How is ISO implemented in digital cameras?

Question

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.

Answer 1

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.

Answer 2

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.)

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

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.

Answer 3

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.

Answer 4

The camera lens projects a miniature image of the outside world onto the surface of a light sensitive image sensor located inside and at the rear of the camera. This image sensor is covered with millions of photosites. Most photosites are overlaid with red, green or blue filters. During the exposure light from scene plays on the photosites. These receive photon hits in proportion to scene brightness.

The photon hits generate an electrical charge within the photosite. The strength of this charge is proportional to the number of photon hits. The hit count is a combination of how long the shutter remains open and scene brightness and the working diameter of the lens (aperture). If the scene brightness is frail the camera automation or the photographer enlargers the working aperture and or the shutter speed is slowed down. If a fast shutter speed is chosen the working aperture must be increased etc.

In every case, the magnitude of the charge at the end of the exposure is puny. Logic built into the chip converts the charge to a weak analog signal that is then amplified and converted to a digital signal. It is the application of amplification that you are asking about.

If the charge is weak, the amplification must be increased. The charge will be weak if the scene is poorly lit. The charge will be weak if the shutter speed used is super-fast. We open up the working diameter of the lens as this allows more light energy in. We can bring to bear artificial light if scene brightness needs bolstering.

Let’s talk about amplification: Amplification in the digital camera corresponds to turning up the volume of a radio or TV. In the digital camera, higher the amplification increases the picture taking opportunity dimly illuminated scenes and or high shutter speeds. Increased amplification is categorized as a high ISO setting. ISO stands for International Organization for Standardization. They are responsible to standardize photographic film speed sensitivity and the digital camera industry has adopted the ISO method of sensitivity labeling.

Increased ISO comes at a price: For most imaging sensors, each photosite has its own amplifier. Each is likely to amplify a little differently than its neighbor. When the amplification is too high sights that should reproduce as black often register as dark gray. This sends false information that we call noise. Additionally if the amplification is too high, some of the charge can leak into the adjacent photosites, we call this blooming. Yes, high ISO can and does degrade but it also allows photography under challenging conditions. Technology marches on the newer imaging chips and camera software better mitigate noise and blooming.

Answer 5

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.

Answer 6

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.

Answer 7

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.