I sorry, I don’t understand 
Then what does the ISO setting do? My photos get very noisy when I increase the ISO 
My understanding is that increasing iso increases amplifier gain. If the amplifier circuit is “clean,” then most noise would come from the sensor. That is the noise that is amplified.
Generally, increasing exposure (when possible) is best for reducing noise.
Edit: last sentence should read … increasing light collection …
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Thank you for your answer! It helped me understand ISO better 
That leads me to even more questions!
What is amplifier gain and what is amplifier circuit? Is it part of the sensor and/or how the sensor captures the light?
Does this help?
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Thank you for sharing the article!
It does help
I will do my best studying it since it’s quite informative yet dense 
Ok, good. Understand that this is a simplified explanation; there is much more to know about collecting light, converting that light to a charge, collecting the charge, then amplifying that as a current and digitizing it, etc.
Here is my take at a simplified overview. I’ve worked a bit on making sensors, but I’m not the expert here on cameras. Anyway -
The lens collects light and projects it onto a sensor. The sensor has pixels (little “light buckets”), and these have either red, green, or blue filters. Once light particles (photons) make it past the filter and into the silicon sensor, the photon creates a charge. This light/charge collection continues as long as the shutter is open.
Now with dim lighting or very high shutter speed, there is very little charge (signal) collected. There is some background charge in the sensor (usually quite small) and micro-variations in the quantity of light (called photon noise). This causes variation from pixel to pixel that is called noise.
With a brighter condition or longer shutter time (or wider aperture on the lens), more light is collected, but the same noise is present. So the ratio of signal to noise is improved.
This charge is collected and amplified. The amplification is set using your iso dial - higher iso gives higher amplification (also called gain).
With a high charge (or current) from the sensor, the signal does not need much amplification (gain). But if you turn up the iso too high, the amplifier could saturate which causes clipping.
But with low charge/current from the sensor, it is necessary to boost the signal by amplifying it, and this is where high iso is used. But remember that sensor signal/noise is low, and both the true signal and the noise are amplified. So, it makes more noise visible in the processed image.
So needing to use a high iso is a consequence of collecting a small signal at the sensor. But the photon noise is about the same regardless of the signal, so the signal/noise ratio is low, and both the signal and noise get amplified.
To improve signal/noise, collect more light. Don’t over-amplify (i.e. use the correct iso).
I hope this helps.
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Also, older cameras did do better for noise as ISO was increased compared to compensating in post. Some modern cameras are considered “ISO less” and there’s no noise improvement from increasing ISO on the camera VS adjusting exposure in post. Some are “dual ISO” where there are two special ISO values, base ISO and another, often around ISO 800 which changes noise performance, but other ISO values don’t impact noise significantly.
Talking about ISO and noise on my Canon R7. I find two sources of noise are underexposure (or exposing to the left) and increasing the ISO number. But I shoot bracketed exposures where the ISO changes between the shots. What I have come to realise with my Canon R7 is that increasing the ISO to very high numbers exposes to the right and produces very similar noise to the under exposed image exposed to the left. So while my R7 is not ISO invariant the effect of ISO on noise is counterbalanced by the brighter exposure in most cases. But under exposure seems a greater source of noise for my R7 than ISO. This is not the case for all brands of cameras and I only reflect upon my experiences with the R7.
Hi @Terry
So in my parlance above, you get lower signal/noise when you collect less light by either: under exposure or by increasing iso (gain), which causes the camera to collect less light. To me that makes sense. There are of course some subtleties that I did not cover (and some I do not even know about).
The bracketing situation may be a result of the same light collection with different gain. If the amplifier circuit is clean you should have about the same signal/noise in each exposure. That is an approximation that (these days) seems to hold over a limited iso range.
Have you tried exposure bracketing at fixed iso in either aperture or shutter priority? There you should see variation in s/n as long as exposure bracket is wide enough. It might be easiest to see such a result at elevated iso, but in principle it should be the case even at base iso.
Of course, turn off noise reduction in your raw developer, and turn off any noise reduction in-camera.
I think that part of the confusion is that what we call “exposure” is determined after amplification. So that term confounds light-collection and amplification. That is how I think of it, anyway.
