To fully appreciate Hosking's black and white bird photographs taken during the 1930s and 1940s, it is useful to understand how difficult it was to take them. Every step of the picture taking process was totally manual, and success relied on in-depth knowledge, experience and calculation. Without through-the-lens viewing (which came later with reflex cameras), the camera was pre-focussed with a distance scale to where the subject was expected to be. The exposure was manually calculated, working out the best F-stop and shutter speed combination, and hoping that the light intensity did not change before the picture was taken. The light sensitivity of the film was very low, about ISO 10. (Later, ISO 400 - and above - film became readily available, allowing much faster shutter speeds). The emulsion was often on a fragile glass base. Only one picture could be taken before reloading with a fresh, unexposed plate. Film holders could only be loaded in complete darkness, necessitating a light-proof changing-bag. It was quite common to have only 12 exposures for a day's photography. The glass negatives had to be developed in the darkroom at a later date and then printed as positive prints.