willie_901
Veteran
For digital imaging very low ISOs (25) are a challenge. The pinned photo-diode electrical charge density (electrons per unit area) must be very high. Few commercial options exist.
A base, or native, ISO of 100 is available for many digital cameras.
I don't think "low ISO noise" is a thing. If we assume maximum sensor exposure, at base ISO the electronic noise is very low. The photon (quantum or shot) noise is also as low as possible.
As ISO increases the meter tells us to decrease exposure. The decreased exposure is purposeful. It becomes possible to use shorter shutter times and, or narrower apertures. But compared to base (native) ISO, exposure decreases. So, the analog DC signals from the sensor are amplified to make full use of the analog-to-difital converter. The best data stream technologies add almost no electronic noise due to analog signal amplification. However the photon noise increases because less light is recorded. In contemporary digital camera images photon noise is much higher than electronic noise. Photon noise is beyond human control since it an inherent property of light.
The maximum analog dynamic range when the shutter is open depends directly on the signal-to-noise ratio – which depend on exposure. The analog dynamic is always highest at base ISO. A sensor with a 25 base ISO would have very high dynamic range because the maximum possible signal level would be four times higher than a camera with a base ISO of 100. This link compared measured dynamic ranges for the Nikon D810 and some Leica M bodies. The D810 has a base ISO of 64. Note how the dynamic range becomes similar for all three cameras by ISO 200. Also, the decrease in dynamic range as ISO increases is almost identical as well.
Here's a comparison of the electronic noise (no signal present) vs ISO for the same cameras. The electronic noise does not decrease as ISO is lowered below the native ISO. The M10 noise level at ISO 100 is barely higher than the D810's at ISO 64. However the signal levels would be different. The increase in signal levels is responsible for the higher dynamic range at base ISOs 64, 100 and 200 for the D810, M10 and M240 respectively.
Practically all digital cameras offer ISO settings below base ISO. These are artificial values achieved by setting exposure above the metered value and then decreasing the raw file numbers by digital division. These lower ISO settings are a convenience to avoid ND filters. The noise vs ISO data above shows for contemporary cameras electronic noise levels do not increase at artificially lower ISOs .
