Comment by dingaling

5 years ago

> Alternatively, we could increase the sensitivity of the sensor which is described using the ISO rating.

Umm well, not really.

The sensitivity of a digital CCD or CMOS sensor never changes; it has one 'native ISO' equivalent sensitivity and that's all. It creates the same output signal in response to a photon every time.

Varying the pseudo-ISO simply varies the gain of the downstream post-readout amplifier. That's where noise is introduced.

Sometimes it's better to take a deliberately underexposed photo and tweak regions of it in post to achieve the desired exposure instead of introducing global noise from in-camera amplification.

6 comments

dingaling

Reply
matthiasv  5 years ago

> Sometimes it's better to take a deliberately underexposed photo and tweak regions of it in post to achieve the desired exposure instead of introducing global noise from in-camera amplification.

That is definitely not my experience for Canon's in low light situations. Here [1] is an example shot at ISO 100 but pushed by 5EV to match the brightness of the same scene shot at ISO 3200. The noise is much more tolerable for the latter.

[1] https://imgur.com/a/Ca9ccbK

P.S.: it's raw, no in-camera JPEG denoising in place.

  • martomi  5 years ago

    Perhaps what we're seeing here is that quantization errors for the non-amplified signal become larger than the noise introduced by the amplification?

rimliu  5 years ago

There are also "dual native ISO" sensors. Still different apmplification levels, but in this case it is before ADC.

See https://www.youtube.com/watch?v=g8hHFt3ChZ8

  • formerly_proven  5 years ago

    "Dual native ISO" sensors don't perform different amplification before the read out amplifier or anything like that.

    With an image sensor you get to decide between high sensitivity and high capacity (how much light it can detect before saturating). So a highly sensitive sensor can handle less light before clipping (which makes sense).

    This trade-off can be tweaked by changing the size of the photodiode (bigger diode -> more sensitive), but also electrically - simply add capacitance in parallel to the photodiode. This requires more energy, i.e. more photons, to change the voltage over the photodiode, which is what the readout measures.

    Dual native ISO sensors simply add another transistor (as a switch) in series to extra capacitance; this allows them to switch between low-capacitance (high sensitivity, but saturates faster) and high-capacitance (lower sensitivity, can handle more light).

    Edit: Haven't watched the whole video but his core explanation in there seems to be that Dual ISO means the sensors has a PGA (programmable gain amplifier) -- almost all sensors use that approach, and that's what is controlled by the ISO setting in them.