Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 42,023
Re: Exposure and brightness.

bobn2 wrote:

Great Bustard wrote:

OK, this is a little more work. Tell me how you like this:

A certain number of photons fall on a pixel releasing a certain number of electrons which generates a charge. A gain may, or may not, be applied to this charge as a function of the ISO setting on the camera. The charge is then converted into a digital number by the ADC (Analog to Digital Converter). The RAW converter (or in-camera JPG engine) processes groups of digital numbers into RGB values for the image file where the ISO setting on the camera maps these values so that they have the brightness that the corresponds to the exposure and ISO setting.

You're making it too complex, getting bogged down on the detail of the technology, rather than what is the process being undertaken.

Is that not necessary, however?  I mean, if people are to understand what an ISOless sensor is all about, for example, then the ADC needs to be discussed, as well as understanding why, for example, a photo at f/2.8 1/100 ISO 1600 is less noisy than a photo of the same scene at f/2.8 1/100 ISO 100 pushed four stops for a sensor that is not ISOless.

The camera counts the photons, in three different bags (red, green or blue). Then it translates those photon counts into grey scale values. The exact translation done depends on the ISO setting, or to reverse that the translation done determines the ISO. The rest is implementation details.

Sure, but at least some of the details are important, methinks.

If we could engineer a sensor that directly output a count from each pixel, the basic process would be the same, and no-one would get bogged down in issues of 'ADC' or 'gain' or any of that stuff. That is just the way we engineer a photon counter at present. Film was also a photon counter. A pair of photons would reduce a reactive spot in a silver grain which catalysed its chemical reduction to silver. Each grain represented two photons collected.

Ideally, of course, we would have a sensor that recorded every photon that landed on it along with the wavelength (and momentum) of each photo that added no additional electronic noise.

However, in my opinion, I think the processing chain, from the capture to the image file, is necessary to describe as it does apply to how current cameras are used.  Like I said, there is a reason, other than operational convenience, to use ISO 1600 rather than ISO 100 and push, on many cameras, and that shouldn't be glossed over.

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