You will recall in previous episodes, we said there were two ways to control the light entering the camera: aperture and shutter speed. But there is a third component of exposure, and that is ISO.
Back in the days of film, ISO (or what was commonly referred to as ASA) was indication of how sensitive the film was to light. Numbers like 100, 200, 400 or 800 designated the sensitivity of the film, meaning how much light was needed.
The higher the number, the greater the film’s sensitivity (meaning able to shoot with less light) but with a tradeoff of more film grain. The lower the number, the less sensitive to light (meaning you needed more light in the scene) but also with finer film grain.
In digital photography, the ISO measures the sensitivity of the image sensor. The lower the number the less sensitive the sensor is to light (just like film) but less digital noise. The higher the ISO number and the sensor becomes more sensitive to light, but also with the tradeoff of more digital noise.
The standard ISO scale of full stop increments are 100, 200, 400, 800, 1600, and 3200. You will notice that moving up the ISO scale, the numbers are a doubling of the previous number.
Each full stop change either increases or decreases the sensitivity by a factor of 2. That means going from ISO 200 to 100 is a decrease in half of sensitivity to light, while going from ISO 200 to 400 is doubling the sensitivity to light.
In digital, the ISO scale reads in 1/3rd stop increments. Starting at ISO 100, the scale reads: 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600, and so on. Do you see how every third number is a doubling of the previous?
While the ISO sensitivity is standardized and the same from camera to camera, the actual performance of the sensor in regards to digital noise will vary from different camera types due to pixel density of sensor. Sensors with lower pixel density have greater capacity to produce images with less noise at a wider range of ISOs.
For example, a point-and-shoot camera that is 16MP with a thumbnail-sized sensor will have more noise than a 16MP DSLR because more pixels are crammed onto the smaller sensor on the point-and-shoot. The EXPOSURES will look the same but the images will differ in the area of noise.
When choosing an ISO, you need to take into account the light on the subject, the motion of the subjet, if you are using a tripod, and how much noise the sensor will produce at higher ISOs. We don’t recommend Auto ISE as you are not in control. Choose the ISO that works best for you shooting scene.