Today we'll be discussing the second of three variables, ISO.
ISO goes back to the film days when different films had different ASA/ISO speeds. You could buy a film with an ASA/ISO rating of 16, 50, 100, 400, 3200, etc. You could "Push" the speed of your film while sacrificing image quality. ISO back then referred to the sensitivity of the film to light and in today's digital world refers to the sensitivity of the image sensor to light.
A higher ISO speed/setting means more sensitive to light (brighter image, all other variables held constant) and a lower ISO implies less sensitivity to light (darker image, all other variables held constant)
You can test this out my setting your camera to Manual mode and setting any aperture/shutter speed. As you increase your ISO you'll notice that your exposure becomes brighter. Of course, as with the other two variables (aperture and shutter speed) ISO has both its pros and cons.
ISO is the first variable that I change whenever I enter my shooting environment. Using ISO 800-3200 when I'm indoors with artificial light and ISO 100-400 when I'm outdoors with plenty of sunlight. I think it's pretty self explanatory why, but if you haven't caught on… Use a lower ISO when there's plenty of light, because you'll be able to maintain a good aperture/shutter speed at pretty much any ISO and a higher ISO when you have poor lighting conditions, because you want your sensor to be more sensitive to light in order to obtain a better aperture and shutter speed.
Pros of ISO
As mentioned earlier, ISO is the first variable that I set. A higher ISO allows me to increase my shutter speed or aperture than a lower ISO would allow. Think about darker environments when you need a 1" shutter speed to get a good exposure at ISO 100. Definitely not hand holdable and if you're caught without a tripod, good luck with the shot! But by increasing ISO to 1600 or so you can maybe squeeze off a 1/15 shutter speed that is handholdable with good technique.
I know you're thinking now, so why can't I just use any ISO that I want and always have a faster shutter speed or stop down my aperture? Well, here come the cons.
Cons of ISO
If we could shoot at any ISO we wanted with no trade off to image quality, all photographer would rejoice and there would be no need for new cameras with better high ISO capabilities and we'd all be a happy bunch. The biggest trade off of higher ISOs is image quality and noise. As you increase your ISO, your image quality will go down and you'll start to see noise or graininess in your photos [especially in shadows and dark spots]. Newer cameras are getting better and better at having better image quality at higher ISOs and that is one of the biggest upgrades that newer, more expensive cameras give over the older cameras [high ISO, low noise]
Although, noise can be decreased by ensuring that you "nail your exposure" (what we're trying to accomplish here). Because perfectly exposed images tend to have less noise than underexposed/overexposed images.
Low ISO, GOOD; High ISO, BAD [in terms of image quality]
Low ISO, darker & less sensitive; High ISO, brighter & more sensitive [to light]
ISO 100-400 outdoors (100 on sunny days, 400 when cloudier or closer to sunrise/sunset)
ISO 800-3200?6400?12800?25600? indoors (unless you have a D3, D3x, D700, or 5dMk2 I wouldn't suggest shooting over ISO 3200 :] )
A quick example:
This image was taken at ISO 800
Get it now? :) Next time we'll be covering the third and final variable to exposure, shutter speed.