Confusion about M43 vs Full Frame ISO Performance

Started 10 months ago | Discussions thread
Horacecoker Senior Member • Posts: 2,164
Re: Let's answer this simply:

FingerPainter wrote:

Horacecoker wrote:

Great Bustard wrote:

Furthermore, the remaining pixels are made with the same amount of light as mFT, thus you lose the noise advantage that you would have otherwise had with FF.

I've been reading though this thread with interested and this strange idea that you lose a stop of light with FF if you crop the image to the same size as the M43 image is the most absurd statement I've heard in a long, long time.

So we have a 20mp M43 shot at 200 ISO and a 42mp FF frame image shot at 800 ISO we open both images and view them at 100% they look the same noise wise, yes?

No. The M43 shot wil look less noisy, because you are looking at a crop that is 4 times the fraction of the whole image as the FF crop is. IF yuor 100% crop is 2MP, then the M43 crop is 2/20 of the image and the FF crop is 2/42 of the FF image. The M43 cop wil have been made with more than twice the light, so it will lok less noisy.

We now crop the the FF image to match the M43, we look at them at 100% again, and guess what, they still look exactly the same,

No, they won't look exactly the same as before. Now they will look equally noisy.

regarding noise nothing has changed whatsoever.

Noise is a statistical property. You have changed the sample size of a bunch of random number. Naturally the larger sample is less noisy.

So how have we suddenly lost a stop of light on the FF sensor

When you crop from FF image size to M43 image size you actually lose 2 stops, because you have thrown away 3/4 of the whole FF image and only retained an image that was 1/4 of the whole FF image. 3/4 of teh light you captured is gone.

and lost the noise advantage when the noise is exactly the same on both images?

Is it me? Am I missing something?

You are probably missing

  • an amount of light can be measured in a quantity of photons, which are discrete packets of light
  • if a photon lands in one part of the sensor, it did not land in another part of the sensor, so when you crop part of the image away, you are discarding all the light that isn't in the part of the image you retained
  • that the noise of a set of photons depends on the number of photons (fewer photons have a lower SNR), and

You might also be missing the distinction between exposure (which is a number of photons per unit area) and total light, which is the number of photons over the entire surface of the sensor.

That's not all I'm missing - see my reply to Bob. 


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