The generation of today knows ISO as a button...the older generation remembers how difficult it was to constantly change the camera spool whether you were inside our outside.
Basically now it is a setting controlled on your camera, that allows you to adjust the sensitivity of your cameras sensor, this sensitivity is the sensitivity to light.
In the old days, you could purchase spools with different ISO’s. Now we do it digitally...spoilt right!
ISO is shown as a number on your camera settings.
Eg: 100, 200, 600, 800 etc.
The value to which you can set the ISO number is determined by your camera.
Eg. Some cameras can go to higher ISO’s than others.
A high ISO is 6400 (this would give you a noisey/grainy image, the effect your cell phone gives when you take a photo at night)
A low ISO is 100 (this would give you a clean crisp image)
So what does ISO do INSIDE your camera?
The lower your ISO, the less sensitive your sensor will be to light and therefore the less grainy your image.
Eg: ISO 100
The higher your ISO, the more sensitive your sensor will be to light, and therefore the more grainy your image will be.
Eg: ISO 1600 or 3200 etc.
If you are taking photos outside and there is sufficient light, keep your ISO as low as possible, ie. ISO 100 or 200 to avoid grain on your images...your shutter speed and aperture should be adjusted accordingly to give you a perfect exposure.
And if you are taking photos inside or outside and its become dark, lift your ISO accordingly (keep in mind your shutter and aperture). A good rule of thumb is to never go above ISO 800.
Even a modern day DSLR like a D810 or 5Dmk3 still gets a little grainy above ISO 800, and if you fussy like me, rather add a little flash to keep the image crisp.
Yes...you can remove Noise in post production, Lightroom or Photoshop...BUT...yes there is always a but, it will never be the same. The software smooths the noise out, making the image a little more 'blurry' kinda like rubbing all the grain with your finger to make it smoother. Its visually better but its not as sharp.
So here is an example, and yes the 2nd image IS taken with flash, but the explanation and results would be the same.
This was taken with natural light as the sun had just dipped over the hill.
ISO 400 shot at f4 - now although this is clear and pretty sharp.
This was taken with off camera flash.
ISO 125 shot at f4 - if you look closely you can see that the clarity of the image looks less disturbed like the eye in the above picture.
Now imagine the difference between ISO 100 and ISO 3400.