Meaning of ISO value in digital photography?

On 06/14/2018 03:29 PM, Savageduck wrote:
On Jun 14, 2018, Jim-P wrote
(in et):

In film cameras, ISO referrs to the sensitivity to light of the emulsion.
Manufacturers formulate different film emulsions with different
sensitivites tarding increased grain with increased ISO speed.

Correct. The important thing is not to conflate analog ISO ratings with
digital ratings. They are two different things.

In a digital camera, presumably the sensor does not adjust itself to have
greater sensitivity. Or does it?

Any given sensor will have a base sensitivety, or ISO. Typically ISO 200, or
ISO 100 depending on manufacturer.

Really? I've never looked into that, but I would have thought that the
base sensitivity would be closer to the mid-point of the camera's ISO
setting range. But that's just electronics design practice.

The sensitivety does not change in the
same way that analog film emulsions can be changed. Any increase from the
base ISO is an increase of signal gain, or amplification of the signal from
the sensor.

So what is happening in a digital camera when I choose a greater ISO
setting? Is more amplification being used?

What you are doing when increasing the ISO number with a digital camera is
increasing the signal gain between the sensor and the camera CPU. This is
effectively the same as turning up the volume (gain) on an audio amplifier.
As gain increases so does amplified noise. How the noise manifests itself
depends on the sensor, the CPU, and the SNR of the pair. That is why some
cameras have better high ISO performance than others.

By using ISO to rate the base sensor sensitivity, and any increases by upping
the gain just gives folks who have made the move from film to digital a rough
guide by referencing film ISO.

I tried to understand this page but it got far too technical....

https://photography.tutsplus.com/art...n--photo-11963

That article is spelling out the fact that digital ISO is not the equivalent
of analoge ISO.

That article does have a high nerd factor. Generally when an article
emphasizes that we are using log-10 not log-e, you need a couple letters
after your name!



--
Ken Hart