f-number equivalent between m43 and FF

Started Mar 25, 2014 | Discussions thread
crames Regular Member • Posts: 192
Re: SNAP! Where it is?

Great Bustard wrote:

crames wrote:

Great Bustard wrote:

xpatUSA wrote:

Oops, the focal lengths were typed back-asswards, should be:

70mm zoom: mean 67.185, s.dev 8.551, min 32, max 104

35mm zoom: mean 77.853, s.dev 16.98, min 32, max 140

. . . making the wider-angle shot the noisier.

Confirmed by a test I just did at HI res (avoids binning obfuscation):

70mm: mean 49.7, s.dev 11.9

36mm: mean 48.4, s.dev 24.4

These last figures straight from RawDigger, whole image, no exporting involved. Numbers are un-scaled raw data values. Much mo' simple!

The text I highlighted in bold was exactly what I needed to know. Excellent! However, I am a bit surprised. That is, which the 70mm photo has about half the noise as the 35mm crop, as predicted,

Since Ted's results are apparently for the whole image, how do you explain the difference in standard deviation? Except for focal length, everything was the same, including the exposure and mean value. Photon noise should therefore be the same.

The 35mm photo was cropped to the same framing as the 70mm photo, thus the crop was made from only 1/4 the light as the 70mm photo, resulting in twice the photon noise.

But Ted said, "These last figures straight from RawDigger, whole image, no exporting involved." I take that to mean uncropped. It's not clear, (maybe Ted can clarify), that's why I presented my own data.

Even if Ted did resample the 36mm crop to the 70mm display size, what effect would that have on the standard deviation?

It would double the standard deviation and double the effective signal, so the NSR (noise-to-signal ratio) would remain unchanged. However, as I said above, the photon noise was twice as much to begin with, so...

You're saying that upsampling increases the standard deviation...

Here's the green channel of a piece of sky shot at 26mm, f/6.3 1/159 (S100):

26mm mean=61.01 STD=2.77

Piece of sky at 5mm, f/6.3 1/159:

5mm mean=60.76 STD=2.53

Did you get 5mm and 26mm reversed?

No, they're not reversed. They are crops representing the same area of sky. The 26mm focal length puts more pixels on that area.

5mm piece upsampled 520%:

5mm X 520% mean=60.76 STD=2.53

Note that although the noise is much more visible when the 5mm image is upsampled, the mean and standard deviation effectively do not change. (Measurements and resampling done in linear space before conversion to sRGB for display).


You just said above that upsampling increases the standard deviation. Now you agree with me that the standard deviation doesn't change?

I think that to get the standard deviation to reduce when upsampling, you would have to somehow redistribute the photons that the pixel values represent, but that's not really possible after the shot has been captured.


Please correct me if I'm wrong about this.

We note, however, that if we crop the photo taken at 5mm to the same framing as the photo taken at 26mm, then it is made with only 3.7% as much light and thus 7.3x more photon noise.

As shown, the 5mm and the 26mm have been cropped to the same framing. The standard deviations are 2.53 and 2.77, respectively. When the 5mm is upsampled to the size of the 26mm, the standard deviation does not change - it's still 2.5.

So where is the 7.3x more photon noise? Would it not show up in the standard deviation?

I would say it appears that cropping doesn't change the photon noise that has been "baked" into the pixel values.

The fallacy is saying in effect that cropping away 3/4 of the pixels is the same as removing 3/4 of the photons from the remaining the pixels.

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