Your ISO setting can be used to manipulate your camera setting for a variety of direct or indirect effects. ISO is simple an indication of light sensitivity. The numbers used are usually something like the following: 25, 50, 100, 200, 400, 800, [1200], 1600,3200, 6400 . . . . Notice that each number generally doubles over the previous number; the exception of 1200 is 1.5 x the previous number. This exception is included because of a common usage for what we called push-processing film in the past. In this case the number means that the film or charge-coubled device electronics have increased by 1.5 times instead of 2x.
Conversly, a preceding number represents a sensitivity of one-half of the next number. So it easily follows that the higher the number, the greater the sensitivity. So why would you not want to use the highest setting for sensitivity. For several reasons, but the main one is that quality suffers as ight sensitivity increases. Although with a higher ISO number comes a greater ability to take photographs in lower light settings without motion blur, the film grain or digital static or noise also increases, reducing image quality. For mostuses, this is not desirable. Sometimes the swap-off is negligible or the quality is "good enough". Sometimes you take what you can get--SOME image being better than NO image.
There are other uses for changing your ISO setting to a higher or lower number. Often these reasons are viewed as an unintended side-effect the photographer has to deal with or are simply not even realized. But by understanding these uses in advance, these side-effects can be used as the primary purpose for changing the setting. Here are a few examples:
1) To blur the background of a portrait or other scene where an otherwise busy background would detract from the primary subject or point of interest. In order to accomplish the blurring of the background, the ISO setting is lowered to the lowest setting available and still maintain an image clear and free of motion-blur.
2) To increase the clarity of a background or to increase the zone or distance wherein the clear focus is visually accepable withut detracting from a close subject over a distant subject--the ISO setting is INCREASED. A case for this might be a picture of your house with your family posed in front of it. You want both to be in focus,so you increase the ISO setting, but no more than necesary so that your image quality does not suffer with undesirable noise.
3) To increase the closest distance that your camera will focus on a subject at a given zoom setting or macros wetting, increase the ISO setting.
4) Another use of an increased ISO setting is to make the background objects smaller relative to the subjects closer to the camera. Thismight be used creatively to distort an image to comical or dramatic proportions for emphasis, such as making a a persons body much smaller in relation to their extended fist--or their nose disproportionately larger than the rest of their face.
Note: This effect increases varies with the distance that the camera is to the subject, or with the zoom setting.
5) You can make distant objects appear much larger proportionately to closer objects than they normally appear with our eyes or at a normal lens zoom setting by decreasing the ISO setting. Again this effect is sometimes seen as an unintended effect, but it is also often used in movies and video production. One common scene comes to mind where an orange setting-sun is made larger than life with an elephant gazelle against the over-sized sun. Or a looming tornado appears much more ominous against the outline of an automobile, line of houses, or fleeing people.
6) A higher ISO setting stops action or decreases blur from an unsteady hand or a moving automobile or from a racing subject.
7) Intentional blug of a running animal or racing car can be exagerated creatively in order to give a greater impression of speed. It can also be used in much the same way to blur the background while stopping a moving object while matching your camera movement with the movement of the object. This is called "panning". Panning adequately freezes the moving object while allowing the backround and/or forground to blur--creating an impression of extreme speed.
8) As alluded to previously, an old-time news photo such as those seen in Life Magaizine or other examples of the great news magazines of the past century by increasing the ISO setting. This increases the noise with a digital camera or film "grain" in the case of film. Although this effect may look artsy today, it was simply the best tha could be delivered with small film format cameras used by those great vintage news photographers. They made lemonade out of lemons by popularizing these "artistic" aspects of their unwanted grainy images. Clever.
9) Increase the light gather ability of your camera lens by increasing the ISO, enabling you to shoot night scenes or creatively over-expose a scene. An example of this is to double your ISO setting when shooting a subject against a brighter window or open sky. You can get a sillouette of virtually any subject even against a slightly lighter backround by increaisng the ISO setting.
10) Decrease your ISO setting in front ofr a window or light background.. This will help keep you from getting an unintended silloutte image of your intended subject.
11) Extend the range of your camera flash at night by increasing the camera ISO (without changing the flash ISO settin).
12) Blacken the background of a night scene by decreasing the ISO setting (without changing the flash setting if offered independently).
There are other effects that can be manipulated by changing the ISO settings on your camera. There are also other ways to achieve these changes by changing your manual camera setting or by selecting presets represented by various icons on cmera setting dials. the biggest benefit of using ISO setting for these purposes is that it is ve3ry quick and not hard for a budding photographer or artist who does not want to be bothered by learning all of the settings and icons inside and out.
These effects are true more-or-less, depending upon different cameras used. They are dependent upon "tricking" the camera into doing what you want it to. Therefore, you should not try to over-think these effects or apply too much logic while using them or you may wind up "correcting" these "desired corrections". Your camera may also try to correct your creativ attempts.
But by changing the ISO settings, what you get is pretty much what you get. Before depending upon the net results of your attempts, you should run your camera through its paces to get a feel what it will do specifically under the aforementioned and other circumstances. Results will vary.
I will drop in a few illustrationsof these techniques later hopefully, but I am rushing this out in order to address a specific readers request. So check back in a few days.