Does ISO 50 on a digital camera reduce the sensor signal or just apply processing?
Asked 3/21/2016
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I understand that digital ISO is related to amplification of the sensor’s readout. Is the lowest ISO setting always just the minimum amplification, or can a camera’s extra-low ISO setting reduce the signal instead? For example, on the Sigma dp1x, if I select ISO 50, is the sensor actually less sensitive, or is the camera exposing at its base ISO and then pulling the image down afterward?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
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Sensors only have the one sensitivity, called native sensitivity, of how many photons were collected.
Higher ISO simply multiplies, to shift the histogram data up, brighter, but bringing low noise up with it.
Lower ISO 50 simply shifts the histogram down, dimmer, which lowers noise too, but also limiting the dynamic range, the top half of range is unused. The visual histogram is adjusted accordingly (blank top half is not shown). Nikon does not ever call it ISO 50 since range does not meet ISO specifications. Nikon calls it LO 1 (1 EV down).
Originally by user38978. Source · Licensed CC BY-SA 4.0
user38978
10y ago
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Generated from our catalog & community — verify before relying on it.
A digital sensor has one actual sensitivity: its native/base ISO. Higher ISO settings generally increase gain on the readout, which brightens the recorded signal but also raises visible noise.
An extra-low setting like ISO 50 is often not a true lower sensor sensitivity. If the camera’s real base ISO is 100, ISO 50 is commonly a pulled setting: the camera exposes as if at ISO 100 (often with 1 stop more exposure) and then reduces brightness in processing. That does not make the sensor less sensitive.
The tradeoff is reduced highlight headroom and dynamic range, because you’re effectively overexposing relative to base ISO and then darkening afterward. Some makers label this as “Lo” rather than true ISO for that reason.
Based on the answers, the Sigma dp1x appears to have ISO 100 as its lowest standard/base ISO, so ISO 50 would likely be an extended pulled setting rather than true reduced analog gain.
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