Are small sensors or pixels fundamentally limited in dynamic range?
Asked 7/4/2022
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I’ve heard claims that phone cameras top out around 11 stops of dynamic range because their sensors and pixels are so small. I understand the general idea that larger sensors often have better dynamic range, but I’m trying to understand the physical reason.
What actually limits dynamic range in an image sensor? Is it mainly the pixel’s physical size and how many photoelectrons it can hold (“full well capacity”), or could a small pixel be designed to hold much more charge in some other way?
In other words: is there a theoretical or physical upper limit to dynamic range for a given sensor or pixel size, even if sensor technology keeps improving?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
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Dynamic range is the difference between the minimum and maximum light (photoelectrons) that can be discerned/detected.
The minimum is determined by the system's noise floor; there must be enough collected for the signal to stand out against/above the noise. The maximum is determined by the photosite's full well capacity; which is just how many electrons it can hold (acting as a capacitor) prior to readout.
A decade ago a photosite's capacity was limited by its' diameter/physical size; and you couldn't really make them deeper due to limitations of the technology/materials.
However, in recent years manufacturers have been adding a second capacitor in parallel with the photodiode. This does increase the photosite's full well capacity (DR potential) without requiring the photosite itself to be larger... it does in effect makes the photosite "deeper." And most manufacturers are disabling it at higher ISOs where it is not needed (lower light levels), or they are making it optional/selectable. In either case, the result is a sensor with dual base ISOs and two levels of maximum DR capability.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
3y ago
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Generated from our catalog & community — verify before relying on it.
Dynamic range is set by two things: the brightest signal a pixel can record before saturating, and the faintest signal that rises above the sensor’s noise floor.
For image sensors, the bright-end limit is the pixel’s full well capacity: how many photoelectrons it can store before clipping. The dark-end limit is read noise and other noise sources. So dynamic range improves if you can either store more charge or lower noise.
Small pixels usually have lower full well capacity, which is why small sensors often have less dynamic range. Historically this was strongly tied to pixel size. However, it is not a simple hard rule that a small sensor can never exceed a fixed number like 11 stops. Sensor design matters too, and newer designs can increase effective charge capacity, for example by adding storage capacitance, while better electronics can reduce noise.
That said, for the same technology and resolution, a larger sensor generally has an advantage because it gathers more light and tends to have better signal-to-noise performance. So there are physical limits for any given pixel size and design, but they are not set by size alone, and they can shift as sensor technology improves.
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