A Brief Introduction to ISO
You've probably heard of the term ISO before. But do you know what it means? If someone told you that you need to raise your ISO settings to compensate for the diminished light, would you know what they're talking about? If you don't know, here is a quick guide, straight from the EDU Advantage Team.
What does ISO stand for?
ISO is a user-defined setting that refers to the light sensitivity of the film emulsion in a film camera, or the sensor in a digital camera. International Organization for Standardization, or ISO, is a consortium that creates internationally accepted standards on everything from socially responsible business practices to scientific methods of determining the sensitivity of film emulsion. The ISO rating system for film combines various sensitivity rating systems of film manufacturers in different countries (ASA for the United States, DIN for Germany, etc...) into a unified and universally accepted scale.
What Do the Ratings Mean?
The higher the ISO film rating number—which ranges approximately from ISO 50 to 3200, and above—the greater the light sensitivity the film has. Film rated at a higher ISO allows a photographer to shoot hand-held or without a flash in low-light environments. High-rated films are considered 'fast-film'. Film manufacturers produce film that has the ability to be 'pushed' and 'pulled', or be under/over-developed, past or below its rated ISO.
ISO 100 (full size here)
ISO 200 (full size here)
ISO 400 (full size here)
ISO 800 (full size here)
ISO 1600 (full size here)
ISO 3200 (full size here)
Not only does light sensitivity rise with higher ISO ratings, but also the appearance of the silver halides that make film work—known as grain—become more prominent. Fast films such as Tri-X 400 or T-Max 3200 work well in low-light situations, but at the expense of being 'grainy'. So film photographers need the foresight of what their lighting situations will be, before loading their cameras. Often they will buy 20-roll 'bricks' of a preferred film type for the sake of economy and emulsion consistency, as emulsion sensitivity sometimes varies from one batch to another.
Film Travel and Storage
This means that photographers were often stocking their refrigerators (film does have a shelf life) and camera bags with a variety of fine-grain (ISO 50-200) and high-speed (ISO 400-3200) films. If you're traveling through airports and other security checkpoints, you also must take care not to let high-speed films go through x-ray machines, lest these films become 'fogged' or exposed due to radiation.
This also means that once the film is exposed, the photographer needs to choose how the film will be processed at the lab. It was common to take one roll to the photo lab, wait for it to be 'clip-tested', or have a short length of shot film taken from the front or the end of the roll, and processed. The results would be analyzed, and the remaining rolls would be processed accordingly.
So what does all this have to do with a digital camera's ISO?
This means that any decisions about what lighting situations will be encountered and what ISO speeds will be necessary will be made when we purchase our cameras. Digital cameras are pre-loaded with one sensor that has the ability of capturing images at a variety of ISO's. The light-sensitivity of these sensors varies depending on the camera manufacturer, model, sensor size, image processors and the on-board software.
Most cameras come with ISO 'settings'. Usually they are found within the Menu Options, or as an assigned button directly on the body. The user can manually adjust the ISO depending on a given lighting situation. Like film, raising or lowering a sensor's ISO will make it more or less sensitive to light. Unlike film, this can be changed, from frame to frame, to expand shutter speed control for stop-action or motion-blur effects in whatever light is available. Changing the ISO also gives more control over the depth of field—or f-stop—by permitting larger apertures (shallow depth of field) in bright light, or a smaller one (greater depth of field) in darker situations.
Unfortunately, as a digital device's ISO is raised, the amount of signal noise in the image will also increase. This means that an image taken at ISO 800 will show more small, colored pixels in the mid-tone and darker areas than an image captured at ISO 100. How much, again, depends on the model and make of the camera.
While there are ISO sensitivity rating standards for digital cameras, the results may differ from camera to camera. Also, as technology improves and camera sensors become more light sensitive, they are capable of capturing images at ISO 12,000 and higher. Some cameras will produce very little signal noise at ISO 1600 while others will receive noise complaints when used above ISO 400. Some models—when set to their full automatic, or 'Program', mode—are adept at setting the correct ISO when the lighting environment changes. Other cameras are programmed to use the highest ISO available when low light is detected, producing a very noisy digital image.
Ultimately, consumers must decide how much 'camera' they will need. Will most of your images wind up on the internet, and grain is not an issue? Will you be printing most of the images you take, and therefore require a camera that can capture images with as little noise as possible in most lighting situations? Is navigating a camera's menu options and experimenting with its various ISO settings enjoyable? Or do you prefer your camera to work automatically right out of the box, so that you can spend more time enjoying the scenery than reading a manual?