Is ISO part of exposure?

Started Sep 23, 2013 | Discussions thread
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blue_skies Forum Pro • Posts: 11,453
Is ISO part of exposure?

YES IT IS, exposure is not a "triangle", it is a "square" (light, film_speed(ISO), time, aperture) relationship. This means that the choice of ISO strongly relates to the amount of light received.

Think of ISO (popular naming) as the old film-speed emulsion. Emulsions were developed to be logarithmically related to each other, so that  e.g. ASA 40 was twice as sensitive as ASA 20, and so on. As a photographer, you could half the shutter time with the 2x more sensitive film, at the cost of more grain with each higher sensitivity emulsion film (coarser, more light-sensitive, grain).

Digital cameras are modeled using the same principle. Choosing an ISO-value affects your other 'triangle' parameters, and is definitely part of the exposure. It also affects noise levels, but this is coming together in a different way than in film days, and part of the reason why we can get such much higher ISO values.

For those that believe that the sensor 'captures' light, and then the ISO is applied afterwards - they are technically correct. That is, the pixels always collect the same amount of light. This date is then  read out using amplification. (Note: some newer sensors seem to be able to boost the pixels 'receive' sensitivity, but photons (emitted light) is a constant factor based on illuminance, so I leave this outside the text).

But staying to close to this "ISO independent sensor" thinking gets you into trouble, as the ISO rating is more complex than that. The camera manufacturer has already made some choices for you, which, effectively, means that you have to see ISO as part of exposure, even in RAW capture.


Anyway, reason for the post: I noticed on the Open Talk forum that there is a lot of misconceptions on ISO and what it means to digital cameras. I just noticed 5(!) threads being filled up, almost 600 posts total.

Ok, that was the short explanations. I now will try a brief summary that explains the above in as few words as possible:

The easiest explanation (I find) is in the Wikipedia link

The (lengthy) Wikipedia article explains the history (particularly ASA and DIN), and the proper ISO definition(s). ISO itself means little, it is just what it it is: International Standards Organization.

Below is not my writing, it is merely a simplification of the Wikipedia article so others can get their head around it, but reasoned from the manufacturers perspective.

1. ISO is a standard.

ISO - in film speed - relates to ISO 12232-2006 standard, which was revised in 2006. It still only affects the JPG (and not the RAW) output of a digital camera, and does allow the manufactures to include FW/SW processing.

The 2006 addition defines the SOS (Standards Output Sensitivity) which greatly helps in understanding ISO in digital cameras.

2. ISO is empirical.

This is the best way that I can put it. In the SOS standard, three measures are taken into effect, taken on an 18% gray level:

  • a) SNR40: this means that only one out of 40 pixels will be wrong (noisy) and this is the high-standard. (popularized, tecnically, this is calculated as a standard deviation). 
  • b) SNR10: this means that one out of 10 pixels will be wrong, and this is the low standard (the picture is still recognizable). 
  • c) SNR_SAT: this is the level at which the image does not yet clip or bloom. 

SNR40 and SNR_SAT are measured at daylight, SNR10 goes through its gain-up stage:

3. Digital Camera control of ISO.

There seems a lot of mystique here, but this is really very simple. The camera has a sensor, which has an ADC (Analog-to-Digital-Converter) which produces digital readout data. Inside the path to the digital readout is an analog amplifier. This amplifier can be set to different levels (gain-stages). Of-course, at higher levels, the amplifier will induce more noise.

Also, at higher amplification levels, light levels are typically lower, meaning that the photo-sites may introduce noise on their own - e.g. by missing to register a photon in the exposure window.

Next, the in-camera processing, applied to the JPG can use different techniques to filter noise. NR control can lead to loss of image detail, as noise and fine detail will end up looking similar at certain control levels (and get removed).  A nickname is the 'smeared' or 'watercolor' look - meaning loss of fine detail.

But back to the amplifier - just to simplify this explanation. If the amplifier has 6 levels, the camera can be measured for the SNR10 noise at 6 different settings. Each level, if full steps, would double the sensitivity to light, and the level when one pixel is wrong for every 10 pixel becomes the higher ISO level that can be used.

The lowest level of SNR10 is effectively one half of SNR40. This give the SNR10 number a range. This range is the range in which the camera can be operated (low to high ISO).

4. Putting it all together.

As you already guessed, SNR40 is the base ISO of the camera, as expressed in JPG output. This number is typical between 50 and 200 ISO for most current cameras.

SNR10 typically starts at one half of SNR40, and, depending on the amplification settings, it can go much higher. ISO levels of 12,800 and higher are no longer unusual on digital cameras.

SNR_SAT is the measure at which the sensor over-exposes under daylight conditions.

Wikipedia shows two examples what this means: The ISO rating of a camera is expressed by taking the SNR40 value as base ISO, if it is lower than the SNR_SAT. If not, the SNR_SAT has to be taken. The SNR10 value (or range) then defines the operating range for the camera.

The two examples:

1. SNR40=107, SNR10=1688 (max), SNR_SAT=49 ==> ISO is 100, speed is 50-1600

2. SNR40=40, SNR10=800 (max), SNR_SAT=200 ===> ISO is 200, speed is 100-800

In practice, the ISO range is not just the gain-up stages, camera mfgrs can add (or delete) speed stages by using in-camera FW, as the standard is measured in the JPG image.

5. Back to RAW (not in Wikipedia article)

In its truest concept, RAW data is just that: it captures the data from the sensor, before any processing is done to it.

So, you can change the exposure in RAW, by simple changing the parameter, and multiplying all the data with an exposure-value. That is, you can decide on ISO 100, ISO 200, ISO 400 during post-processing, right?

Well, yes and no. A manufacturer may allow for 'in-between' iso values (360, 480, 620, that are not real ISO values. These are SW processed, and applied to the JPG - so you see a different exposed image. In RAW such ISO values do not exists, as only the gain-stages make a difference to the captured data.

Depending on the camera model (and vintage), ISO settings may affect all, some, or none, of the RAW data values. I would suggest that some is most often correct.

This implies that choosing ISO does  have an affect on RAW capture, but to a lesser degree as it has to JPG images.

Of-course, you can re-expose a JPG image in post, but since the image is already processed in-camera, the noise is already 'embedded' in the image, and therefore you bring in more noise than a RAW process would do.

In both JPG and RAW, an over-exposed image (blow-out) is unrecoverable, and sometimes unavoidable (in certain areas).

Simple, right?

(I hope this is simple, and still 'correct enough' to make sense) - please add/delete/flame sections that are incorrect or confusing - I am merely adding to the debate.

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