How far can ISO levels be pushed?

[donboyfisher]

I can understand the need to higher and cleaner ISO abilities in camera and to me its a great thing. As I understand it, lower noise is a combination of accurate sensor cells and then applying a gain factor to the sensor readings.

But is there a point at which ISO can no longer be pushed?

Surely you must get to a stage where ( either through shutter speed / low light ) you simply dont get many photons onto the sensors and so the readings are so low that now matter the gain, there will always be noise through there only being a limited number of photons hitting a limited number of sensor cells.



Or...

In a typical photograph on todays camera . . . lets say a typical portrait shot, is there any idea how many photons actually hit the sensor cells? If a cell can output to 16bit values . . . does that mean that there is a significant amount more amount of photos hitting the sensor? ( i.e. more than 2*16 ?? )

Basically, how many photons are required to hit a cell before its reading is changed, and will there be a point where the reading can change on a photon by photon basis ?


And if it could, would there be any point pushing a sensor to be better than that?



Sorry if thats all drivel, but i'm at the desk daying dreaming at the mo!

[gjl711]

There is no theoretical limit to which ISO can be pushed. It is theoretically possible to create a sensor capable of triggering when a single photon hits it. Practically however it’s a completely different story as there are many more factors than just photons hitting the sensor. If you look at the astronomical applications they play all kind of games like super cooling the sensor to increase the sensors sensitivity to photons and reduce its sensitivity to other types of triggers like electrical noise. As to the number of photons hitting a specific pixel, there really is not a way to specify that as a photon isn’t really there. It’s just how we describe the particle nature of light. But take for example a shutter speed of 1/60. Just doing the math a column of light 3100 miles long just slammed into your sensor.

[StewartR]

There was a discusssion here (http://photography-on-the.net/forum/showthread.php?t=316340) a few weeks ago that seemed vaguely relevant. In the course of it I did some research and discovered that the human eye can function at levels between 100 photons per second and (at least) 1,000,000,000,000 photons per second. (Admittedly at the lower end it doesn't function very well!)

I'll see if I can dig out some information about what's going on inside a sensor...

[StewartR]

There is lots and lots of interesting (and highly technical) information at www.ClarkVision.com (http://www.clarkvision.com/index.html).

For example:
Digital Cameras: Does Pixel Size Matter? (http://www.clarkvision.com/imagedetail/does.pixel.size.matter/index.html)
Digital Camera Sensor Performance Summary (http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary/index.html)
Digital Cameras: Counting Photons, Photometry, and Quantum Efficiency (http://www.clarkvision.com/imagedetail/digital.photons.and.qe/index.html)
Analysis of Canon 1D Mark II Sensor Noise, Dynamic Range, and Full Well Capacity (http://www.clarkvision.com/imagedetail/evaluation-1d2/index.html)
Notes on the Resolution and Other Details of the Human Eye (http://www.clarkvision.com/imagedetail/eye-resolution.html)

I feel sure the answers must be in here somewhere. More later after I've wrapped a cold towel around my head...

[gjl711]

There is lots and lots of interesting (and highly technical) information at www.ClarkVision.com (http://www.clarkvision.com/index.html).

For example:
Digital Cameras: Does Pixel Size Matter? (http://www.clarkvision.com/imagedetail/does.pixel.size.matter/index.html)
Digital Camera Sensor Performance Summary (http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary/index.html)
Digital Cameras: Counting Photons, Photometry, and Quantum Efficiency (http://www.clarkvision.com/imagedetail/digital.photons.and.qe/index.html)
Analysis of Canon 1D Mark II Sensor Noise, Dynamic Range, and Full Well Capacity (http://www.clarkvision.com/imagedetail/evaluation-1d2/index.html)
Notes on the Resolution and Other Details of the Human Eye (http://www.clarkvision.com/imagedetail/eye-resolution.html)

I feel sure the answers must be in here somewhere. More later after I've wrapped a cold towel around my head...
WOW!! Lots of fun stuff. I think I have smoke comming out my ears. ;)

[donboyfisher]

oh . . . thats tomorrow's reading material sorted then :)