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Film speed

Film speed is a measure of light sensitivity


A film’s speed is a measurement of its sensitivity to light and is expressed by a number that appears after the letters "ISO" - e.g. ISO 25, ISO 100, ISO 200, ISO 400 and so on. This ISO number is shown on both the package and the film canister itself.

In digital photography, "sensitivity" is the equivalent to film speed. It is also sometimes referred to as "ISO equivalency."

The higher the number, the “faster” or more sensitive to light is the film or your digital camera's sensor, meaning you need less light to get a properly-exposed image. Faster film requires either a faster shutter speed or a smaller ƒ-stop than slower film to properly expose a scene. (There are actually two numbers in a film's speed using today's ISO rating system. See our sub-section entitled Film speed rating for more information on numbering for various film speeds.)


CHOOSING FILM'S SPEED

Your choice of film speed will depend on the lighting conditions in which you will be taking pictures, and whether it is important for your images to stop action (fast shutter speed) or show blur (slow shutter speed), and also will depend on the amount of depth of field you wish to have in your image. As a general rule, use a fast film (ISO 400 or higher) when shooting indoors without flash or on a darkly overcast day. Use slower speed film (ISO 100 or less) on brightly overcast or sunny days. A more detailed look at the characteristics and performance of different speeds of film is provided under the heading Film speed characteristics.

Generally, you will use slower speed film (ISO 100 or less) on bright, sunny days when there is ample light for good depth of field or for stopping action with a fast shutter speed.
Generally, you will use slower speed film (ISO 100 or less) on bright, sunny days when there is ample light for good depth of field or for stopping action with a fast shutter speed.

In darkly overcast shooting conditions, a fast film speed is usually essential since there is so little light compared to the light of a bright sunny day.
In darkly overcast shooting conditions, a fast film speed is usually essential since there is so little light compared to the light of a bright sunny day.

ADVANTAGES OF FASTER FILM

There are several advantages to using a faster film. The fundamental effect is that photographers are given a wider choice of apertures and shutter speeds. For example, ISO 400 film needs half as much light as ISO 200 film, so you can set the shutter one speed faster or shut the aperture down one stop when you switch from film with a speed of ISO 200 to ISO 400 film. Since a film rated at ISO 200 needs half as much light to form the same image density as one rated at ISO 100, then a film rated at ISO 400 needs one-quarter as much light as a film rated at ISO 100 (two times as fast a shutter speed or two stops less.)


Other advantages? There are plenty to choose from. A faster shutter speed can be used to “freeze” motion in action shots, reducing blur. Scenes with reduced lighting can often be captured with a hand-held camera without the need for a tripod. Increased depth of field can be achieved. Super-fast film’s graininess can add mood to a picture; and exposure latitude is generally increased.

A fast film speed permits you to use a fast shutter speed (even on cloud-shrouded, overcast days), which can be used to sharply freeze motion such as this storm-driven wave, reducing blur.
A fast film speed permits you to use a fast shutter speed (even on cloud-shrouded, overcast days), which can be used to sharply freeze motion such as this storm-driven wave, reducing blur.

DISADVANTAGES

There is a price to pay for faster film speed. Among the disadvantages of higher-speed film are tonal gradation decreases (which is another way of saying that colors are often weaker), and images appear more grainy.

Fast film creates the image using larger grains of silver than slower films. Clumps of these grains form the image, and their larger size can sometimes be seen in a print. Such a print is said to have poorer definition. Many of today’s film manufacturers have improved their fast films so that these characteristics are less evident. However, there is no question that the sharpest-looking, most color-saturated, smooth-toned images are those taken on slower-speed films. The quality of slower film, in terms of sharpness and tone, is better than that of higher-speed film, so the best film to use is the slowest-speed film that will still give adequate shutter speed and aperture settings for the task.

EFFECTIVE FILM SPEED

Film speed numbers - which are not to be taken as perfectly accurate in every case - are the film maker’s recommended film speed settings, and should be treated as a starting point for modification by a photographer. For instance, some photographers may like the effect of purposefully overexposing or underexposing a certain film, and may always shoot that particular film at a different film speed setting than the manufacturer’s recommended ISO number. For that photographer, the film’s “effective speed” is different from its maker’s rated speed. Determining a film's effective speed for your own tastes is a matter of experimentation. If you always set your light meter for the manufacturer's film speed rating as shown on the package or film canister (some cameras do this for you automatically if your film is DX-encoded), you should always have acceptably-exposed pictures.

LIGHT METERS MUST BE SET FOR DIFFERENT FILM SPEEDS

To ensure proper exposure, your light meter must be set for the speed of the film you are using. Many automatic cameras do this for you when the film’s canister is marked with DX-encoding. If you have such a camera (and you probably do if you acquired it in the past several years), it has electrical contacts in the film chamber that not only read the film’s speed and automatically set it for you, but also determine the number of exposures on the roll and the film’s exposure latitude. If you are unsure whether your camera reads DX-encoded film, check your camera’s manual or have knowledgeable personnel at a camera shop look at it for you.

With older cameras that do not have the ability to read DX-encoding (or with film that may not be DX-encoded, such as bulk film you load yourself into non-marked canisters), you will have to set film speed manually. For ISO 100 speed film, set your light meter to 100.

Some cameras may show film speed settings in DIN numbers, another film speed numbering system. For example, the DIN number equivalent to ISO 100 film is 21. See our section on “Film speed rating” under the paragraph heading Equivalencies for ISO/DIN number comparisons.

MOST COMMON SPEEDS OF FILM

ISO 25, 50, 64, 100, 160, 200, 400, 800, 1000, 1600, 3200 and 6400 are the most common film speeds, meaning that most good photo retailers, particularly those used by pros, will stock them in fresh supply, with the possible exception of the less-frequently-used ISO 6400 film, which may have to be ordered in.


Further information...

Film speed rating

Pushing/pulling film
Related topics...

Film speed characteristics