Oskar Barnack (November 1, 1879 – January 16, 1936) was a German optical engineer, precision mechanic, industrial designer and the father of 35mmphotography.

In 1911, he was in charge of microscope research for Ernst Leitz at Wetzlar. He was an enthusiastic photographer, but the heavy equipment of the day was difficult for him to handle due to his poor health. In 1912, he constructed a 35mm movie camera.

Between 1913 and 1914 he was head of development of the camera company Leitz in Wetzlar, Hesse, Germany. He was the driving force behind the making of the first mass-marketed 35mm camera. Barnack suffered from asthma, and sought to reduce the size and weight of cameras and supporting equipment used for outdoor photography. His 35mm design helped introduce the concept of exposing a small area of film to create a negative, then enlarging the image in a darkroom.

The onset of World War I kept the first Leica from being manufactured until 1924, and it was not introduced to the public until 1925, when Leica's chief, the optician Ernst Leitz, took a gamble and authorized the production of 1,000 cameras.

Leica stood for Leitz Camera. Instead of the exposure plates used in past Leitz cameras, the Leica used a standardized film strip, adapted from 35mm Eastman Kodak roll-film. Barnack decided that the 18 x 24 mm (3:4) standard movie frame was not large enough for good still photo quality with the films of the day and doubled the frame size to 24 x 36 mm (2:3), with the image horizontal instead of vertical.

Lynow, Oskar Barnack's birthplace, and currently a municipality of Brandenburg, Germany, has a museum to Oskar Barnack.