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If I change the ISO settings on my camera, obviously the gain of the system is increased, amplifying the signal from the sensor. What's not clear to me is where the amplification takes place. I see several possibilities:

  1. In the sensor, by increasing voltage or some other mechanism
  2. Via an analog amplifier outside of the sensor
  3. Digitally, after the signal has been digitized, but before storing data in the RAW file
  4. As a parameter applied solely in creating an image from RAW

If #4 is true, then you could take a 4-stop overexposed RAW picture at ISO 1600, and then in post processing produce a JPEG at ISO 100 that would be the same as if the original photo had been shot at ISO 100.

If #1 or #2 is true, then a RAW file shot at ISO 1600 would actually contain more information about shadows, and an ISO 100 RAW would contain more information about highlights.

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1~2 and 3. On CCDs, the amplifier is effectively in the corner of the sensor, but on CMOS, there is an amplifier built into each photosite, dispersed throughout the sensor. See here.

As mentioned in one thing I recently discovered, most DSLRs have an amplifier before the ADC (Analog-to-Digital Conversion). They tend to max at 800 or 1600 ISO and are all digital amplifications afterward. The following paragraphs assume a camera that maxes out its analog amplification at 1600:

Unfortunately, the 12 or 14 bit RAW files prevent you from doing what you describe. The digital amplification takes place before the RAW files are stored. There is a maximum value that can be stored, so when you shoot 4-stops overexposed, even though the ADC is not saturated, the RAW file will probably be clipped. However, the technique that overexposes just as much as to not clip highlights is effective at reducing noise, and known as ETTR (Expose To The Right).

Yes, due to the analog amplification, RAW files at higher ISO do contain more detail. However, ISO 1600 and ISO 12800 should contain the same amount of shadow detail (unless there is some additional special processing OR the ADC has effectively more precision than whatever bit depth your RAW files are stored in).

Even though #3 is true above ISO 1600, an ISO 1600 RAW may contain more information about highlights because they can still be clipped through the digital amplification process. For this reason and perhaps others (battery life, effective buffer size), when shooting RAW, it may be beneficial to shoot ISO 1600 and simply post process later. Again, I have not tested this, and if the effective ADC bit-depth is higher than the RAW format's bit-depth, it will not be true.

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On Canon cameras, they have "expanded" ISO settings (H) and (L). I thought it was only the expanded settings that were digitally amplified, and that all of the "normal" ISO settings were done with the analog amplifier. That would mean that most of the higher end (and newer) Canons have a real ISO range up to 3200 at least, if not more. I think the 1D Mk IV has a normal ISO range from 100-12800, with 50, 25600, 51200, 102400 being the digitally enhanced ones. Is that not actually the case? –  jrista Sep 1 '10 at 5:56
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I've heard that one of the reasons to have L and Hx instead of actual numbers is that they're not calibrated, at least not as well as the common ISO values. Should be taken with healthy doze of skepticism. –  Karel Sep 1 '10 at 12:16
    
@jrista, my conclusion (not confirmed by companies) is that the (L) is may be sincere, and (H) is a marketing term (its the same as the others but looks bad so we'll separate it). I have not seen anywhere in literature where it claims the expanded modes is actually a line between amplification methods. Here is research that shows on a Pentax, 1600 or so it is analog, and after that it is ambiguous: forums.dpreview.com/forums/… forums.dpreview.com/forums/… –  Eruditass Sep 1 '10 at 12:33
    
Now, with my K-x, I have used anecdotal evidence to infer that above 1600, it is all digital. The main two observations are that above 1600 ISO, my K-x JPEG buffer size drops significantly. The other is that video is limited to 1600 ISO. While the latter may be a marketing decision (too noisy above 1600), the former shows there is some inherent difference between 1600 and above 1600 ISO. When combined with the information in the link above, I conjecture that the processor on the K-x is amplifying the data instead of clearing the buffer. –  Eruditass Sep 1 '10 at 12:34
    
On the K-x, the (L) equivalent is half sincere, and I believe they were just trying to copy other manufacturers. Much evidence points toward the base ISO actually being between 100 and 200, closer to 125, due to the DR differences. If it really had a base ISO of 200, there would be no way to get more DR at ISO 100. –  Eruditass Sep 1 '10 at 12:35
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Yes. The ISO setting is the sensitivity of the sensor -- in practice the amount of amplification applied to the signal from the pixel on the sensor to get a real result.

In theory, you could post process to produce a similar result to that of a lower or higher ISO using exposure compensation - I've always found I've got much better results shooting it right "in-camera", which leads me to believe that the amplification is done in an analogue manner prior to digitising.

