.....
That's a common problem in this discussion, although also an irrelevance. When you realise what the photographic process is, and what ISO actually does, you realise that whilst where the sensor starts and stops is of academic interest, it's irrelevant to the discussion. ISO says what lightness should result from a given exposure.
This is one of the clearest , most understandable short definitions i read about this subject .
Simplified but accurate enough.
Thanks for the feedback. I'll remember it and use that text again
I will explain why i find this to the point, most of the time ISO value is connected to "gain" of voltage or current or digital gain in threads and webpages which brings your mind to a "amplifier". Which is a trap of misconceptions and assumptions. (I did) most internet searches don't help to untangle the confusion either, on the contrary it tighten the knots even more.
That's why this simple text don't distract you and don't sent you in a maze of half truth's. and keep you on track for the next information.
So as far is i remember all the basics:
While ISO isn't a Gain/Amplifier but more a (wanted) result of the several gains (to get the signal in range of correct processingvalues for readout and conversions) along the road of capturing photons towards a image represented in a rawfile. This in order to get the image correct(lightnes) represented on screen or paper. By getting a underexposed sensor/pixel , low amount of photons on a given shuttertime, and by setting a ISO value you define the lightnes you want to correct that (under)exposure in the camera to get a correct lightness in your jpeg/tiff.
A word of caution there, the term 'underexposed' is something of a trap, too. The pixels's job is to measure exposure, whatever it is. 'Underexposed' is a relative term, relative to the exposure you want, and what exposure you want depends on what you're trying to do and towards what end you're trying to manage exposure. It doesn't relate to any requirement in the pixel or any need for some absolute level of exposure. A low exposure doesn't 'need' boosting, it just needs the desired lightness value assigned to it.
new question:
What if you alter a rawfile of a 1600iso image and change the stored value of 1600iso in the exif in lets say 200iso? (lets say this is "base iso") drops the lightnes of the image in a rawdeveloperapplication 3stops down? so it looks like a -3EV Exposurecorrection?
Think of the ISO development process as a table of lightness values against exposure values. (What a processor is doing is like 'painting by numbers'). By changing the ISO you shift all the lightness values up or down the table. If you set the ISO three stops lower. then all the lighness values end up against three stops larger exposures, so if you dindn't actually change the exposures, the image will be rendered three stops darker.
(highlights stayed clipped so those are gone and black is black so you scrunch the DR of the tonecurve. )
The sensor tone curve is pretty much a straight line, unlike film. It doesn't get 'scrunched'
and this altered exif shows the "real image" what's captured by the sensor showing the real lightnes it would show on base ISO? (except the clipped data)
There isn't a 'real image' in the sense you mean. What the sensor captures is what in film days would be called a 'latent image'. The image you see is a perceptual illusion, tailored to human vision, not a direct funelling through of the light from the original scene. It doesn't exist until you've processed the latent image.
Or are some "gains" non reversible? (i think simplified that all gain before ADC is irreversible, after the ADC it's just "software calculation" and fully reversible. )
I think it's best to forget about 'gain'. It has to do with optimising the camera electronics, and nothing to do with photography. If you don't optimise the capture electronics for low exposures (which is what happens when you raise the ISO) you won't recover the shadow information lost in the resultant noise. If you optimise for low exposure, then use a high one, you won't recover the clipped highlights.
To determine the lightness in the output file, all the camera needs to be able to do is measure the exposure in each pixel. 'Gain' doesn't change the exposure in the pixel, but it can change how accurately it is measured.
Is this the reason that dynamic range changes when ISO raises? less accurately means less steps between black and white (full exposure)
So far as DR is concerned, the problem is that if you apply voltage gain before the ADC you boost the higher signals up above the ADC maximum, the larger exposures can't be digitised. Thus each stop of gain means a loss of a stop at the high end. Once that loss stops lowering the noise floor, there is no point applying any more voltage gain. There's another question of just why manufacturers continue unnecessarily clipping the raw file, which we can have if you want.
People have difficulty with this concept because they tend to call both the input and output 'exposure', in which case they think of 'gain' as giving a bigger 'exposure' from a smaller one, but that is not what is happening. Exposure and lightness are different things, which is why two words are needed.
Extended ISO at the high end i can think of: Just add measurement's at the lowest part of the sensor output voltages until noise overgrows mesurable signal and higher iso values are redicules because you can't separate random noise from the actual latent image capturing enough to get a reasonable clear image at the end anymore.
The real problem is that the noise is in the signal, which is why photography is somewhat different from common electronic applications. The SNR goes as the square root of the number of photons captured. The problem with very small exposures is just not many photons.
But at the low end below base iso. Why don't use that as base iso?
See upthread. The thing,about th Lo settings is you can meter as normal, get higher exposures, thus more photons, thus less noise.
Got it. metering is for the determination of the shuttertime to get a proper exposure and shuttertime increases when lowering iso so more photons are able to reach the sensor/pixel with in the shuttertime.
Not shutter time, exposure. Remember, exposure is controlled by shutter, aperture and the light coming from the scene. The ISOI that you set provides your meter with a target exposure, that's the exposure that the meter measures when it's centred. You can get that target exposure by changing any of the shutter speed, aperture or light coming from the scene (if you have control over that). This is hard to conceptualise if you're thinking of 'exposure' as the output of the process, how light or dark the final image is. Many people do this, including supposed experts that write web sites (for instance, I've caught the Northrups doing this, left a comment, not responded to). Exposure is what you control at the
input. Lightness is what you get at the
output.