In the film days, of course, there was no such circuit, just film types with different sensitivities. So, once you load a roll of film, the only way to change exposure is to change the amount of light that is collected. Digital is different.
So, to beat this dead horse, collect more light to improve signal/noise.
Yes, there are some subtleties (e.g. with dual-amp circuits), but the general trend follows.
FYI I also think that this explains much of the “dynamic range vs iso” trend chart that was shared earlier. As iso is increased, for the same “exposure,” less light is collected, reducing s/n, and effectively raising the “noise floor.” Curves from most camera sensors are parallel in these plots, with offsets that come from pixel size and sensor/amp technologies (sensor mostly). My thinking, anyway.
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The only time I used P mode was for the first 2yrs I used a DSLR then I switched over to Aperture when I changed over to Raw from Jpeg. I use Manual mode when I do long exposures.
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I like [P] mode in that the camera can do most of the work but on my Canon R7, after trying it several times, I have now switched to [Fv] mode. With this you use 1 dial to switch between what you want to change - an icon moves between SS, f/, ISO, and Exp Comp. And the other dial changes that setting. A single press on the “delete” button sets that feature to Auto, and the display shows this with a line under it, and a long press sets all to Auto. In this mode you can effectively switch the way of working normally given by P or Tv or Av with or without auto ISO. I am still getting used to it - I still often press the ISO button on the top to make changes, and sometimes have to stop and think. But I don’t think I will go back. I guess it will allow me to assign other uses to buttons. I have 2 back buttons for auto focus and the video button for switching between full silent electronic shutter and 1st curtain shutter.
The way you describe this approach, it doesn’t look much quicker. For example, to change aperture, you first have to switch from auto to aperture mode and then dial the required aperture. Same for shutter speed.
In P mode the step of switching is removed.
Yes, if you know in advance exactly what aperture or what shutter speed you need, you can do it faster because you can see already the amount of rotation needed to get to those value (here I’m assuming that the camera has physical dials).
But auto mode, of which P is a particular case, exists precisely because you don’t always want or need to choose an aperture or shutter speed, because it doesn’t really matter.
These are not just cases of point-and-shoot, but also cases where you might want, for example, to test a different depth of field, and you are not necessarily pressed for time, so you just turn a dial which advances through the available apertures for you, while adjusting in real time the shutter speed.
The fully auto mode leaves you with no control at all. The aperture, shutter speed and (not fully) manual mode requires that you know already what these parameters need to be.
This is a bit of a necro post, but I’ve just found out about this mode on Pentax cameras, and it sounds great.
Basically, when in P mode – what Pentax calls Hyper Program mode – you can choose a “program line” which adjusts the exposure to the best settings based on your creative vision or scenario. I quite like the MTF priority program line, which automatically chooses the lens’s sharpest aperture.
But what I especially like about the P mode on Pentax is that you automatically shift into Aperture priority if you rotate the front dial and automatically shift into Shutter Speed priority if you rotate the rear dial. Then you can quickly revert back to the original P mode settings with one press of the “green” button.
I have never owned a camera that has a P mode like this. All of my cameras have taken control away from me in P mode, and even though you can “shift” the exposure settings, you are essentially just adjusting the shutter speed. When looking a bit deeper, it seems that Canon implemented something similar with its Fv mode. It still doesn’t sound as easy and neat as the Pentax solution, but it has that same workflow of shifting into A or SS priority when you start rotating dials.
I have given a lot of thought to how I could achieve something similar with my Sony and OM cameras, and basically it’s not possible. The closest is perhaps Manual with auto ISO, but the problem with this is that M mode automatically defaults to the last settings used, which could be totally inappropriate for the scene in front of you. So, the beauty of P mode is that it will default to some optimal settings, so you have a good starting point from which to adjust further.
For now, my solution has been to save some commonly used starting points in my 3 custom banks, so it will default to those settings instead of the last settings used. I just then need to switch between 1, 2 or 3, leaving all other modes alone. But it still requires a mode dial switch, which is not very ergonomic.
