At base iso many cameras apply no gain and still output an image.
In the PAS modes raising iso maintains the same image lightness because either shutter speed or aperture are adjusted to reduce the exposure* by the same amount thus maintaining the same image lightness.
* exposure. - amount of light that struck the sensor per unit area while the shutter was open.
Oh yes, certainly some 'gain' is used in the electronics if a camera, there's charge gain, current gain and sometimes voltage gain.
"ISO is the gain that is applied to the amount of light" - so it is light that is gained? This is impossible without a laser somewhere in the system. This is one of the problems with the statements that ISO is 'gain'. It leads people to conclude that ISO creates more light, which is not the case. Moreover, if ISO was a gain in light, then the ISO standard (12232) would define how much light is to be gained, and it doesn't.
So somewhere in the camera is a device that results in the 'gain' of light after the shutter has closed. That's not the case.
On image lightness. Lightness is not brightness. It's important to understand the difference between them to understand what ISO is. Refer to another reply I gave to you somewhere about painting by numbers.
someone with more knowledge than me will be able to answer that. Im just not sure how much gain is added at the first amp stage.
I don't know of any, but that's in part what's so lame about this 'gain' analogy - it's used by people that don't really know what 'gain' is and don't have any clear idea about how imaging works. Moreover, the idea seems to be that a lay person will somehow understand what 'gain' is, and know what ISO is. Generally it leads to the impression that light is what is 'gained'. I've seen this said explicitly in quite a few 'ISO' tutorials.
Just to be clear, the photoelectric effect results in a charge collected in the pixel. In the pixel is a charge amplifier, which results in a gain of charge between the pixel and the column amplifier. The column amplifier is generally a current amplifier, which results in a current gain between it and the VGA (variable gain amplifier) the VGA is a voltage amplifier with programmable voltage gain. In general its gain might be adjusted between 1 and a larger number. Many cameras have two VGAs in series. Very few cameras adjust the gain of the VGAs proportionally to the ISO setting over the whole of the ISO range, and not many over the whole of the normal (non-extended) ISO range. The ADC's will often apply voltage and current gain of their own, depending on the exact circuit of the ADC/ Thereafter we're in the digital domain, and what's happening is multiplication as part of a rendering function, not 'gain'
What do you mean by 'the first amp stage'? The first stage of 'amplification' is a source follower, which acts as a charge amplifier. Then there's a column amplifier, which acts as a current amplifier, and sometimes also a voltage amplifier. Then there's a variable (voltage) gain amplifier (or often two). Often, but not universally, that is programmed to give unity gain at 'base ISO' - but the fact is that manufacturers do not release that information, and there's no way to determine exactly how that VGA is programmed without putting probes into the circuit or getting access to the code that programs it (and that means all the code, because the API might specify unity gain, but without the drivers you don't know whether it actually is). You can also get the same effect, in terms of changing the relationship between charge input and digital number output by changing the reference voltage (or current) or the VGA. Then, there are plenty of cameras that don't adjust the 'gain' proportionally to the ISO.
All very interesting, but a beginner will just see that using a higher ISO number means they can leave the speed and aperture alone and take pictures in lower light at the expense of a bit of noise.
What you are explaining is WHY it happens - but does a beginner care, or indeed even need to know?
Alan
Im a radio ham guy 😁 dont ask to many tech questions its been 40 years.
How is it a "cheat", when there is a cost of noise and/or noise reduction artifacts?
IMO knowledge does never hurt, unless it is based on inaccurate models and understandings.
I agree, but too much detail can put beginners off. Introduce it slowly, let it sink in and then progress to the next level.
Alan
We're on the Open Talk forum, not Beginners' Questions.
Not sure which post you're replying to there Alan.
Anyhow, you're right. All the beginner sees is exactly waht you see. What the beginner doesn't need is a faulty explanation as to why that happens, especially if that faulty explanation engenders mental models which impede further learning, should the beginner choose to learn a bit more.
In the ned, better to stop at 'what', and if you try for 'why', get it right. It's not hard to explain to people who haven't had their thinking clouded by faulty models.
Shouldn't this entire thread be in the dumpster?
So, isn't the net effect of whatever is going on in the camera is that the raw data values are increased over what would be provided at base ISO? Use of the word "gain" may not be strictly correct, but analogous to what happens in an audio device when the volume knob is turned up. Maybe more properly, "amplification" ??
This whole pedantry over terminology is diverting attention from the net effect, which is what most folk are trying to understand.
It isn't even true. Electronic gain is always used. If a camera uses twice the PGA gain at ISO 200 as at 100, and twice the PGA gain at 400 as at 200, that is the only difference; a quantitative one. There is no qualitative difference. ISO 100 uses 1/4 the gain of ISO 400 with that camera, and we don't need to know what the actual gain is at ISO 100, as the user of the camera, but even with amplifiers that amplify voltage or current by 1.0, there is still buffering and impedance matching provided by the amplifier, which is often as important or more important in that position in the signal chain than actual gain ratios.
You can see this in some of Canon's older DSLRs, where the raw files had fully-populated histograms at 1/3-stop intermediate ISOs, leading some people to say that the first PGA had 1/3-stop gain granularity, but when you looked at the read noise, you could see that this 1/3-stop gain granularity happened too late in the signal chain, because the read noise trends vs ISO were in sawtooth triplets, just like the cameras that used "Digital" or arithmetic 1/3-stop granularity. And, unlike the "digital" approach, which used both plus and minus 1/3 stop, the ADC input gain method used +2/3 for ISOs 160, 320, etcetera, making the analog approach much noisier than the digital approach at those ISOs.
'Scaled' would be the appropriate word. The point is that what the raw numbers are is of zero significance. It's an arbitrary code that represents something - and it's what it represents that matters. You could translate them all to ASCII strings. It would make them 'bigger' but wouldn't change what they represent. The numbers represent per-pixel charge, which in turn measures per-pixel exposure. The various settings in the signal chain affect the precision and noise in that measurement, but they don't change what is the per-pixel exposure. If you do change them, then the rendering engine needs to scale them differently, but again this is arbitrary, it doesn't stop it haveing to do all the processes requited to turn that set of per-pixel exposure measurements into a viewable image.
It's not really analogous, for (at least) two reasons. Firstly, your audio system's volume control occurs after the coded audio signal has been decoded into a representation suitable for listening, not before. Secondly, it's a completely different situation. In an audio system the input and output represent the same type of quantity - instantaneous sound pressure. In photographic imaging the input and output are completely different types. The input is measurements of localised light energy, the output is locations in a colour space, which in turn is a description of how a human being should see the image. Turning the one into another requires much more processing than 'gain', and ascribing 'gain' as being solely responsible for one component of the colour space is misleading. Variable gain or no, the output value of all of the components need to be computed.
I don't think it's pedantry. When people are striving to understand something the make inferences from the words that are used to explain them. If you say 'ISO is gain' the general inference seems to be that ISO 'gains' light in some manner. This of course is completely wrong and leads to further misunderstandings, thus the word is to be avoided, especially as the statement that ISO is gain is factually incorrect.
