is ISO limited by hardware or software?

im curious as to why some camera offer ISO expansion.

My understanding is that "native" ISO is hardware-capable ISO, and "expanded" iso (50, 3200, 6400, etc...) is software driven. You could technically make any camera do ISO 6400, but it may look so poor that Canon would be embarrassed to make it public - which is why they limit it on some cameras with sensors that are not up to the task.

Erik explained it pretty well. Theoretically you could continue to amplify the signal via hardware until the noise floor becomes the same as the signal and the image would disappear in a sea of speckles.

ah gotcha! thanks guys!

so its like point and shoot cameras offering digital zoom.

Then more capable image processors may be able to better deal with the noise they get from the hardware, i.e. the sensor and its amplifiers. Thus a camera with the same sensor and associated hardware may produced inferior images, if it had a Digic II image processor, compared to one that has a Digic 4 connected to that hardware.

To a certain extent. When the noise is at the same level as the signal, then not much helps. Not in a case like this, where you don't know anything about what to expect. It's different with things like GSM or GPS signals. They are well below the noise floor, but can be detected anyway, since they contain certain patterns to look for. In this case that principle would require that you took photos of the same subject all the time, hence it's not applicable for cameras.

polobreaka wrote in post #10400479
ah gotcha! thanks guys!

so its like point and shoot cameras offering digital zoom.

kind of/sort of, but not really.

The sensor pixels have a fixed sensitivity to light. It is a property of the material that the sensor is made from. From there, the pixel signal is amplified via the electronic circuitry. For a ISO100 shot some level of amplification is applied. For ISo200, 400, 800 The gain of the amplifier is increased in the circuitry itself.

Once you get to the expanded ISOs, the camera no longer has the hardware to amplify the signal so some games are played. The picture is taken with some level of exposure compensation applied at the time the sensor is exposed. This tricks the camera into thinking that a higher ISO is available so the shutter speed and aperture are adjusted accordingly. Then vie some fancy software, the data is manipulated applying the opposite exposure compensation to the image. The end result is as if you would have taken the image with the expanded ISO.

So like the digital zoom feature, the camera is inventing something that it is not physically capable of delivering.

apersson850 wrote in post #10400515
... It's different with things like GSM or GPS signals. They are well below the noise floor, but can be detected anyway, since they contain certain patterns to look for....

This isn't really the case. A GSM mobile signal, GPS or even a CDMA signal still has to be above the noise floor. Once the signal falls into the noise, even patterns cannot be invented. Noise is the bane of photographers and RF engineers alike.