Photographic film: Encyclopedia II - Photographic film - Film basics

Photographic film - Film basics

There are two primary types of photographic film:

• Print film, when developed, turns into a negative with the colors (or black and white values, in black and white film) inverted. This type of film must be "printed" — either projected through a lens or placed in contact — to photographic paper in order to be viewed as intended. Print films are available in both black & white and color.

• Color reversal film after development is called a transparency and can be viewed directly using a loupe or projector. Reversal film mounted with plastic or cardboard for projection is often called a slide. It is also often marketed as "slide" film. This type of film is often used to produce digital scans or color separations for mass-market printing. Photographic prints can be produced from reversal film, but the process is expensive and not as simple as that for print film. Black and white reversal film exists, but is uncommon — one of the reasons reversal films are popular among professional photographers is the fact that they are generally superior to print films with regards to color reproduction. (Conventional black and white negative stock can be reversal- processed, to give 'black & white slides', and kits are available to enable this to be done by home-processors - however, the gamma required for an effective slide is high, and more easily achieved with a slower film like Pan-F).

In order to produce a usable image, the film needs to be exposed properly. The range of tones that a given film can accurately record is called its exposure latitude. Color print film generally has better exposure latitude than other types of film. Additionally, because color print film must be printed to be viewed, some after-the-fact correction of the exposure can be made during the printing process.

The concentration of dyes or silver salts remaining on the film after development is referred to as density. A dark image on the negative is of higher "density" than a more transparent image. If part of the image exceeds the maximum density possible for a print film, then it is overexposed and will appear as featureless white on the print. Likewise, if part of an image is beneath the minimum density possible on a film, the area will appear as featureless black. Some photographers use their knowledge of these limits to determine the optimum exposure for a photograph; for one example, see the Zone system. Most automatic cameras instead try to achieve a particular average density.

Film speed describes a film's overall sensitivity to light. The international standard for rating film speed is the ISO scale (also known as ASA, since it it was initially developed by the American Standards Association). Common film speeds include ISO 25, ISO 50, ISO 64, ISO 100, ISO 160, IS0 200, ISO 400, ISO 800, ISO 1600, and ISO 3200. Consumer print films are usually in the ISO 100 to ISO 800 range. Some films, like Kodak's Technical Pan, are not ISO rated and therefore careful examination of the film's properties must be made by the photographer before exposure and development.

ISO 25 film is very "slow", so it requires much more exposure to produce a usable image than ISO 800 film. Films of ISO 800 and greater (referred to as "fast" films) are thus better suited to low-light situations and action shots. The benefit of slower films is that it usually has finer grain and better colour rendition than fast film. Professional photographers usually seek these qualities, and therefore require a tripod to stabilize the camera for a longer exposure. Grain size refers to the size of the silver crystals in the emulsion. The smaller the crystals, the finer the detail in the photo.

A film with a particular ISO rating can be pushed to behave like a film with a higher ISO — that is, exposed for a shorter period of time than would normally be used. In order to do this, the film must be developed for a longer amount of time than usual. This procedure is usually only performed by photographers who do their own development or professional-level photofinishers. More rarely, a film can be pulled to behave like a "slower" film.

Adapted from the Wikipedia article "Film basics", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki