What is ISO speeds? ISO speeds is a setting determine the quality of picture. Do not get me wrong, quality does not equal to number of Pixels.
Traditional film camera do not have ISO speeds setting. It is a setting for digital camera's Sensor (A Sensor is a "film" of Digital Camera). Low ISO speeds obtain higher picture quality, and high ISO speeds obtain lower picture quality.
Some digital camera have a lower ISO speeds, and higher ISO speeds up to ISO 12800.
As you can see, the picture get blurred by increasing the ISO speeds. But It is not "Blur", it is "Noise". Let's make it easier. Sensor is a device (oh god, not again...) to collect light. ISO 100 means the Sensor is a lazy guy and it collects light in slower speed. ISO 1600 means the Sensor is a hard working guy and it collects light in faster speed.
Therefore, a hard working Sensor will collect whatever comes in, even the reflection of dust in the air. Picture with higher ISO speeds will appear white spots and probably you will think the Lens is cover by dust, so you clean it up quickly. Well, you might be correct, but it might just dust in the air.
We are surrounded by dust all the time, you just do not notice. To prove there is dust , turn off your room lights and turn on your torch light. Do you see dust right now?
Someone gonna ask why we need to set higher ISO speeds if it will gain Noise? Do you remember maximum Aperture but still not bright enough in chapter "How does Aperture work?" ? ISO speeds can help you. Higher ISO speeds work well for dark scenes.
As I told you, a hard working Sensor collects more light. In dark scenes, you can capture a better picture, without Flash, by setting a higher ISO speeds. Also, higher ISO speeds setting save more battery than a Flash.
This picture is captured in different ISO speeds. If you take a closer look, you will see the ISO500 is actually the clearest and got a the best texture of all. Do not get me wrong, I mean ISO500 is suitable for this picture ONLY because the brightness of environment, the object's colour, the shooting angle and etc might be the factors. Although ISO200 will reduce the noise, it does not capture the best texture. While ISO3200 does not capture the best texture as well. This also proof selecting a suitable ISO speeds will also affect the quality of picture.
However, Do Not Use ISO 6400 and above! Noise increase at this level and the texture is reduced. Most photography at this speeds is more to "document" than actually "use" the picture.
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