But I’m left baffled why only Pentax and Canon seem to have embraced this flexible approach, because it sounds ideal to me, unless I’m missing something. In the case of Canon, Fv mode is apparently still fairly niche, and most users have stayed with Av, Tv or Manual.
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Fuji sort of pioneered that flexible priority mode. It’s been a thing since the X series appeared.
I also don’t understand why Fv on Canon is not more popular, my guess is resistance to change (and gatekeeping “real photographers use M mode”, we all know someone who uses that argument). I never use any other mode (except for astro, which I barely even do these days).
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Exactly.
As only ever owning Fuji, these mode talks hurt my head. I just put stuff on Auto when I don’t want to think about them, and take them off when they are a physical or creative requirement, such as shutter speed for birds or aperture for bokeh/DOF, it’s as simple as that, why over complicate?
Exposure comp on the front dial, Auto ISO till 6400, and just roll with it.
Sometimes during cloudy or low light days for birds, I’ll just use ISO locked at 800 (dual ISO second threshold), aperture wide as possible, and then adjust with shutter speed for not blowing out highlights.
Sometimes I get some imposter syndrome reading stuff like the mode talk above, as if I am missing something when photographing 
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Yes, you’re missing something, as I have. The pain of fiddling with buttons and dials and aching over having used the wrong setting when you could’ve just been taking pictures
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Well, that depends on the situation.
Close-up/macro need high F-numbers if you want something resembling depth of field. Flying birds are easier to get sharp with short exposures. Not sure how a landscape looks at F2.8.
I.e. as soon as I want or need to control one or two of the three factors, P-mode is not my first choice.
As close-up makes up a large part of my images, I prefer selecting the F-stop myself, and limit the max ISO. And when working from tripod, either fixed diaphragm and ISO, or manual (flash, so then exposure time is more or less fixed)
Of course, if you’re into specialised stuff like macro, astro or motorsports it doesn’t work for you for obvious reasons. But the majority of use cases it covers it perfectly.
Yes, I deliberately didn’t mention my Fuji in the longer post because it works differently and does offer nice flexibility. But some people gripe about how top physical dials slow you down, and I get their point if you’re the type of photographer who is always glued to the viewfinder. For example, if I quickly want to adjust shutter speed with the dedicated dial, I need to take my camera away from my eye and look at the dial because the options either side of Automatic are 8000 and Bulb, which is rarely what I want!
With my X-T5, I tend to use the dials and the lens aperture ring, so I’m not actually sure if the camera retains the same mode flexibility if you use the command dials instead. That’s something for me to experiment with.
Simon d’Entremont on YouTube has a good video on that Manual mode myth. He’s a bona fide professional, and he argues that most professionals actually use one of the priority modes or at least M with auto ISO. It obviously depends on your genre, but Manual mode is certainly not the ultimate mode. It just offers the most control, but you often don’t need to control every setting for every shot, and all it will do is slow you down.
Similar to video game characters, people like to obsess over creating the optimal “build”. It’s a meta game, and I think photography is similar. 98% of the time, I’m just happily in A priority mode and enjoying my photography. But every now and again, a scenario pops up when you struggle to get the right setting dialed in at the right moment. It’s those times when I start to wonder “Is there a better way?”. But it’s not a major source of frustration. It’s a challenge I enjoy solving. I can’t speak for everyone, but that’s how I see it.
I’m lucky (and broke) in that I own 3 different cameras from 3 different manufacturers, and they all have their unique quirks. But none of them are better than the others at everything, which is why I use all three and not just one. So I take the best features from each one and try to mimic them on the ones lacking them. Right now I’m trying to see how close I can get to Canon’s Fv mode on my cameras. The Fuji is obviously the closest.
If you’re completely happy with your workflow, then you’re definitely not missing anything. But personally, I’m always on the lookout for new tips and hidden features that I may not have known about, even after several decades of this being my hobby. It’s also a way to keep things fresh when I’m feeling a little stuck in my ways. Admittedly, I get a bit too obsessive about these things and always think there’s a better way, even when there’s not
I had previously dismissed P mode entirely, but this thread made me look at it more closely, and I’ve discovered some very interesting things about how Pentax and Canon do it, which has made me get to know my own cameras much better.
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