What is ISO? It stands for International Organization of Standards. Yes, I realize that would be IOS, but it isn't, so there. This is a group that makes up some standards for different things and they have come up with standards for "film speeds" in photography. People that have been around in the photography world for a long time, probably aren't even reading this because they already know, will also know this as ASA which is what it used to be called.
You have all probably seen this ISO number and not even realized what it meant. Remember when you had those old film cameras and you would buy film and there would be a number on the bottom right corner, something like 100 or 200 or maybe 400. The number would also have a strip of a different color from the box, maybe blue, or green. This was the ISO or film speed that you were seeing. The ISO doubles for each stop of light, so from 100 to 200 is a full stop of light; 1600 to 3200 is a full stop of light. This is important to keep in mind with shutter speeds and aperture values. The values for those settings we covered were also listed in full stops.
So we actually have three things now that determine the exposure in the camera. I promise, there isn't anything else hidden, these three things are all you need to determine the proper exposure.
So what are the different values in ISO doing for exposure? With the other two elements of exposure I mentioned they control two things each, and ISO is no different. ISO first, for exposure purposes, controls how sensitive the film is to light. Well, now it isn't film but your digital sensor. So if you film, sensor, is really really sensitive to light, then it doesn't take very much light to get the information it needs. So the more sensitive the sensor is you can think of it as your sensor getting more light. So this would be the same as slowing down your shutter speed or opening up your aperture. The second thing that comes with ISO is what is called noise in the digital world. Due to the sensor needing to be more sensitive, there is actually more of a charge on your sensor now, and it actually gets hotter too, this will cause some little disturbances in the image that looks like little pieces of grain, and it is called grain, or noise. So the more sensitive your film gets you can now have faster shutter speeds, but you are now going to start getting noisy pictures. So lets look at some values you may see, all cameras are different, but most will have at least 200-800.
50 Lowest sensitivity - less light(requires slower shutter speeds), also less noise
3200 Highest sensitivity - more light(can use faster shutter speeds), also more noise
So what do we want to use for our ISO settings? In the earlier exposure posts I said to leave it at the lowest setting you could. The reason for this is because you get less noise. I personally always try to keep my camera set to its lowest setting, 100, and only raise it when I have no other option because I don't want to deal with the noise. Sometimes you just need to raise it to get good pictures though. One good example is when you are inside. If you are in your house taking pictures of your kids, or at a sporting event, you don't usually get a lot of lighting. In most low light situations I just use a tripod so I can just use a longer shutter speed and can keep my ISO at 100, but in situations where you have to keep a high shutter speed, because you want to stop the motion, you can't keep your ISO at 100. Even sports photographers with their lenses that shoot at f/1.4 can't get quite enough light to stop a basketball player with a fast enough shutter speed indoors. So to keep their shutter speed where they want it the option is to raise your ISO. Another time you might want to raise your ISO is for artistic purposes. Janae likes to raise her ISO sometimes when she knows she wants to turn the picture into a B&W. The reason, is because in some cases, especially in some B&W shots, the noise actually adds a cool affect to the picture.
So a quick re-cap. Lower ISO means less noise, but it means you can't use faster shutter speeds, and higher ISO means you can use faster shutter speeds, but you are now getting more noise. We now have 3 things working together to get the correct exposure. As we saw in the other exposure posts, if you have a certain reading you can change any one of the 3 elements to get you more light, as long as you change one of the other 2 to get less light, and you will have the exact same exposure. So for example if the correct exposure is f/11 @ 1/4 ISO 100, you can change one setting to get the most important thing to you in the shot. If the most important thing is to freeze the motion, we can up our shutter speed by 6 full stops to get 1/250 so we can stop the action. Now to get the proper exposure we need to compensate by adding light, so lets lower our f-stop by 4 stops, our lens can open up to an f/2.8, now we still have 2 stops of light we need to gain. So we move our ISO to 400 to gain those two stops of light. We now have f/2.8 @ 1/250 ISO 400 which is the exact same exposure, but now we have a fast enough shutter speed to stop the motion, which was the most important thing for us in this shot, and we have a smaller dof and a higher ISO, which means a little more noise.
Okay, your assignment for this tutorial is simple. Take a picture of the same object at all your possible ISO settings. Lets compare the noise in the different ISO values for your camera.