A developing tank is a light-tight container used for developing film

Photographic processing

Photographic processing is the chemical means by which photographic film and paper is treated after photographic exposure to produce a negative or positive image...

. A developing tank allows photographic film

Photographic film

Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...

 to be developed in a daylight environment. This is useful because most film is panchromatic

Panchromatic

Panchromatic film is a type of black-and-white photographic film that is sensitive to all wavelengths of visible light. A panchromatic film therefore produces a realistic reproduction of a scene as it appears to the human eye. Almost all modern photographic film is panchromatic, but some types are...

 and therefore can not be exposed to any light during processing. A developing tank can hold one roll of film, or it can hold many.

A film reel holds roll films in a spiral shape. The film is held evenly spaced so that the chemicals in the developing tank reach all of the film.

Types

Developing tanks and film reels for roll films come in two varieties: plastic and stainless steel. With stainless steel reels, the film is clipped to the center and then gently pinched while the reel is turned so that the film falls into the reel's grooves. With a plastic reel, the film is loaded from the outside and then wound onto the reel by rotating the reel with a back-and-forth motion.

Use

The user begins by opening the film canister (in the case of 35mm film) or separating the film from a paper backing (in the case of medium format film, e.g. 120/220 format). The film is then loaded onto a film reel in a completely dark environment; this can be a light-tight room or a changing bag

Changing bag

A changing bag is a photographic bag specifically designed to be light-proof while in use. It is required for certain applications involving photosensitive materials when a darkroom is not available, like in the field. Common usages include removing film from its canister to put it into a...

. Care must be taken during this step, as improperly loading the film may result in parts of the film not getting developed. Once the film is on the reel it is put into the developing tank, and then a lid is placed on the developing tank. Because the lid prevents light from reaching the film, the rest of the film developing process may be carried out in daylight. In addition to protecting the film from light, the lid contains an opening for rapidly pouring liquids into and out of the developing tank. Finally, a separate cap seals the canister, which prevents the contents of the tank from spilling when agitated by inverting the tank.