Digital noise and the rise in ISO

Written by 杨 道波 Monday, 08 November 2010 14:24

Any photographer using a digital camera will face at one time or another to the digital noise generated by the increase in sensitivity (ISO) to light in the picture too low.

Here is an attempt to explain the phenomenon of increased sensitivity = higher noise ...

1. Some reminders about the operation of a CO

To simplify very hard, here's an NPC (for games that affect us):

Light passes through the iris (iris) that controls the amount of light to pass through its opening. Then it passes through the shutter open (shown closed here) during the installation. She finally falls on the pixels of the sensor that turns it into electrical energy. This electrical signal is amplified by the electronics of the device.

ISO sensitivity in apn is the amplification gain of the electronics of the NPC.

more iso, the higher the gain is high. The gain to ISO 400 is 4 times the gain to ISO 100.

In this diagram, we see in particular that if there is little light, once you open the aperture to the bottom, once you've reached the maximum exposure time that is tolerable (depending on the movements objects in the scene or on the basis of tremors if the photographer does not use a tripod), it remains only to increase the iso.

2. Noise

Electrical noise is formed at each pixel during the shooting. More exposure time is long, and more noise at leisure to form.

But this noise does not depend on the amount of light that reaches the sensor.

Note: the noise depends on the temperature, which explains why the sensors of some scientific imaging systems are cooled with liquid nitrogen

So we place ourselves in the case of a given exposure time, at a given temperature, with a given sensor (each cell has its own noise depending on the type and design).

If there is a lot of light, the electrical signal created by the pixel is very strong and will not need much amplification (low ISO). The noise is very low compared to the signal created by light. we do not distinguish on the final image. The signal caused by light is predominant.

If there is less light, the electrical signal created by the pixel is low. Against him by the noise remained the same. In the signal that is sent to the amplifier from noise is stronger.
For a well-exposed image, it is mounted iso and thus gain power is stronger. The problem is that this is not the signal (low) that is amplified, but also noise. And noise, as the signal is much magnified iso strong, which explains that a picture is much more noisy for large iso ...

I hope the lack of illustrations you will not be too disheartened, and that little explanation is understandable ...

Feel free to ask questions, to the extent possible, try to answer them as clearly as I can!


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Last Updated on Monday, 08 November 2010 14:40