ISO Numbers
Film speed is designated by an ISO number, which denotes a numbering system developed by the International Standards Organization. Because ISO numbers were developed to replace two older film speed rating systems — ASA and DIN — they're written as a combination of the older systems, such as ISO 50/18°. However, most people use only the first number when talking about film speeds.
Film speeds fall into three categories:
Slow: Rated at ISO 50 or less. Film in this category is not very light sensitive and is sharp and fine-grained.
Medium: Rated at ISO 64 to ISO 200. More light sensitive and still sharp and fine-grained, but not as fine and sharp as slow film.
High (or fast): Rated at ISO 400 to ISO 3200. Very light sensitive, not as sharp. Grain is larger and more visible, especially when enlargements are made.
The ISO numbers also indicate each film's light sensitivity when compared to other films. ISO 400 film, for example, is twice as fast as ISO 200. While it's not extremely important to know all of this in great detail, these facts do come into play if you aren't shooting in automatic mode. Film speed affects the two exposure controls on your camera: the shutter speed and the aperture, or f-stop. If you're shooting with slow shutter speeds in bright light situations with your lens wide open, you can use slow film. Fast shutter speeds, low light, and narrower apertures, which allow less light to enter the camera, require faster film for good exposures.
To give you an idea of how film speed will affect your picture taking, here are some examples of common conditions and shutter speeds (with reasonable f-stop settings for each).
TABLE 8-1 FILM SPEED CHART
For proper exposures, your camera needs to know what film speed you're using. Automatic cameras with built-in exposure meters (most cameras today) will electronically program film speed by reading the code on the film cassette. If you're using a manual camera, you'll need to set the film speed yourself.
The best way to choose film speed is to know the conditions in which you'll be shooting. In general, the more light you have to work with, the slower the film can be. If you're shooting outside on an average day, medium-speed film — ISO 200 — will probably be your best bet. If it's really sunny, you might want to switch to slow film, say ISO 100 or even ISO 64 if you're using a tripod. On overcast days, high-speed film — ISO 400 and up — will make the best of the light you do have.
Many beginning photographers are tempted to use the fastest film possible because faster film is more versatile. But increased light sensitivity comes at the cost of lower sharpness. In general, use the slowest film that allows a shutter speed you can work with.
There's always something of a tradeoff involved when choosing film speed. No one film will deliver everything you want in every shooting situation. Slow- and medium-speed films need longer exposure times, which limits their use when shooting action or using a telephoto lens. High-speed film is great for getting those action shots, as it allows for faster shutter speeds, but it's also grainier and less sharp. If you're not going to make prints larger than 4″ × 6″, grain isn't a factor; if you are going to make enlargements bigger than this, it is.
Unless you're shopping at a pro shop, you'll usually see film with ISO speeds of 100, 200, 400, and 800. In general, 100 is a good selection for shooting nonmoving subjects, brightly lit outdoor shots, and for tripod work; 200 works for daylight print film; and 400 and up is good for indoors, outdoors low light, and action situations.
There are times when even the fastest film isn't fast enough. You might be shooting with existing light that's too dim, using a small aperture for good depth of field in a low-light situation, or using a fast shutter speed with a small aperture. If this is the case, it's possible to make your film even faster by pushing it.
Will the X-ray machine at the airport affect my film?
For the little X-ray machine you put carry-on luggage through, probably not, unless you have to have it done repeatedly. However, checked baggage is subjected to a much more intense level of X-rays, which can be damaging. The safest approach is to ask to have your film hand inspected.
Film is pushed by setting your camera's ISO meter at a higher number than what's printed on the box, such as setting your camera to 800 for 400 speed film. You can also push film by setting your camera's exposure compensation dial to underexpose the film. Film that has been pushed needs special handling when it's processed. If you're going to try this technique, be sure you mark the film's cartridge with the speed it was pushed to. Take it to a lab that will process pushed film, and be sure to tell them what speed you used.
Film that has been pushed will result in pictures with greater contrast and more grain. Colors are altered as well. Such changes aren't necessarily bad; in fact, many photographers push their film so they can achieve stunning images with these effects.
Film can also be pulled, or shot at a slower speed than what it's rated for. Keep this in mind if you're using a manual camera and you've shot a roll of film at the wrong speed because you forgot to reset the ISO meter (which can happen far too easily). Be sure to advise the lab of what you have done when getting the roll processed.
