Are bigger pixels less noisy?

Started 4 months ago | Discussions thread
spider-mario
spider-mario Contributing Member • Posts: 897
Re: Are bigger pixels less noisy?

JahnG wrote:

But how about the combined read noise of 50M pixels compared to the combined read noise of 20M pixels. One might thus think that the sensor having only 40% pixel count would have much less combined read noise? (Or shouldn't we sum read noise?).

One would typically sum it in quadrature (sqrt(RN₁² + RN₂² + …)). So if you combine k pixels, each with read noise RN, into one combined pixel, you would consider it to have read noise sqrt(k · RN²) = sqrt(k) · RN.

So, if you apply this to an α7R IV at base ISO to reduce it from 61 MP to 24 MP, you arrive at sqrt(61/24) · 2.9 ≈ 4.6 e⁻ of read noise for the 24 million “new” pixels, which compares favorably to the 6.4 e⁻ of an α7 III and its native 24 MP.

At high ISO, the situation reverses somewhat: at, say, ISO 3200, you would be looking at sqrt(61 / 24) · 1.1 ≈ 1.8 e⁻ for an α7R IV reduced to 24 MP, or ~1.2 e⁻ for an α7 III.

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