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I do believe you're right: on my Canon DSLR, RAW files with ISO1600 can get a lot of noise –  Danny T. Aug 31 '10 at 19:40
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+1 for "shooting it right in-camera" –  Alan Aug 31 '10 at 20:40
    
@Alan You can't beat getting it right in-camera. RAW is great for white balance issues, but I try not to rely on it for exposure/metering as the results are never quite as good. –  Rowland Shaw Aug 31 '10 at 20:50
    
"In theory, you could post process to produce a similar result to that of a lower or higher ISO" ... Surely not? You can't undo the noise created by higher ISO. –  AJ Finch Sep 1 '10 at 10:32
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@Rowland I'm not sure I fully understand your comment, but yes the type of scene will strongly influence your exposure decisions, my point was just that I don't believe that exposing-so-you-don't-have-to-shift-in-post is necessarily the best choice. I agree that doing things right first time is preferable, however when it comes to exposure I don't think there is a "right". There are settings that preserve highlight detail settings to reduce noise etc. being able to adjust exposure in post is a useful tool, not just something to correct "mistakes". –  Matt Grum Jan 24 '11 at 15:25
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Warning: My answer is 100% anecdotal, and is based on results from a Panny FZ-28, which very possibly doesn't handle RAW's the same as a DSLR.

Having said that, I've struggled with noise levels in low-light conditions since day 1 with this camera. There's only so much performance you can wring out of a small sensor, but I've tried a number of things to try to get the best results I can.

Since I see a pretty dramatic increase in noise as I increase ISO, I usually try to keep the ISO pinned at or near 100. One of the things I tried, though, was shooting RAW at ISO 100 with the shot underexposed to maintain the shutter speed I wanted. Post processing lets me fix exposure (to some extent), but when I do that, I pick up an amount of noise that seems to be about the same as if I'd just set ISO higher to begin with. I see a very comparable noise level when I increase the ISO and exposure (so that I don't have to boost exposure in post-processing), leading me to believe that there's little to no room to "cheat the system" this way.

I haven't done any reading to support this, but based on what I've seen, I'd be inclined to believe that either #1 or #2 (personally, I believe it's #1) above are true.

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If #3 is true, then you could take a 4-stop overexposed RAW picture at ISO 1600, and then in post processing produce a JPEG at ISO 100 that would be the same as if the original photo had been shot at ISO 100.

No. If you take a picture with the ISO 100 setting you get a different exposure (more light on the chip) than with the ISO 1600 setting. No matter how the amplification (in hardware or software) is achieved, your ISO 1600 picture will carry more noise than an ISO 100 image.

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If you take two pictures of the same scene with the same aperture and exposure time, only varying the ISO, the same amount of light will hit the sensor during each picture.The photo produced by the ISO 1600 capture would be 4 stops brighter than the one at ISO 100, hence "4-stop overexposed". –  Evan Krall Sep 6 '10 at 5:45
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ISO has no affect on the amount of light hitting the sensor. ISO affects the resulting digital image brightness, whether it has been amplified before or after digital conversion. Shooting the same exposure with ISO 100 verse ISO 1600, however intrinsically means you have to have more light on the sensor, if that's what you're saying... –  Nick Bedford Jan 24 '11 at 1:18
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Yes, ISO setting does affect the image data stored in the RAW file. Here are two images, shot at the same time/place (almost - this was shot hand-held, without much attention to composition), with the same aperture (f/2.8) and shutter speed (1/100).

This first image was shot at ISO 100 and adjusted +2 stops in ACR to an effective ISO of ~400. All adjustments besides exposure were zeroed (No noise reduction, sharpening, contrast adjustments, etc.)

alt text

This image was shot at ISO 1600 and adjusted -2 stops in ACR to an effective ISO of ~400. Again, all adjustments were zeroed.

alt text

Notice the blown-out highlights in the second image. Same amount of light hitting the sensor each time, but different information recorded in the RAW file.

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RAW is pretty much a direct dump of sensor output to storage, so #1 and/or #2 are the obvious choices (though I think Canon does some manipulation of data before writing their RAW, and maybe so do others).

A good way to increase the sensitivity of a sensor is to increase the voltage applied over it. This both increases the output for given input, and increases noise (which are both indeed observed to happen when upping the EI on a digital camera).

I've difficulty in envisioning how you'd apply some kind of light amplification before the sensor though that can be adjusted on the fly (sensors do have microlenses over each element to collect light to that element, which determines the element size and have a small, but fixed, effect).

Option 3 would modify the RAW data after collection, which is the same as modifying the RAW file after reading it from the camera. Some cameras (see above) might attempt this, especially to filter out noise. Most will not and leave it to the user operator (in fact years ago there was a battle between Canon and Nikon where Canon seemed to produce much more vibrant images compared to Nikon, which were found to be caused by Canon applying in-camera manipulation to the RAW data, which caused quite some uproar from users who didn't want their cameras to do that).

Mind all this is for mid/highend DSLRs, with entry-level models there's probably a lot of in-camera filtering and enhancement going on, but people using those aren't typically RAW shooters and their RAW modes may well be clean too.

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