Why change ISO Settings.

"Whats with the ISO stuff on all the cameras?" This was a casual question directed to nobody. Rafeeq, Amol and I were toying around with a Canon A95 when this question came up.
"Well" I replied "Its something to do with light sensitivity. The higher the ISO, the more sensitive the camera becomes to light."
"Then why bother to keep ISO 50, 100, 200 on the camera instead of simply ISO 200."We are a bunch of super smart kids. We can come up with smart questions all the time.
"I guess it has something to do with noise. Higher ISO introduces a lot more niose." Amol replied. We are a bunch of smart kids who can come up with smart answers to smart questions.
"If higher ISO signifies higher sensitivity to light, then naturally you must have lower noise." I already told you that we were were a bunch of smart kids.This final question created a rare silence among us. A silence introduced by deep introspection.

So what really is this ISO.
ISO, which stands for International Standards Organisation (formerly expressed as ASA: ) is basically a numbering system that expresses the speed of photographic negative materials. Now that the techno geeks are satisfied with the definition, lets put it in another way. ISO basically tells you how sensitive your film(or sensor, if you are using a digital camera) is, to light. A higher ISO signifies a higher sensitivity to light whereas a lower ISO signifies a low light sensitivity.
More Explanation: A high ISO like ISO 3200 makes sure that you get a clear photo in low light conditions, whereas a low ISO like ISO 100 requires you to have a LOT more light for a sharp and clear photo. In fact, ISO 3200 films are used for photography in near darkness.

Why give so many ISO options
ISO affects exposure. The two basic things that control exposure are
1. Shutter speed
2. Aperture size.

The ISO settings affect this shutter speed/aperature combinations so as to decide the exposure.
More Explanation: You are about to take a very nice photograph, but your camera says that there is no enough light for a decent exposure. To make things worse, lets say you don't have a flash or a tripod to help you. This is where ISO comes handy. Just increase the ISO settings, and you are good to go.
Now if you are wondering as to why that happened, allow me to explain:
Doubling your ISO settings will double the sensitivity of the film (or sensor, in case of a digital camera) to light.
Switch to ISO 200 and you will only need half the light at ISO 100 for a clear picture. ISO 400 will require only a quarter of light that ISO 100 requires.
Pretty cool huh.


So why use low ISO anyways?
Just one answer: NOISE !!!! Whether its a film camera, or a digital camera, higher ISO settings will introduce a lot more noise in the picture.
In high speed films, there are more layers of silver halide. This tends to make the light sensitive grains of silver halide more noticable. Now you know why some pictures look grainy. In case of a digital camera, this effect is in terms of random pixels of color.

However, at times, you can use the noise introduced by higher ISO to give a mood or atmosphere to your images; especially black and whites.

When to change ISO
1.Clear hand held photos.
When you dont have a flash, or a tripod handy, simply increase the ISO for to get a nice and clean picture. Higher ISO settings ensure higher shutter speeds thus minimizing the risk of camera shake.

2.Adding to mood.
At times "graininess" can contribute to adding an atmosphere or quality to black and white images. But you can also do that while post processing.

3.Motion Shots
When you are taking shots that require fast shutter speeds, like say sports, boosting ISO can really help. Most of the time, there may not be enought ambient light to take pictures with fast shutter speeds. So increasing the ISO will be the only option left.

To conclude, always remember:
You obtain the best image quality by using the lowest ISO possible. So if you have enough light to shoot something at ISO 50, then do it by all means. Increase the ISO only when neccessary.

Those who would like to know more about this can go here.