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

Started Feb 23, 2014 | Discussions thread
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Anders W OP Forum Pro • Posts: 21,465
Re: In which ways, and why, are smaller sensors more efficient than larger? Part 2

This is a response to Chris's (cpw's) post here in a recently expired thread on the same topic as the present one.

cpw wrote:

Anders W wrote:

As already acknowledged, yes, although I'd express it slightly differently: My normed SNR data takes the negative impact of PRNU noise on the SNR into account whereas a QE figure doesn't.

cpw wrote:

If you do a series of calculations of your normedSNR vs. ISO for the D4, you will see the number monotically increase as you go up in ISO. This is because the PRNU noise is becoming less influential, and you are moving closer to your normedSNR becoming proportional to sqrt(QE). (If you go up really high in ISO, I guess the read noise would start to be involved). But it's not as if any QE number was changing. Bob's peak QE evaluations look solid to me, and show modern sensors holding steady in the 50-60 % region. Also, remember if you want to use your normedSNR data for comparisons, for equivalent comparisons of say a crop x4 sensor at ISO 100, that more properly gets compared to a FF at ISO 100*4^2 = ISO 1600.

Well, those are my thoughts on this.


I will attempt to cover yours and Joe's points at the same time. The end of my post says what I was getting at. You have obviously not continued evaluating your normedSNR data for D4 at higher ISOs, so I will:

I certainly have. It's just that I didn't have the opportunity to post the results of that evaluation before the previous thread expired.

for D4 I get a number of 140 at ISO 1600. Also, from Bob's QE estimate, it's peak QE = 0.53. Now I'll use S100 data, because it has a known QE of 0.52 (I don't think Bob did the S120 yet), but this is ok. For S100 at ISO 100, I get a normedSNR of 130 from your equation.

In what way can you claim a higher efficiency for the smaller sensor?

While I am glad that you like the idea I introduced in my reply to Joe (Great Bustard) here and presented in greater detail in the OP of the present thread, illustrative examples like the one you provide above can be slightly misleading. First, they have to be done right and this one isn't. The ratio of the D4 sensor area to the S100 sensor area is about 21 whereas the ratio of the ISOs for the SNRs you are comparing (DxO measured ISOs of 1192 and 85, respectively) is only about 14. It follows that the comparison is biased against the smaller sensor. Furthermore, the difference between the S120 and the S100 isn't entirely negligible (more than a dB with the measured ISO kept constant at a point close to 100), which introduces additional bias against the smaller sensor. Second, a systematic analysis across the entire sample of sensors/bodies is of course preferable to a single illustration. You find such an analysis in the OP of the present thread.

What the data really says is: When comparing sensors at equivalent operating points (higher ISO for the FF and near base ISO for the smaller sensor as I wrote above), both large and small sensors with similar QEs operate with similar SNRs.

The question of QE aside, smaller sensors do better for SNR in this case too (although that may be because they have higher QE). See the results presented in the OP of this thread.

When we try to operate the large sensored camera at its lower ISO, an operating point that the small sensored camera can't operate at, then it's achievable SNR, while being larger than at high ISO, is not as high as it could be due to PRNU. This last point, of PRNU limitations, can be ameorilated by use of flat-fielding, which would allow the sensor to operate at its shot noise limit.

It's an ameliorative procedure all right, but would you say it is easily accomplished (and worth doing) for the ordinary photographer (and outside special applications like astrophotography)?

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