As I mentioned earlier, that ISO is one of the most tricky aspects in digital photography and it is analogous to the film speed in the old school cameras. But that definition doesn't help us much. Recently I was able to get my hands on a really good book called Understanding Exposure by Bryan Peterson. Its really good and highly recommended if you wish to gain insights in your equipment's operation. As Bryan mentions, we should consider the ISO number as the number of worker bees in the camera.. What worker bees? But trust me stay with me on this discussion it will make sense. We all know that if we have to shoot in low light situation, we crank up our ISO to 400 or 800 to have a correct exposure and in bright sunshine we can get the gig done with a much lower value of ISO...
Analogy: Lets consider the camera's aperture as a water tap from which water is flowing continuously and there are bees working on your camera sensor who are ready with their buckets to collect the light (water) and each bee has bucket with same capacity as any other bee. Now, no matter where or when you shoot, you need to have a correct exposure for shooting tach sharp images. So the total amount of light required for making a good image is constant. When you have more light (bright sunshine), you employ lower number of bees to fetch the light to your sensor thus a lower ISO, but in lower light situations if your shutter speed is high, for carrying the same amount of light you need more worker bees so a higher ISO number. So we crank up the ISO. Its all about getting the correct exposure.
Thus, in a low light situation if you are taking pictures using a longer exposure, the number of worker bees required is less, because they spend enough time on the sensor to obtain the required amont of light, where as in bright light their photo-buckets gets filled in a relatively less time, thus enabling higher shutter speed.
Now, a little experiment as suggested by Bryan. If you point your camera to a subject read the exposure on the camera metering. Say you read an ISO of 100 at an aperture of f/11. Crank up the ISO to 200, you will find that the aperture size required for making the correct exposure is f/22. Similar, trials can be conducted by setting the camera on shutter priority. If a shutter speed of 1/250 at ISO 100 gives you the correct exposure, a shutter speed of 1/500 with an ISO of 200 will give you the correct exposure..
Over exposed photos are examples of photos where the buckets of your worker bees are overfilled and they spilled the water (light) from their buckets. Similarly, under exposure refers when their buckets are not completely filled with light. Thus making the photo underexposed or darker because there was not enough light..
Things to remember:
1. Its all about getting a correct exposure.
2. Remember this formula for the photographic triangle
ISO X Aperture X Shutter Speed = Constant. (Rules of inverse proportion)
For a given ISO, you will have at-least 5 to 6 camera settings for the correct exposure. It depends what you want to do with your shot. Control your depth of field or freeze the motion or get some creative blur. They determine your shutter speed and accordingly aperture size is determined by the camera and vice versa depending whether your camera is on Shutter priority or Aperture priority.
I learned all these from The above mentioned book. Its really good and really cheap. worth a purchase. I hope I was able to make ISO clear to some extent. Any further questions let me know. Keep Clicking... :)
