In which ways, and why, are smaller sensors more efficient than larger? Part 2

Started Feb 23, 2014 | Discussions thread
OP Anders W Forum Pro • Posts: 21,468
Re: Question about DR for various sensor sizes

Lab D wrote:

Could you briefly explain why dynamic range for smaller sensors is often very close to what it is for larger ones? For example, IIRC, the E-M1 at most ISOs has DR within 1 EV of D800 (And almost identical at ISO12800). This goes against the myth that there should be a 2 stop difference between the M43 and FF sensors. Is this due to pixel size?

What I have done in this thread so far is only to show that the premise of your question is largely correct, i.e., that the difference in DR between sensors of different sizes is often significantly smaller than the one we would expect if the sensors were equally efficient. Exactly why that is the case, is something I can't yet speak of with much certainty. So what I say below is at least partly speculative.

What we know is that DR is by definition a matter of the strength of the signal (determined, up to the clipping point, by the total amount of light that falls on the sensor and its quantum efficiency) plus read noise. Differences in quantum efficiency is likely to be part of the explanation although primarily, I would guess, because the smallest sensors use BSI technology whereas the larger don't. However, read noise is likely to be the more important factor.

The data we have suggest that if pixel size is kept proportional to sensor size (as it is if we compare sensors of different size but with the same pixel count), it is difficult to keep the read noise constant as you increase the pixel size. The read noise tends to be bigger for bigger pixels, especially at low ISOs (where more of the FWC is utilized).

If instead the pixel size is kept constant as sensor size expands so that larger sensors have more pixels rather than bigger pixels, this problem can obviously be overcome. Imagine, for example, a 64 MP FF sensor made up of four 16 MP MFT sensors. The sensor technology would thus be the same. It's just the sensor area and the pixel count that varies. Now let's ask ourselves what the relative DR of these sensors would be like.

If we compare them at their original resolution, it would obviously be exactly the same at the same ISO/exposure. The FF sensor would in this case have higher resolution (at the plane in perfect focus) but less DoF. For equivalent images (same DoF, different exposure), the FF image would still have higher resolution but 2 EV worse DR (if we compare the sensors over that part of the ISO range where read noise stays constant as you change the ISO).

Now if we try to address the noisiness of the 64 MP FF image by downsampling it to 16 MP, one might at first be inclined to think that the two sensors would do equally well with regard to resolution and DR alike. Such isn't the case however. At the same ISO/exposure, the FF sensor would be one EV ahead with regard to DR and two EV behind with regard to DoF. For equivalent images, the MFT image would be one EV ahead with regard to DR and the two images the same with regard to DoF.

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