D600 High ISO in DX

Started Nov 23, 2012 | Questions thread
bobn2
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Re: pixel pitch and SNR
In reply to Leo360, Nov 23, 2012

Leo360 wrote:

bobn2 wrote:

Leo360 wrote:

There are two different things here. There is photon count per pixel and number of photons collected per unit area.

Those are indeed two different things.

The latter does not depend on the pixel pitch but the former does. And SNR per pixel gets larger with more photons collected by that pixel (photon shot-noise per photon gets weaker). For the same exposure larger pixels capture more photos and, thus, have higher SNR. This is why pixel peeping reveals more noise-per-pixel for smaller photosites.

Indeed, bt that is of little relevance to what we are actually trying to do in photography, which is make a picture that we can look at.

The price to pay is reduced resolution.

The resolution is identical, if you look at individual pixels, because a pixel just describes the value of light where it is. There is only 'resolution' when you look at an area, and if you want to compare 'resolution' it makes sense to compare the same area (or equivalent areas when magniified to the size of the final image). So, the bottom line is that 'resolution' makes no sense at the pixel level, and nor, in terms of image quality, does the SNR.

The resolution of a digital image depends (among other things) on your sampling rate (Nyquist theorem) and pixel size has A LOT to do with it.

Yes, but you were talking of the resolution of a pixel. There is no 'resolution' at a pixel level, only when you look at areas containing many pixels.

With proper down-sampling (bicubic, etc) to the same level of detail one can hope to recover the SNR back by effectively combining outputs of multiple smaller pixels into an aggregate one but doing so does not entirely compensate for read-noise increase.

The 'downsampling' argument is a red herring. All that is required is to look at the images produced the same size.

It is not a red herring. Producing images at the same size with the same dpi means that we have to resample an output of hi-res camera to match the sampling rate of a low-res camera. Otherwise your prints will be of different size (dpi is the same, right?)

That is untrue. You do not resample the output of a high res camera to match the sampling rate of a low res camera. Generally you resample it to match the sampling rate demanded by the output image. You will generally resample all cameras so.

Correct me if I am wrong, but I think that you will have to jump through several hoops to match 4x4 um pixel SNR to 6x6 um one.

You cannot measure SNR in a single pixel in a single photograph. Now think on the implications of that.

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Bob

Did I say anything about a single photograph? To measure pixel readings statistics one shoots repeatedly in controlled environments with constant light, exposure, etc. You get mean, variance, probability distribution etc in due course of statistical analysis.

This now becomes throughly artificial, that you make the multiple observations of the same pixel over multiple exposures. It is not even guaranteed that the noise so observed would match the noise observed in the spatial domain. A thoroughly artificial result of no interest.

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Bob

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