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Raw files are supposed to contain mostly unprocessed data from the sensor. What's the point of having an ISO setting if it doesn't affect the amount of photons counted, but only provides amplification? Couldn't everything done in the camera be done better in post-processing?

In other words, why not keep ISO at 100 and adjust exposure later (aside from in-camera previews)?

Update: The three answers given to date are complementary to each other, as is the question given by mattdm. That means I've got a hard time selecting the correct answer... Will be glad for a summary. I'll do it myself in a few days if nobody steps up.

What still isn't clear to me is ISO on a CCD vs CMOS. Guess that's a different question.

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possible duplicate of How is ISO implemented in digital cameras? –  mattdm Oct 14 '12 at 21:07
    
Do you want to know more about ISO or RAW? Because the answer in short is because it still is unprocessed data even when ISO is applied. –  BBking Oct 15 '12 at 21:38
    
I think that what I wanted to know is really the thing that actually changes when you change the ISO and, importantly, how it affects photon counting in a way that makes ISO impossible to adjust in post. I gather that it's about voltage levels in photodiodes in CMOS - it gets lower with increased ISO, so it's more likely to count things that aren't photons entering through the lens (i.e. noise). The question then becomes why can't RAW files record voltage levels instead of photon counts? –  Baczek Oct 16 '12 at 20:58
    
Because RAW is compiled after the signal has gone through the amplifier. ISO doesn't affect the photon count solely, however for appropriate exposure, the shutter/aperture will need to be changed which then does affect the photon count. What changes is the output level of the amplifier. As explained in my answer, ISO is like the volume knob for that amplifier. It doesn't matter if it's a CCD or a CMOS sensor. You can't adjust the ISO in PP, however it's like the equivalent to ISO. The sensor has only one sensitivity level essentially. –  BBking Oct 23 '12 at 3:04

6 Answers 6

The ISO function on most digital cameras amplifies the analog signal prior to readout and digitization, which itself is a source of noise. If you just apply the correction digitally you amplify the read/quantisation noise as well as the signal.

Increasing ISO in camera to account for lack of light actually reduces the overall noise seen the image.

Here's an overused example comparing analog amplification vs. digital correction:

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Not to mention that you lose out on dynamic range because a weak signal doesn't take full advantage of all of the analog-to-digital converter's bits. –  Blrfl Oct 14 '12 at 17:33
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@Matt: Your sample image of the books comparing ISO 100 boosted in post vs. ISO 1600 would probably be helpful here. (I think that may be one of the most ubiquitous photos on PhotoSE now ;) –  jrista Oct 14 '12 at 18:14
    
oh go on then... –  Matt Grum Oct 14 '12 at 23:24
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My understanding is that one factor affecting the sensor over different ISO settings is that the operating charge voltage is different. That would affect how many electrons get kicked out by a photon. –  Skaperen Oct 15 '12 at 2:18

Turning up the gain in the sensor is not the same thing as lightening a dark picture in your image processing application.

You cannot restore in post-processing resolution which was not captured.

Suppose that the dark picture uses only four bits for each color: the values are in the range 0-15 for red, green and blue. Then, it is effectively a 12 bit image, with only 4 bits for each color channel.

No image manipulation can fix that.

Which is not to say that applying gain at the sensor will necessarily give full resolution to the image: there is the question of noise. But, in any case, the result will be different.

The "gain structure" of the signal processing pipeline matters. This is well known in audio. Turning up the gain on a microphone preamp is not the same as turning up the master volume on the PA instead even if the effect on overall loudness is the same.

"Raw" just means that the data from the sensor isn't put through lossy compression like JPEG. It has no direct bearing on these issues. Without adequate gain, the photons will not be counted. Gain is the sensitivity to the arrival of photons, so to speak.

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It's almost the same as asking Why is there ISO graded film?

Couldn't everything done in the camera be done better in post-processing?

Well, yeah. But aperture and shutter speed are forms of exposure control, just as ISO is. Would you ask the same thing about Depth of Field? What's the point of changing aperture when shooting in RAW if I can digitally enhance/change it? Because it's about the photographer's control, really.

if it doesn't affect the amount of photons counted

ISO actually does affect the amount of photons collected for "correct" exposure. The ISO/shutter/aperture all compensate each other. You increase one, another one falls.


Consider you have a consistent light source and fixed aperture/shutter speed values. Changing the ISO will not change the amount of light/photons captured. However, the resulting image will be either over/under exposed.

