We’ve seen how information about a photographic scene is collected in the ISOless/invariant range of a digital camera sensor, amplified, converted to digital data and stored in a raw file. For a given Exposure the best information quality (IQ) about the scene is available right at the photosites, only possibly degrading from there – but a properly designed** fully ISO invariant imaging system is able to store it in its entirety in the raw data. It is able to do so because the information carrying capacity (photographers would call it the dynamic range) of each subsequent stage is equal to or larger than the previous one. Cameras that are considered to be (almost) ISOless from base ISO include the Nikon D7000, D7200 and the Pentax K5. All digital cameras become ISO invariant above a certain ISO, the exact value determined by design compromises.
In this article we’ll look at a class of imagers that are not able to store the whole information available at the photosites in one go in the raw file for a substantial portion of their working ISOs. The photographer can in such a case choose out of the full information available at the photosites what smaller subset of it to store in the raw data by the selection of different in-camera ISOs. Such cameras are sometimes improperly referred to as ISOful. Most Canon DSLRs fall into this category today. As do kings of darkness such as the Sony a7S or Nikon D5.
Refer to the earlier article for a more in-depth description of the diagram above and remember that this is a simplified model chosen for ease of getting the point across. Many cameras do not actually work like this, but in many cases this model represents a decent and intuitive approximation.
Non ISOless: Downstream DR Lower than Sensor’s
For a given exposure this is the path that faces scene Information collected in the non ISO invariant range of a digital camera on its way to being converted to digital numbers and written to the raw file.
Contrary to the situation in the earlier ISOless case, where the noise floor* of each stage was the same or lower than at the photosites, by the time the signal of this imaging system as set up reaches the ADC and related circuitry the noise floor is two stops higher than at the photosites. This means that for a given Exposure the photosites can collect more information about the scene than the electronics are able to process into raw data in one go. This effectively reduces the information carrying capacity of this system by two stops from 14 to 12 bits (photographers would call it Dynamic Range, in stops).
Same IQ as ISOless in Highlights
At base ISO such a system records highlights like its ISOless counterpart does, all else being equal. But as the Signal dips into the shadows the camera’s noisier ADC input results in SNR worsening faster. By the time the Signal gets into the deepest shadows random digits are encoded into the raw data where information from those last two stops should have gone. The system is not able to transfer the last two stops of tonal information from sensor to digital numbers properly, so while ‘Dark Detail’ tones in the diagram above are acceptably present at the photosites they are gone forever by the time the relative information is written into the raw data.
Tone Selection by ISO
Since deepest shadow tonal information is present at the sensor’s photosites, however, the photographer can choose to amplify it before it gets to the ADC’s noisy surroundings so that it is no longer below the noise floor. They do this by raising ISO, in this example by two stops. As explained in the earlier article, if ISO is raised with a fixed Exposure highlights are lost to clipping stop for stop, so the DR of the resulting information will still only be 12 stops. But at least the photographer will have chosen what 12 stops out of the 14 available at the photosites to save in the raw data. This is what raising ISO from base to 400 does to tone information transfer:
The entire Signal out of the photosites is increased by two stops, including related noise. Therefore the Signal to Noise ratio (SNR) out of the amplifier remains the same or similar as at the photosites. In so doing, however, the top two stops of highlights were pushed beyond the ADC’s range, clipping them off. But at the same time the desired ‘Dark Detail’ information was encoded with an acceptable SNR, therefore being saved usefully into the raw data. In addition every other tone from the photosites was amplified, therefore increasing the Signal relative to noise at the input of the ADC thereby improving relative SNR in the shadows.
Improved SNR in the shadows: As at the Sensor
That’s why when, for a given exposure, one raises ISO in a camera in a non ISO-invariant range the deep shadows recorded in the raw data appear cleaner: they have higher SNR, better IQ. In fact what the photographer has done is made a conscious choice to capture in the raw data the better SNR available at the photosites in the deep shadows, giving up in exchange highlight headroom.
We could have raised the ISO to 200 only, thus compromising just 1 stop of highlights but then saving just one stop of shadows, final DR unchanged at 12 stops. And you can see that in this example raising the ISO above 400, say 800, would have lost three stops of highlights but only netted two stops of SNR improvement in the shadows, thereby reducing DR to 11 stops. In other words, this exemplified camera is ISOless above ISO 400, where raising ISO reduces highlights stop for stop without providing an improvement in SNR.
Information about the scene (IQ) is best right at the photosites and can only be degraded from there by imperfect information transfer. Ideally all properly designed** imaging systems would be ISO invariant, that is capable of storing all scene information collected by the photosites during Exposure into the raw data. That typically requires short paths from pixels to ADC, which today means bringing the ADC on board the sensor chip forcing compromises elsewhere (for instance readout speed). We can’t have our cake and eat it too, yet.
* I know this is overly simplified, but follow along for the sake of clarity.
** In a well designed imaging system the encoded bit depth needs to be maintained higher than the ratio between the capacity of the ADC and read noise at its input, in the same units, expressed as a power of two logarithm.
Only the manufacturer can measure the actual noise level at the input of the ADC. What we can estimate instead thanks to Photon Transfer Curves is the random read noise referred to the output of the photosites in physical units of photoelectrons (e-). If analog amplification and transfer of the e- to the ADC adds little noise, we can assume that the estimated noise out of the photosites is about the same as that at the input of the ADC. That is not always the case. This subtle difference can sometimes result in interesting PTC responses near base ISO for overly clean sensors, even with an estimated input-referred read noise larger than 1 DN/LSB (latest Exmors, see for example some curves at Jim Kasson’s).