When a digital photo is recorded in the camera, it is the result of light striking the image sensor. The camera settings determine how much light enters the camera to strike the sensor, the three main settings being aperture, shutter speed and sensitivity (ISO).
A photographer should know what the three major settings are and what will change if the settings are modified. In particular, sensitivity is a bit more difficult to understand, but a good understanding of the ISO settings on a digital camera will help the user make wise choices about how to select the correct ISO number in order to get the best possible picture.
ISO sensitivity of the image recording surface has always been a part of photography, even as far back as when photographic plates were used. Then, when film cameras came on the scene, sensitivity was measured in ASA. One could purchase film in several ASA sensitivity values. ASA is essentially the ISO of film, with the numbers being somewhat equal. "Normal" film had an ASA value of 50, 100, 200, or 400.
Carrying over to digital camera sensors, the sensitivity (ISO) settings are generally normal at about 100-400 ISO.
What actually happens when the ISO setting for the camera is changed, the image sensor is more or less sensitive to the light that is entering the camera. As the number goes higher, the image sensor becomes more sensitive, thus allowing for a faster shutter speed or a higher aperture, or both.
In terms that all can understand, raising the ISO setting by a factor of 2 (i.e. from 200 to 400) means the camera needs only half as much light to record the same image. So, if the required shutter speed for an image at ISO 200 is 1/100 sec, the shutter speed at 400 ISO could be boosted to 1/200 sec, allowing for better control of a hand-held camera or the ability to stop action without a blurred image.
This sounds like raising the sensitivity ISO sensitivity setting is the ideal way to get better photos, but there is a down side when it comes to high ISO settings. The more the ISO is raised, the more digital noise there is in the picture.
Simply put, high ISO photos have much more visible grain, and most photographers will do anything to avoid grainy or noisy pictures. Or course, there are plenty of photographers who specialize in grainy photos as a preferred style, but they do it on purpose rather than as a result of an ISO setting that is too high.
The most recent digital camera models have much improved ISO capabilities compared to their older siblings. This is true in both compact cameras and digital SLR cameras, but it is more obvious in the higher end models.
It must be mentioned that ISO quality is a function of two factors. The first, as mentioned is that the lower the number, the better the picture. And the second factor is related to the size of the image sensor. Image sensor size is not measured in megapixels for this comparison.
Rather, it is the actual physical size of the sensor. The sensor of a typical compact digital camera is about 1/10th the size of a digital SLR camera. You may be able to imagine that there can be much more technologically developed pixels if they are 10 times as big, which is the case for DSLRs.
Digital cameras of a few years ago struggled to get acceptable images at 800 ISO, but now technology has improved in the ISO department so that 800 ISO is now in the "normal" range for many of the new, high-end digital cameras.
As you look for a new camera, make sure you know what happens to the quality of the images as higher digital sensitivity ISO is used.