A Guide to Understanding ISO in Photography
Learning photography is a difficult process and requires a lot of patience and practice. This photography lesson concentrates on the basics of exposure, it’s definition, importance and finally it’s implementation. Light is the most important criteria in learning photography. Light creates the texture around you, it defines the composition of not only the object but also it’s surroundings.
The whole idea of photography basically revolves ISO顧問 香港 around light and its laws of reflection and refraction. Exposure means the amount of light that your camera gathers while taking a photograph. It’s the foundation of digital photography.
One of the most important exposure parameter is called the ISO, and it’s truly the key to solving some difficult exposure problems. But it is very important that you have this aperture and shutter speed thing under your belt before you move on to ISO.
ISO is a measure of the sensitivity of your image sensor. If you have a film background, then you are probably already familiar with ISO as a measure of film sensitivity–sometimes referred to as film speed–and you may even remember ISO being referred to as ASA. ISO is the same thing. By default, your camera is probably set to an ISO of 100 or 200.
Now, for the sake of example, I am going to say that I don’t have a tripod here. Let’s say I stumbled into this scene while I was out walking around shooting hand-held, and I am ready to take a shot of it. So I am lining up my shot, and I am half-pressing my Shutter button to meter. But when I looked down at my shutter speed; it’s saying somewhere between a 10th and a 15th of a second. That’s way too slow for hand-held shooting. Now, there are times when you can stabilize your camera but still, a 10th of a second is too slow.
So what I am going to do is increase the ISO on my camera. I am going to dial it up from 100 to 400, and now when I meter, I see I am at a 45th of a second. I am going to dial it up a little farther. I am going to go to 800, and now I am at a 90th of a second. That’s plenty for hand-held shooting. I can take my shot. As ISO increases, it takes your camera less time to gather light. This means that you can get away with shorter shutter speeds, which is what just happened here.
Now, the question is how it works? When data is first read off of your cameras image sensor, it’s in the form of tiny, little electrical charges. Before those charges can be analyzed, they have to be amplified, because they are very minute signals. When you increase the ISO setting on your camera, all you are doing is turning up that amplification. Now, because it is more amplified, weaker light levels are more significant, so you can get away with less light, which means shorter exposure times or smaller apertures.
Now think about what happens when you turn up the volume on your stereo. As you amplify the sound more, it gets louder, but you’ll also hear more noise, a hissy sound. Electrical components in your amplifier, other gizmos in your house, cosmic rays passing through the room–these all generate electrical noise, and as you amplify your sound, you also amplify that noise, and so you hear a noisy hiss as your volume gets louder. Your image sensor works exactly the same way. As you increase the amplification of the signals that come off the sensor, you exaggerate any noise that the sensor might have recorded from the other electrical components in the camera, or those cosmic rays that might be passing by. And you find that that noise will appear in your image, as speckle-y patterns that look like this. Now how much noise will be generated will depend on your camera.
Obviously, you’d prefer not to have noise in your images, so you should always try to keep ISO as low as possible. That said, the ability to change ISO from shot to shot is one of the great advantages of digital over film, and something can really nag films norms about. Here is why. First of all, as you have seen, when I get into a situation where I am trying to shoot hand-held, and the light is so low that I can’t because my shutter speed will go too low and my images will be shaky, all I have to do is turn up my ISO.
Now, if you take a look at standard ISO numbers. Each one is double the previous. You should know by now that what that means is each one is one stop apart. So if I meter a scene at ISO 100, and my camera recommends a shutter speed of 1/30th of a second–a speed that’s a little too low for hand-held shooting–then I can turn my ISO from 100 to 200, which is a difference of one stop. One stop is a doubling of light, and that means my sensor is now twice as sensitive to light, so when I meter now, my camera chooses a 60th of a second. With that single shift, I am back in the realm of safe hand-held shooting.