Digital ISO

[oldgit]

Something I've been mulling over for a while and not been able to find a satisfactory answer to is...

I know that the ISO rating of a digital sensor is it's correspondance to the equivalent film speed and that higher ISO means more noise that kinda equates to higher digital noise.
All sensible straightforward and easy to understand, but...

What is happening inside the camera to adjust the sensor sensitivity?
Clearly it's the same sensor so there is some sort of jiggery pokery going on, but,
What:
- Is it a different pre-charge on the sensor drive lines?
- Less time allowed on the A-D convertors?
- Different back bias or bulk node voltage?
.... how does the thing work?

I'm also making the assumption that the sensor is "equivalent" to a RAM without the lid on, and therefore there are row and column drivers/decoders..

I've tried googling for "CMOS Image Sensors" "FET image sensors" etc, but have not found out a decent reference.

Anyone help?

 

[CT]

As I understand it Paul it's just increased voltage to the sensor which effectively increases the ISO. It's actually the increased voltage which is also responsible for the noise, and it becomes worse the more pixels that are crammed on the sensor - so more pixels isn't always a good thing as far as noise is concerned. - this week anyway.

 

[pxl8]

In practice a single camera can achieve multiple different ISO ratings by applying some form of amplification to the signal coming off the sensor. This can be done by applying analog amplification to the signal before it hits the A/D converter, or by bit shifting the results after they have gone through the A/D converter. Cameras may apply a combination of these approaches, depending upon the desired ISO. Which is best will depend upon whether amplifier noise or A/D converter noise is larger.

Taken from:
http://www.cs.duke.edu/~parr/photography/faq.html

 

[oldgit]

Taken from:
http://www.cs.duke.edu/~parr/photography/faq.html

Ahh that makes some sort of sense, but doesnt explain the problem completely

As I understand it the process is (with amplify box added):

|precharge| -> |capture (shutter open)| -> | amplify | -> | A-D | -> | Save RAW |

So I'm guessing that the higher the ISO the larger the precharge and sensor voltages = more current/leakage and therefore increasing the sensitivity and noise, which is then amplified before hitting the A-D amplifiying the image and noise..

Plausible..

 

[pxl8]

The article suggests the increase can be done after the A-D stage as well. I don't think there is a set method, each maker/model might use a different approach and probably keeps it a secret to some degree.

You could also dismantle your camera and reverse engineer it

 

[Jonnyreb]

Dont forget that a major cause of noise is heat. Erg; higher sensitivity = higher voltage = more heat = more noise = irate photographer / (Photoshop + Noiseware) = greater cat life expectancy