How Film Responds to Light

Recording an image on film involves a reaction between light and SILVER HALIDE crystals. The crystals are suspended in the gelatin that is the emulsion In the crystals, electrically charged silver ions move around when light strikes them. The ions combine to create specks of microscopic silver metal. Millions of these specks record the light that will eventually become a photograph. Chemical developer uses the specks to build density. The denser the buildup, the darker the image.

A black-and-white negative is formed when millions of exposed crystals are converted to silver metal by the developer. The parts of the film struck by the most light are darkened by metallic silver (which, in crystal form, is black). The parts struck by no light remain transparent after processing because they contain no silver. Intermediate exposure produces varying amounts of silver, creating shades of gray.

Structure of Black-and-white Film

Chromogenic Film Chromogenic film has a combinaton of silver halide crystals and dyes. The silver ions react to light, and the dyes react to the silver to enhance density. Most color films today are chromogenic, and are developed in Kodak’s C41 chemistry. Some chromogenic films use only one dye and produce black-and-white results when processed in C41 chemistry. Among these films are Ilford’s XP2 Super, Kodak’s T-max 400CN and Kodak’s Professional Portra 400BW.

Different types of black-and-white films react differently to colors in a scene. They record colors as various shades of gray. The silver halide crystals respond primarily to ultraviolet, violet, and blue wavelengths. Dyes added to the film can increase the film’s range of sensitivity.

Types of Black-and-white Film PANCHROMATIC is the most commonly used BW film. Dye is added so the film is sensitive to some ultra-violet radiation and all visible colors of light. They are slightly more sensitive to bluish colors than the human eye, which can cause a blue sky to seem too light in a photo.

Types of Black-and-white Film ORTHOCHROMATIC is sensitive to ultra-violet, blue and green. It is used primarily for copy and graphic arts work.

Types of Black-and-white Film INFRARED Records invisible infrared wavelengths in addition to all the visible colors. The results can be unusual and beautiful.

Push Processing Push processing enables you to increase the exposure of some films from one-half to 3 stops more light than the ISO rating suggests and increase the developing time to compensate. For instance, ISO 400 film could be pushed to ISO 800 or ISO Increase the processing time 2 minutes for each stop pushed. Pushing film increases the contrast and grain, and is usually used only when you need the extra speed.

Infrared Photos Infrared film is sensitive to visible light AND to invisible infrared wavelengths. Objects that appear dark to the eye may reflect a great deal of infrared radiation and appear light in infrared photos.

Infrared Photos Leaves, grass and skin will be very light because they reflect infrared strongly. Water particles in clouds reflect infrared, making clouds very light. Blue sky becomes very dark, especially when using a deep red filter on the lens that block most visible wavelengths and emphasizing infrared wavelengths.

Infrared can reduce haze in landscapes, enhance lava flow, track water pollution, decipher charred documents, detect forgeries, and much more. Longer exposures will create a dreamlike “bloom” or glow in an image.

Infrared Film SPECIAL CONSIDERATIONS Refrigerate until used. Very sensitive to heat. Load in total darkness. Focus, then back off the focus. A red filter on the lens enhances the effect. Bracket. Infrared wavelengths are not accurately measured by ordinary light meters. Add a filter to your flash unit or buy an infrared-only flash unit. Process your own film. Most commercial labs aren’t set up to prevent infrared exposure in processing.

Read pages – Selecting and Using Film, and – Special-Purpose Films