Film in General
This discussion will center on black-and-white, negative films. Most of the information is applicable to other films, with some variations.
Physical Characteristics
1. Format
Format is simply the film size. Film ranges from miniature (110, Minox 8mm), to small (35mm), medium (6 x 6cm to 6 x 9cm), and large (4 x 5in to 16 x 20in and larger). Film formats are synonymous with camera formats.
2. Structure
Film is constructed in layers. These layers consist of, from top to bottom, an anti-scratch layer, the gelatin and silver layer, an adhesive layer, the film base, another adhesive layer, then an anti-halation layer:
In detail, the anti-scratch layer is just that, a coating on the image side of the film to help protect against scratches.
The gelatin/silver layer is of particular importance, as this is where the image happens. Physically, the tiny particles of silver are suspended in gelatin, which is coated on the base. The gelatin is just that, a natural emulsion. It is the characteristics of this gelatin that it can be dried and form an flexible, resistent layer. When soaked in water, the gelatin becomes permeable, like a sponge, so that chemistry can enter and react with the silver. It can then be dried and will close up again.
The silver at this point is silver bromide. When light hits it, it forms a latent image, which is made visible and intensified during the development process.
The film base is polyester, which has replaced glass and celluloid. Polyester is flexible but very dimensionally stable (menaing it doesn't expand or contract much with moisture and temperature changes).
The anti-halation layer is a light-absorbing coating on the back of the film which prevents halos from forming in the image by absorbing light which may otherwise bounce back up into the silver layer.
The adhesive layers simply help the other stuff stick to the polyester.
3. Packaging
Film comes in two basic forms, roll and sheet. Roll films are available up to medium format sizes, and sheets generally from 4 x 5in up.
Professional outlets offer film in bulk packs (5-10 rolls or more) at discount, guaranteeing that it is all from the smae maunfacturing batch to insure consistency. Also available is bulk rolls, 50 or 100 meters of film which can be loaded on cassettes by hand. While very inexpensive, bulk filmis subject to scratches and other handling errors (not the least of which is reusing cassettes too much— they’ll leak when they get old).
A small note. When buying roll film, get 24 exposure rolls rather than 36, if they are available. Those extra 12 exposures seem to take forever to use up, and you can only get 7 strips of 5 in standard binder-sized negative sleeves.
Photographic Characteristics
1. ASA rating/speed
This is a rating, by the manufacturer, of the particular film's relative sensitivity to light. Another term for this is the film's speed. Films are rated as slow (ASA 25-75), medium (around ASA 100-125), and fast (ASA above 125). So, the higher the ASA number the faster (and more sensitive to light) the film. When you set the ASA number on your camera, usually by adjusting a dial on the top left of the body (or letting the camera automatically set it for you), you are telling the meter how fast your film is. So the first step to good exposures is setting the ASA on your camera correctly.
2. Contrast
In general, the slower the speed of the film, the more inherently contrasty it is. Thus, a 100 ASA film is more contrasty than a 400 ASA film. This characteristic can be controlled/manipulated by developer, developer/time combinations and agitation during development.
3. Grain
Grain refers to the ability to see individual bits of silver. A grainy image simply lacks crispness. Excessive grain looks fuzzy. As for film, the slower, the less inherent grain. Grain is quite important with 35mm photography, as making even an 8 x 10 inch print requires quite a bit of enlargement—some 50x—whereas an 8 x 10 from a 4 x 5 inch negative only needs 4x. Grain can be manipulated in development like contrast.
4. Exposure Latitude
Exposure latitude is a film’s ability to be under- and over-exposed, and still produce a printable image. In general, slower films have less latitude than faster films, black-and-white films more than color, and transparency films (slide film) very little at all.
5. Acutance
Acutance is a measurement of a film’s ability to record edge sharpness, particularly between differing tonal areas. Slow films do this better than fast films, producing more appprent sharpness in the image. Acutance is actually measured by photographing a knife edge.
6. Resolution
Resolution is the ability of a film to resolve detail. Related to acutance, resolution is measured by photgraphing tightly-spaced horizontal lines. High resolution film can ‘see’ several hundred lines per millimeter, low fewer than fifty. Grain structure, exposure and development all play a role in resolution.
7. Characteristic Curve
A characteristic curve is a graph of density vs. log exposure for a particular film/developer combination. What this means in English is that a characteristic curve gives you an idea of how a film reacts to a given exposure and a given developer.
The characteristic curve consists of a toe—film base + fog desity, a straight-line portion, and a shoulder—where density reaches D-Max. It can be looked at as a cross-section of the film, the least exposed is the thinnest, and most the thickest. The angle of the curve is an indication of contrast, with a steeper curve meaning more contrast.
