Understanding ISO Sensitivity

In our course on understanding the exposure triangle, we spoke of the importance of light in photography and about controlling how much light gets exposed to the digital sensor in your camera to produce an image. You learned how ISO, along with the aperture setting and the shutter speed control how much light enters the camera to make you image. These three pillars work together to control lightness or darkness of a photograph. Today we’re going to talk specifically about Understanding ISO Sensitivity.

What is ISO Sensitivity?

ISO measures a cameras sensitivity to light. Digital cameras convert light that hits the image sensor into electrical signals. Those signals are used to process the image. ISO sensitivity is increased by amplifying the signal. The ISO measurement was created by the International Organization for Standardization.

Film ASA

It’s important for photographers of all skill levels to have a solid grasp of ISO sensitivity. Before we jump into ISO in modern day digital photography, we’re going to take a look back at how ISO was used with film cameras. In the era prior to the invention of digital cameras, photographers bought film based on the ASA or film speed.

Film ASA at the time was a numeric measurement of film’s sensitivity to light and was represented in numbers ranging from approximately 100 to 800 (though slower and faster ASA were available). You also had the choice of using slide film or negative film in either color or black and white for prints.

Film was either slow, fast, or high speed. If you were going to shoot out in the daylight on a bright sunny day, you used a slow film speed of 100 ASA. There were 4 popular film speeds in the pre-digital age:

• ASA 100: Slow speed – Use for daylight, bright sun, beach, snow
• ASA 200: Slow speed – Best for overcast days and in the shade
• ASA 400: Fast speed – Good for sports photography and rainy days
• ASA 800: High speed – Used in low light, evenings, sunsets, inside with a flash

Film Grain

Another consideration was grain. Film consists of millions of light-sensitive silver halide crystals that we call grain. Slow films used a fine grain and faster films used a larger grains which have greater light sensitivity. So images shot with high speed film were known to appear grainy compared with pictures shot with slow speed film.

ISO Replaces ASA

ASA was eventually replaced with ISO, . ISO, like it’s ASA predecessor, measures a cameras sensitivity to light. The average ISO sensitivity range is approximately 100 to 1600. Some digital cameras have an ISO range of 100 to over 100,000 ISO.

ISO sets the amount of light needed for a good exposure. Just like with film, the lower the ISO number, the more light you need to get a properly exposed image. When you need more light, you can use a slower shutter speed. So you use a low ISO range of 100 to 200 when you have plenty of bright light (like sunlight) or if you are mounting your camera on a tripod. If you need a faster shutter speed for something like sports photography, you need to use a higher ISO (even if it’s a sunny day). And in low light, you need a higher ISO.

When you double your ISO setting (let’s say from 100 to 200) your camera needs only half as much light for the same exposure. If you had a shutter speed of 1/125 at 100 ISO, increasing your ISO sensitivity to 200 and changing your shutter speed to 1/250 will get the same exposure (provided you don’t change the aperture). Photographers will use a higher ISO setting for sports photography and at sporting events. If you need to stop the action of a tennis match, you probably need an ISO of 1600 or higher.

Noise

Using a higher ISO will also cause noise in your image. Noise, or digital noise, in a digital image is similar to noticeable grain in higher ASA films. Generally speaking, visible grain is usually undesirable and degrades the quality of an image. Most photographers strive to take images with the least amount of noise possible.

The size of the pixels in the cameras digital sensor is a factor in digital noise. A DSLR will produce better images than a point-and-shoot because the size of the pixels on a DSLR sensor are larger. Large pixels produce less noise than smaller pixels. Digital SLR cameras perform better at higher ISOs than compact cameras. Fortunately noise reduction processing built into digital cameras continues to improve allowing photographers to shoot at higher ISO’s with less noise than ever.