book", of course with Jon Fauer's permission.