what's wrong with pixel peeping?

Started Nov 28, 2008 | Discussions thread
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ejmartin Veteran Member • Posts: 6,274
Actual images for the data challenged

Here are the Imaging-Resource test images I used to generate the power spectra (at least, jpegs generated from the TIFFs; I did the analysis directly on the TIFFs, of course):



50D, downsampled to 40D dimensions:


Warning: these are 4-6MB files.

They were generated from the RAWs posted at Imaging-Resource using DPP with noise reduction turned off. My noise spectrum analysis was taken from the wall on the RHS, just to the right of the yarn skeins. The standard deviations in luminosity (as measured by PS CS3) of those patches are

40D: 3.23
50D: 4.28
50D dowsampled: 3.08

These are rough averages over several small windows, to mitigate the effects of overall tonal variation across the patches yet have a statistically valid sample. A conservative estimate of the error in these figures would be + - .05.

The theoretical reduction in noise from downsampling the 50D to the size of the 40D is the ratio of vertical pixel dimensions 2592/3168=.8182; the reduction in the measured patch is 3.08/4.28=.72. The additional reduction is due to the additional smoothing applied by PS Bicubic resampling relative to an ideal resampling (with corresponding loss of detail, which is why one typically needs to sharpen after downsampling).

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