ISO Speed in Digital Cameras

Xiney

Ok so as I lay awake last night listening to everyone in my house (husband, cat, dog) snore, something occured to me:

I understand the basic idea behind film iso, higher iso having larger grains which means the film is faster but more noisy... but... how does it work in digital cameras?

enda1

I think its to do with light amplification and filtering.
Like audio amplification, you can a clean signal (good sensor) so that you amplify the music (picture) and not signal noise (grain/noise).

Also advanced filtering comes in to lower this noise in the picture I guess.

johnmcdnl

When you raise ISO from 100 to 200, you double the output of the CCD sensor.
Raise it from 200 to 400 you double it again.

It is much like raising the volume on a small portable radio.
The higher you raise the volume, the more distortion you get.
Turn that small radio all the way up and the distortion is overwhelming.

ISO speed, shutter speed and aperture all work together to get the correct EXPOSURE.

So it all just comes down to how much light the CCD sensor is capturing basically... the higher the iso the more light it captures


so basically if you want to capture a ahot at say... 1/500 but the auto settings or whatever say 1/250 with iso 200.. then if you double the iso to 400 the shutter speed will only have to be 1/500

but by raising the iso there's going to be more distortion

The good old reliable wikipedia can explain more

Xiney

I understand exposure

I just wanted to know how the ISO speed works in a digital camera.

So it's basically, it takes the light that comes in and multiplies it or something?

5uspect

It's much the same. In tech speak you increase the gain of the sensor.
This makes the sensor more sensitive to light but at the expense of signal to noise ratio.
The electronic signals have more fuzz in them.

The difference you see is in the type of grain.
Film grain has a certain aesthetic quality. Digital grain, or noise, is quite harsh and ugly.
In film the grain is related to the grain size of the film medium. High ISO film has bigger crystals. On a digital camera the amount of noise is dependent on the amount of gain. It is always per pixel.

There is an excellent explaination here:
http://www.cambridgeincolour.com/tutorials/image-noise.htm
Much better than what I can tell you!

Ghost Train

johnmcdnl wrote: »

It is much like raising the volume on a small portable radio.
The higher you raise the volume, the more distortion you get.
Turn that small radio all the way up and the distortion is overwhelming.

Distortion is a bit different, less light on the sensor is more like in a weak radio station, the more you amplify it the more you amplify the background noise

charybdis

As I understand it, digital camera sensors are composed of millions of light-sensitive cells. When the sensor is active, the light-sensitive cells become charged proportionally to the amount of light falling on them. The charge on the cells is allowed to accumulate for the duration of the exposure and when the shutter closes the cells are allowed to discharge in a measured way and the resulting voltages created are converted into a digital image file. By applying an amplification or gain operation between the cells discharging and the conversion of the information to a digital file you can increase the apparent amount of light in the scene at the expense of also increasing the amount of noise in the signal. This is effectively what increasing ISO over a sensor's native sensitivity is.

DotOrg

johnmcdnl wrote: »

So it all just comes down to how much light the CCD sensor is capturing basically... the higher the iso the more light it captures

just fixing this point. the sensor captures the same amount of light no matter what iso you use. it's how sensitive it is to the light that it captures that makes the different.