Are bigger pixels less noisy?

Started 4 months ago | Discussions thread
OP JahnG Veteran Member • Posts: 3,323
Re: Pixel size comparison & Aptina DCG

alanr0 wrote:

JahnG wrote:

Iliah Borg wrote:

JahnG wrote:

how about the combined read noise of 50M pixels compared to the combined read noise of 20M pixels.

The combined read noise can be marginally, and I mean marginally, higher than the noise of a single pixel. Decreasing the maximum charge opens a possibility to improve performance in terms of noise.

Mr. Claff kindly keeps the original Aptina white paper on dual gain, it might help explaining the relationships:

Thank you for your comments and for the Aptina white paper.

I'm not sure but I think that I have been told that (newer?) Sony sensors might utilize the Aptina aproach of using both HCG and LCG? But even if that is the case, probably not the sensor of my 6 years old DSLR, the Pentax K-S1 APSC camera? (BTW, if I didn't get it wrong i understood that Aptina finds it better to use larger pixels?)

My take from the Aptina paper is that dual conversion gain allows large pixels to be used without impairing low ISO dynamic range.

It may help to take a closer look at Bill Claff's results for the Sony cameras reference by spider-mario.,Sony%20ILCE-5000_14,Sony%20ILCE-7M3_14,Sony%20ILCE-7RM4_14,Sony%20ILCE-7SM3_14

Note that the vertical scale is base 2 logarithmic, so each division is a factor of 2.

I have added the results for your Pentax K-S1. Results for the 2014 vintage Sony A5000 are very similar, with around 3 electrons RMS read noise around ISO 1000. The curves diverge where noise-reduction kicks in at ISO 5000 for the Pentax (open symbols).

The more recent Sony cameras all show much less high ISO read noise, at around 1 e- rms. They also show a steep step in noise near the unity gain ISO, where the gain corresponds to 1 ADC unit for each electron captured.

The step is where the dual conversion gain switches in. At lower gains, voltage noise from the source follower transistor connected to the floating diffusion is pretty much the same, but conversion gain is reduced by the additional capacitance switched in (Aptina paper, figure 5). The input-referred noise increases by an amount proportional to the extra capacitance.

The increase is largest for the large pixel 12 Mp A7Siii (which needs a greater capacitance to avoid saturation at ISO 100), lowest for the small pixel 61 Mp A7Riv, with the 24 Mp A7iii intermediate.

Sony A7Siii (12 Mp, 8.4 µm pixels, FWC 187 ke-, 10.8 e- rms noise at ISO 200).

Sony A7iii (24 Mp, 5.96 µm pixels, FWC 96 ke-, 5.4 e- rms noise at ISO 200)

Sony A7Riv (61 Mp, 3.76 µm pixels, FWC 36 ke-, 2.9 e- rms noise at ISO 200).

The ISO 200 performance of the Pentax K-S1 and Sony A5000 fall between the A7iii and A7Riv results, but input-referred noise rises more steeply at lower ISO.

Pentax K-S1 (20 Mp APS-C, 4.28 µm pixels, FWC 28 ke-, 3.7 e- rms noise at ISO 200)

Full well capacity and other parameters derived from DxO results on Bill's site are here.

Down-sampling to similar pixel count

If you are shooting at low ISO, and down-sampling to 6-12 Mp, then the A7Riv should deliver better dynamic range and sharpness.

Assuming noise adds in quadrature, I estimate 6.5 e- rms input-referred noise at ISO 200 after down-sampling from 61 Mp to 12 Mp, compared with 10.8 e- from the A7Siii.

In practice, there may be a futher improvement, since the down-sampled image will need less sharpening to compensate for Bayer interpolation.

To summarise and simplify for these 5 specific cameras:

  • At low ISO (100 - 200) larger pixels are noisier in term of input-referred noise per pixel.
  • With dual conversion gain, there is less variation of high ISO noise with pixel size. Larger, fewer pixels, gives lower noise per unit area.
  • More recent cameras have lower noise than older cameras.

If you are shooting at high ISO and only need 12 Mp or less, then the large pixel Sony A7Siii wins.

If you need high resolution, then high pixel count is the way to go.

Thank you very much for your detailed information!


 JahnG's gear list:JahnG's gear list
Canon PowerShot A650 IS Panasonic ZS100 Pentax K100D Super Pentax K-r Pentax K-S1
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