ISO: Friend or enemy?

There were several concepts written in a previous post: now, it is time to understand what do each of them mean, and how can we take advantage of it.
One of the most important questions is: what happens with ISO?
This question comes to our heads constantly: I bought a camera with capability of ISO values of 6400, 12800, 102.400...
Why should I use "low" ISO values? Usually we work with ISO values of 2000-3200 as a maximum, not much higher than that. Why?

The main reason to use "low" ISO values is mainly the noise, or as it was called in the analogic photography, the grain. The higher the ISO the higher the grain; and furthermore, you will lose contrast and detail, rendering your pictures flat and colorless (or pale-colored)

On the other hand, higher ISO values allow for shorter exposure times: it is even possible to take a handheld picture of a northern light with high ISO, but everything comes at a cost.

This issue is a decision of each photographer: how much grain are you willing to accept? Do you want longer exposure times and vivid colors, using low ISO, or do you prefer  shorter exposure times and pay the price of a lower contrast?

See the examples below, and judge yourselves. Click the pictures to enlarge and see the differences!
The pictures were shot one after each other, so the northern light was the same.

ISO 1000, 10"

ISO 1600, 5"

ISO 6400, 0.6"

It is quite obvious that the picture with ISO 1000 (left picture) looks best, but it required 10 seconds of exposure.
ISO 1600 (center), on the other hand, required only 5", but the colors are more pale and contrast is lost. It is also important to see that there is more noise (grain).
ISO 6400 required just 0.6", it is almost possible to shoot handheld. However, the colors are not so clear anymore, and noise becomes a big issue.

None of these pictures have been enhanced nor corrected, to show the differences.
The camera used was a Canon EOS 60D, with a Samyang 14mm (f=2.8) lens.

Do you have any questions? Need help with pictures? Just send me a message to my facebook page, my e-mail or leave a comment!