Why can a Sony sensor’s PTC roll off at “1x” analog gain instead of clipping normally?
Asked 8/5/2026
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2 answers
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I measured a photon transfer curve (PTC) on a Sony CMOS sensor and saw that at the nominal 1x analog gain setting, the curve starts to roll off around 700 LSB instead of behaving like a normal clipping point. I’ve heard that on some Sony sensors the true base/native gain may be higher than the lowest labeled setting, so the lowest setting may be implemented similarly to a “low ISO” mode rather than true lower analog gain. Could that explain the unexpected drop in the PTC, and how does this relate to base ISO/native gain versus digitally reduced low-ISO settings?
Originally by Landon Ham. Source · Licensed CC BY-SA 4.0
Landon Ham
1mo ago
2 Answers
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I don't know if the base/native ISO should be 142 or some other value... it is not always the lowest selectable numeric value.
But the ISO 100 PTC looks something like what you might expect if the lower ISO is achieved digitally. I.e. Nikons use Sony sensors and have "lo" ISO settings. These settings do not reduce the sensor's gain below base. So every step lower results in that much more sensor overexposure. Which results in the sensor saturation point being reduced. The photosites approaching/reaching saturation causes the gradual falloff rather than the hard drop of ADC clipping shown in the other plot.
The overexposure is corrected digitally during processing by the camera, or by the raw converter if it honors the meta tag directions (most do).
Originally by Steven Kersting. Source · Licensed CC BY-SA 4.0
Steven Kersting
28d ago
0
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Yes — that behavior can be explained if the labeled 1x setting is not the sensor’s true base/native analog gain.
On some Sony-based designs, the lowest numeric ISO or gain setting is effectively a pulled low setting: the sensor is exposed as if it were at a higher native gain, then the output is reduced digitally afterward. In that case, you are not actually lowering analog gain below the sensor’s base.
The result is less highlight headroom. Photosites begin approaching saturation earlier, so the PTC can show a gradual roll-off or drop before the ADC’s hard clipping point. That looks different from a normal top-end clip because it is driven by pixel saturation from overexposure relative to the true base setting, not by the converter simply running out of code values.
So if the true base gain is closer to something like 1.42x, the “1x” mode may behave like a low-ISO pull mode, which would fit the PTC shape you observed around 700 LSB.
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