What determines a camera sensor's lowest native ISO?
Asked 10/6/2010
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I understand how ISO affects exposure, but I’m asking specifically about the hardware limits on a camera’s lowest native (“true”) ISO. For example, if one camera has a base ISO of 100 and another has a base ISO of 200, what design factors in the sensor or imaging pipeline set that minimum native ISO?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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In principal the base ISO is determined by how much light can hit the sensor before the individual photosites become saturated (i.e. the signal they produce doesn't not increase in response to extra light). This is in turn affected by factors such as the electron well capacity of each sensel (how many electrons can be stored before saturation), the efficiency of the microlenses, transmission rate of UV, IR and low pass filters in front of the sensor etc.
High base ISO is not necessarily a bad thing, it can indicate the sensor is very efficient at gathering light. Likewise a low base ISO (along with allowing longer shutter speeds / larger apertures), may be good as it can indicate a high well depth which will allow more photons to be captured for lower noise / higher dynamic range.
edit: all things being equal fill fraction (the percentage of the CCD or CMOS real estate that contains light gathering circuitry) doesn't affect the base ISO.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
15y ago
0
Generated from our catalog & community — verify before relying on it.
A camera’s lowest native ISO is mainly set by how much signal the sensor can collect before its photosites saturate, plus how efficiently light reaches the sensor.
Key factors include:
- Full-well capacity: how many electrons each photosite can hold before clipping.
- Sensor/light-gathering efficiency: how effectively incoming photons are converted into signal.
- Optical stack transmission: how much light is lost through microlenses and the filters in front of the sensor (UV/IR/low-pass, etc.).
If photosites fill quickly, the sensor reaches saturation with less light, which tends to push the base ISO higher. If they can hold more charge, the camera can support a lower native ISO.
A lower base ISO is not automatically “better,” and a higher one is not automatically “worse.” A higher base ISO can reflect a very efficient sensor, while a lower base ISO can indicate larger well capacity, which may help dynamic range and allow longer shutter speeds or wider apertures in bright light.
So the minimum native ISO is essentially a consequence of sensor saturation limits and overall sensor/optical efficiency, not just a menu setting.
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