Example: ƒ/5.6, 1/125 shutter speed, ISO 100

If you increase the ISO but still want the same exposure, you'll have something like:

ƒ/5.6, 1/250, ISO 200 or ƒ/2.8, 1/125, ISO 200

In both cases, you've increased the ISO while decreasing the amount of light/photons captured.


When the light hits the sensor, it's still capturing analogue signal/s. After the analogue signal/s leaves sensor's output it goes through an amplifier. This is where the value of the ISO comes in, it's essentially the volume knob for the amplifier.

After this it goes through an analogue to digital converter. It can then be digitally processed, adjusted, compressed or whatever according to your camera's feature functions. Before any of this digital process, it's in the form of a RAW data.

why not keep ISO at 100 and adjust exposure later (aside from in-camera previews)?

Because a low ISO increases the shutter time (in auto mode) and you might want a faster shutter speed to prevent shake/blur or what ever else.

Deleted as I now think it's excessive information that I don't know enough about.

The basic answer is because it hasn't done any processing yet. It goes through the amplifier (that you control with the volume knob) first. RAW is the unprocessed image.

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I do not agree with the film analogy. Sensors use electronic amplification of the collected analogous signal while film changes overall sensitivity by actually changing the size of grains collecting the photons. If we apply this analogy to digital photography, we should change the sensor and use a lower resolution one (lower Mpx) to achieve higher sensitivity. –  Vincent Robert Oct 15 '12 at 9:07
    
Sure. But the thing about digital is that you don't need to do that. The only reason I said that at the start is because ISO was a photography concept before it even become digital. –  BBking Oct 15 '12 at 21:32
    
Minor note: increasing the ISO does not increase the number of photons hitting the sensor. It increases the strength of the resulting signal from those photons. Only the lense aperture and shutter speed can change the amount of light you're getting - but you can be more sensitive to that light. –  Jasmine Jul 22 at 16:21
    
I was saying it in a sense that if you change the ISO, you also change the aperture/shutter speed, for correct exposure. So, it affects the amount of light captured. However, if in manual mode and in settings ƒ/5.6 1/125 shutter speed fixed, but change the ISO... Yes, you are correct. If you change the ISO and nothing else, this does not change the amount of light/photons captured (if you have a consistent light source, of course) but the resulting image will be over/under exposed. –  BBking Jul 23 at 0:04

Hmm, I think ISO is the control to how much electric potential you put through the sensor.

Think of it like photovoltaics(solar cells), when it stores up an electric potential into a capacitor or battery, eventually the resisting potential stops more charge from building unless a stronger light source illuminates the photovoltaic.

The resistance of low ISO (meaning high resistance) is to eliminate uneven light strength from activating adjacent light sensors (which could introduce noise).

When you raise the ISO, it is something like lowering this potential and hence allows lower light to trigger an acceptable signal. What happens is, when a strong packet of photons hit the sensor, it illuminates and also trigger surrounding sensors, hence introducing noise.

Noise is visible in low light exposure pictures, but instead results in over exposure in high light exposure pictures(due to many adjacent sources lighting up... Think heavy rain on a swimming pool's water surface vs a few drops here and there. You can't tell where the ripples are forming in heavy rain, but they are pretty obvious in light rain).

So therefore, ISO is a physical attribute, not a digital amplification or whatnot. With low ISO, the light isn't even picked up as a signal. In high ISO, surrounding sensors pick up light sources that are unnecessarily strong.

Because of this, it is best to set the ISO just right to pick up weak packets of light, but not high enough that the surrounding sensors pick up that same light packet

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In other words, why not keep ISO at 100 and adjust exposure later (aside from in-camera previews)?

  1. The effect depends on implementation in your particular camera. Some cameras implement analog gain (of various extent) before the signal is digitized, some don't. You could shoot a set of exposures at various ISO, then look at the noise level and determine what is best for your particular camera model. Some details here. It seems that newer higher end cameras are often designed as ISO-less.

  2. Shooting all at base ISO and underexposing instead of using higher ISO in camera is a pain for reviewing the images. Ability to see my images right after exposure is one of the most important benefits of digital cameras compared to film. My camera may give marginally better results if shot at ISO 100 and "pushed" in editor, but the benefit of proper preview is more important for me than a little bit of noise.

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In other words, why not keep ISO at 100 and adjust exposure later (aside from in-camera previews)?

Simply - because you don't have enough lights. When you have enough lights it is a good choise to keep iso at 100. But, when you have less lights, you have three options to keep the right exposure: 1. You can change shutter speed to some value that is still ok, then 2. You can change aperture, and than and often 3. You have to change ISO value.

So, changing ISO value is needed when you don't have enough lighting.

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