What ISO isn't

Started Sep 27, 2013 | Discussions thread
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bobn2
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What ISO isn't
Sep 27, 2013

In the series of very interesting threads on exposure, the subject of 'ISO' has been raised frequently. Sometimes ISO os referred to as 'sensitivity', sometimes it is referred to as 'amplification', sometimes it is referred to as 'gain'. It is none of these things. To see why, let's first consider the process of taking a photograph.

The input and output of a photograph

The input of a photograph is a stream of photons, which when incident on the sensor, builds up an image. The image area is divided into a grid of pixels, each of which counts the photons which hit it during an exposure. This count of photons represents the luminous energy incident on the pixel (SI units lumen seconds). Luminous energy is the energy that we can see, that is electromagnetic energy discounting those parts to which our eyes are not sensitive. It is derived by weighting the total photon energy by the luminosity function. In a camera the luminosity function is realised by the combined characteristics of the colour filter array and the infra-red cut filter placed in front of the sensor. It is conventional not to measure the amount of light as the total luminous energy collected by the whole sensor, but in terms of the intensity of luminous energy across the sensor. This quantity is called exposure, and is measured in lux seconds. However, whatever is the convention of measurement, what the sensor actually measures is photon counts in each pixel (scaled by some arbitrary measure), which is an indication of luminous energy, not exposure.

The output of a photograph is produced by the device or medium that is used to display it. For instance, an LCD panel has pixels that emit more or less light. The ones that emit the most light will be seen as 'white', the ones that emit least will be seen as 'black', with shades of grey in between. There is no absolute level of 'black' or 'white', indeed displays have different contrast ratios and are capable of emitting different amounts of light. A print works by means of reflected light, and in this case the amount of light that will be emitted by the print is determined by how much light is incident on it.

For this reason, the output of a photograph does not determine the absolute amount of light that will be emitted. Instead it represents a position of an arbitrary scale from 'white' to 'black'. The absolute luminosity of 'white' and 'black' will be determined by the display device, and it is left to the human visual system to interpret the brightest as 'white' and the darkest as 'black'.

ISO

The ISO exposure index represents a function that maps the input of a photograph, the exposure, to the output, the arbitrary scale from white to black. The Standard Output Sensitivity ISO specification dictates that the ISO rating is equal to 10 divided by the exposure (in lux seconds) required to produce an 18% grey in the white/black scale (in this case, the specific white/black scale defined by the sRGB colour space). The ISO specification says nothing about how the function will be realised. To illustrate this, at 100 ISO, and 18% grey is the result of an exposure of 10/100 lux seconds = 0.1 lux second. At 1000 ISO the same 18% grey is the result of an exposure of 10/1000 lux seconds = 0.01 lux second. Remember above that a camera counts the photons which hit each pixel of the sensor (times an arbitrary scaling factor). To produce an image at a specified ISO from an array of photon counts, the following operations are necessary:

1. Calculate the exposure represented by the photon count.

2. Calculate the resultant black/white value in the output 'space' (according to ISO, sRGB).

As an intermediate step, the photon counts may be stored in what is called a 'raw file'. It has become conventional in raw files to change the arbitrary scaling factor according to the ISO setting of the camera. This, however, does not change the quantity of photons represented by each pixel, all it changes is the conversion constant that will be required in the calculation in (1) above.

What ISO isn't

All definitions from http://www.thefreedictionary.com/

Sensitivity (electronics)

The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.

Note that the definition of ISO includes no specified SNR, and also that there is strictly no 'output signal' (since the output is purely relative), so ISO is not 'sensitivity'. As a side note, on some cameras the OSO control increases the analog gain before the ADC, which effectively increases the SNR of the system (but not the sensor) for low outputs. This action may indeed be seen as increasing 'sensitivity', but is not necessary for the definition of ISO.

Gain. n

1.

a. Something gained or acquired: territorial gains.

b. Progress; advancement: The country made economic gains under the new government.

2. The act of acquiring; attainment.

3. An increase in amount or degree: a gain in operating income.

4. Electronics An increase in signal power, voltage, or current by an amplifier, expressed as the ratio of output to input. Also called amplification.

Amplification. n

1. the act or result of amplifying

2. material added to a statement, story, etc., in order to expand or clarify it

3. a statement, story, etc., with such additional material

4. (Electronics) Electronics

a. the increase in strength of an electrical signal by means of an amplifier

b. another word for gain [13]

5. (Life Sciences & Allied Applications / Genetics) Genetics Also called gene amplification the production of multiple copies of a particular gene or DNA sequence. It can occur naturally or artificially, by genetic engineering techniques.

The operative definition here is 4 (gain) and 4 (a) (amplification) - an increase in strength of an electrical signal by means of an amplifier. But this is not what has happened with ISO, all that has happened is that one quantity has been quantified and translated to a number. The set of numbers is an arbitrary code, nothing has been 'increased in strength' by making them larger (since we don't even know what the 'strength' is until some output device is deployed. So, ISO is not amplification or gain.

What ISO is.

It's what ISO says it is, the 'Exposure Index'.

index n.

1. Something that serves to guide, point out, or otherwise facilitate reference, especially:

a. An alphabetized list of names, places, and subjects treated in a printed work, giving the page or pages on which each item is mentioned.

b. A thumb index.

c. A table, file, or catalog.

It is, in the sense of c, a table or catalog of sRGB grey values indexed by exposure.

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Bob

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