Difference between revisions of "ISO (light sensitivity)"
| Line 2: | Line 2: | ||
ISO is usually specified on a logarithmic scale like 100, 200, 400, 800. Each step doubles the light sensitivity of the camera so ISO 800 is eight times as sensitive to light as ISO 100 and the [[exposure]] time would be 1/8. It is quite common for cameras to have itermediate steps for finer adjustment. The relationship between [[aperture value]] and ISO is: | ISO is usually specified on a logarithmic scale like 100, 200, 400, 800. Each step doubles the light sensitivity of the camera so ISO 800 is eight times as sensitive to light as ISO 100 and the [[exposure]] time would be 1/8. It is quite common for cameras to have itermediate steps for finer adjustment. The relationship between [[aperture value]] and ISO is: | ||
| − | + | :relative xposure time = aperture value / (ISO ^ 2) | |
The common way to adjust sensitivity in a digital camera would be like this: | The common way to adjust sensitivity in a digital camera would be like this: | ||
| − | + | :Pixel --> variable gain analog amplifier --> analog to digital converter --> digital multiplier | |
Each colour channel have a separate gain setting which would be part of the [[white balance]] system. | Each colour channel have a separate gain setting which would be part of the [[white balance]] system. | ||
Revision as of 13:23, 20 October 2009
ISO stands for International Organization for Standardization. The standard that regulates the light sensitivity of digital cameras is ISO 12232:2006. It is common to use ISO for short when discusing the light sensitivity of cameras.
ISO is usually specified on a logarithmic scale like 100, 200, 400, 800. Each step doubles the light sensitivity of the camera so ISO 800 is eight times as sensitive to light as ISO 100 and the exposure time would be 1/8. It is quite common for cameras to have itermediate steps for finer adjustment. The relationship between aperture value and ISO is:
- relative xposure time = aperture value / (ISO ^ 2)
The common way to adjust sensitivity in a digital camera would be like this:
- Pixel --> variable gain analog amplifier --> analog to digital converter --> digital multiplier
Each colour channel have a separate gain setting which would be part of the white balance system.
The digital multiplier is often referred to as uncalibrated or HI setting. Usually it multiplies the pixel value by a power of two, the same effect can be had by adjusting the brightess with image processing software. There may or may not be an advantage of letting the camera do it because it may affect how the noise reduction algorithms in the camera works. It needs to be tested on every camera model to find out what works best.
The number of photons that hit a pixel is a digital value so in some sense the analog amplifier is not analog at all. When the number of photons is large the quantization of values is completely hidden in noise and the signal seems analog, but as the number of photons decrease there will discrete values. For example if each pixel registers 0 or 1 photon the image is digital from the very moment the shutter closes.
The noise is determined mainly by how many photons each pixel registers. Since the photons spray randomly from an object it will hit the pixels in the camera in a random order. If the scene is completely evenly and brightly lit, each pixel will still contain significant random noise just from random chance. If each pixel registers 65536 photons on average the average noise would be sqrt(65536) = 256 but there is nothing to stop some pixels to be completely black by random chance, it is just very unlikely for it to happen.
