ISO in ISO Speeds stands for International Standards Organisation and a few decades ago they came up with a standard for measuring the speed of film called the ISO Speed.

As with aperture and shutter speed, film speeds (and now sensor speeds in digital cameras) are governed by a factor of two rule. There have been some films that broke that rule along the way but, in general, those were new films that were pushing the boundaries in speed.

Film and sensor speeds speeds come in the following ratings:

25, 50, 100, 200, 400, 800, 1600 and 3200, 6400, 12800

The speed of a film or sensor is the measure of how fast it responds to light. A low ISO speed means a film or sensor responds slowly to light, a fast ISO speed means it responds quickly. Those silver squares you see on the film cases (if you still use a film-based camera) tell the camera what the film's ISO speed is (for cameras that can read them).

An ISO 100 film responds to light twice as quickly as an ISO 50 film but only half as quickly as an ISO 200 one. So it goes for each speed in the above sequence. In digital cameras, sensors don't go below ISO 100. Film, however, is available in ISO ratings of 50 or 25.

Why have different ISO speeds? Well, you use different speeds of film/sensor for different lighting conditions. In sunny climes, a slow film or sensor setting like ISO 50 or ISO 100 will allow you to take photos with a good depth of field while still being able to hand-hold the camera (i.e. not having to mount it on something like a tripod to keep it steady and reduce hand-shake and consequent blurring).

In low-light conditions such as late afternoon, early evening, etc., you would need to use a tripod for long exposures. In such conditions, or if you're in a far-northerly or southerly country where light levels are generally low, a faster film or sensor speed will be called for, typically ISO 400.

Really low light conditions, say for night-time photography or to freeze the action at a concert would require using the faster ratings such as ISO 1600 or ISO 3200.

All films suffer from grain. These are the parts of the film emulsion that react to light. The larger the grain, the faster it responds to light. So the faster the film, the bigger the grains it uses. For really fast films such as ISO 800 and above, this grain becomes apparent in the photographs you get.

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Digital camera sensors suffer from noise. It can be thought of as somewhat akin to grain on film. As you increase the ISO rating of your camera sensor, noise becomes more apparent. So if you want fine detail, stick to using low ISO speeds.

If you're after gritty realism, fast films and sensor settings can provide that type of atmosphere in your results, even when used in daytime conditions, but landscape photography is more about recording the fine details in a view and so slower ISO speeds are recommended.

But that doesn't mean the rules shouldn't be broken! The best advice is to experiment to find out what works for you.

If you use a digital SLR camera, then it will have an ISO setting as well. The range of options may be a little different to the speeds available for film. A typical range of digital ISO settings is:

100, 200, 400, 800, 1600

More expensive cameras may offer speeds up to ISO 12800.

Higher ISO speeds on digital camera introduce noise rather that grain into an image. The effect is similar in that noise breaks up the fine detail in images and produces a grainy appearance in the final photo.

On digital cameras, the ISO speed determines how fast the sensor responds to light. The ISO speed cannot be changed with film-based cameras but, on digital cameras, it can be set to any ISO value the camera provides. So you could take one exposure at ISO 100 in good daylight conditions, the next one might be at ISO 1600 to bring out detail at night and so on. Digital cameras are much more flexible with regard to the lighting conditions they can handle.

Tip: If you use a film-based camera and generally use slide film and your camera has a facility for Exposure Compensation, setting it to -1/3 of an f-stop will provide slides that are very slightly underexposed but have a greater color saturation which may be more pleasing to the eye.

Setting ISO Speed Videos:

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