 BetterPhoto Member |
Film I was just wondering if you are able to tell me the technical differences between fast and slow b&W film. Cheers
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Maynard McKillen |
Dear Karen: Hmmmmm...technical differences between fast and slow black and white film...Here are just a few. Maybe some of them will turn out to be the answers you're looking for. SPEED- You've seen Fruit Roll-Ups, that snack (junk) food kids like to eat? Imagine a Fruit Roll-Up with sand in it. That's right, sand. Film is a bit like that. The Fruit Roll-Up is somewhat like the film base. It's job is to hold the particles of light-sensitive silver halides (the grains of sand). "Fast films" (say, films with an ISO of 320 or higher) have large particles (grains) of silver halide in the base, but also tend to have a wider range of grain sizes than "slower" film. The particles of silver halide in slower films tend to be smaller, there tend to be more per unit area, and these particles (grains) don't vary in size as much as the grains in a faster film. Fast films, with those larger grains of silver halide, catch more light than slow films, which have smaller grains. Fast films need smaller amounts of light to cause that physical/chemical reaction called exposure than do slower films. BUT, you may not always want to shoot with a fast (more light sensitive) film because of GRAIN- Most of the prints you make from a negative are larger than the negative, i.e., they're enlargements. As you make larger and larger prints from a negative, you begin to notice "grain," which makes the photo look as if it were made up of tiny dots that are black, white and shades of gray. Slower films tend to allow you to make larger prints (before grain gets too noticeable) than do faster films. The apparent grain in a print is also influenced by the kind of developer that was used to develop the negative, and how that developer was used (What temperature was it at, how long was it in contact with the film, how was the developer agitated (moved across) the film.). Graininess hides fine detail, so when you choose a Black and White film, think about how large you'd like to make your prints, how much light you will have available to take the photos, whether you must try to freeze fast moving subjects, and how the film will be developed. RESOLUTION- How well does a film record fine detail, and how well does it separate areas of the image that have almost the same, but not identical, density? Fast films tend to have less resolving power than slow films. Resolution is also affected by the film developer and the type of agitation the film receives during development. CONTRAST- Each film, by brand, by speed and by design, has a certain degree or level of contrast. This could be called the designed or inherent contrast. To generalize, slower films tend to have more contrast than faster films, BUT, film contrast is also influenced by the length of time it is developed, the temperature of the developer, the way the developer is agitated (moved across) the film, and by the type of developer used. COLOR SENSITIVITY- I know we're talking about Black and White film, but that film is translating the colors you see (and some colors you don't see) into tones of gray. And B&W film isn't necessarily democratic about that translation. Some early black and white films were called orthochromatic. These films were very sensitive to blue, but blind to red. Prints made from the film, especially outdoor scenes, often show the blue sky as virtually white. Because these films were blind to red, anything that color tended to record very dark on a print. You could also develop the film under red light. Newer panchromatic films are somewhat more democratic in their sensitivity to the spectrum of light. Photographers sometimes tinker with the way B&W film translates colors into tones of gray by placing red, orange, green or yellow filters in front of the lens. Ah, but that's another story. These comments merely scratch the surface of the topic. Other visitors to this site may need to amend the accuracy of the properties I've mentioned...
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John A. Lind |
Karen, "Fast" and "slow" are relative terms. Faster film responds faster to light and requires less exposure . . . shorter shutter speed and/or narrower aperture . . . than "slower" film under the same lighting conditions. Photographic film consists of minute grains of silver halide suspended in a gelatin coating called the emulsion that is layered on top of a polyester base. There are typically two other layers added, but they're to protect the film from scratches and prevent "halation" and are not relevant to what makes one film "fast" and another "slow." Faster film has larger grain and clumps of grain in the emulsion which allows it to respond to light faster. This is the downside of fast films, their graininess, and they don't have the resolution of slower film. It also makes the emulsion relatively thicker. Film is three dimensional and faster film with very slightly greater thickness has a secondary effect in also lowering resolution as oblique ray paths travel not only through the emulsion, but very slightly across it. Again, this is a much lesser, but measurable, secondary effect and is of more interest when extreme enlargements are made. -- John
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